diff --git a/src/main/java/net/onrc/onos/datagrid/web/DatagridWebRoutable.java b/src/main/java/net/onrc/onos/datagrid/web/DatagridWebRoutable.java
old mode 100644
new mode 100755
index e0e1666..90c7a18
--- a/src/main/java/net/onrc/onos/datagrid/web/DatagridWebRoutable.java
+++ b/src/main/java/net/onrc/onos/datagrid/web/DatagridWebRoutable.java
@@ -18,6 +18,7 @@
         Router router = new Router(context);
         router.attach("/get/map/{map-name}/json", GetMapResource.class);
         router.attach("/add/intent/json", IntentResource.class);
+        router.attach("/get/intents/json", IntentResource.class);
         return router;
     }
 
diff --git a/src/main/java/net/onrc/onos/datagrid/web/IntentResource.java b/src/main/java/net/onrc/onos/datagrid/web/IntentResource.java
index 0d50865..0f552d3 100755
--- a/src/main/java/net/onrc/onos/datagrid/web/IntentResource.java
+++ b/src/main/java/net/onrc/onos/datagrid/web/IntentResource.java
@@ -11,6 +11,7 @@
 import net.onrc.onos.intent.ShortestPathIntent;
 import net.onrc.onos.intent.IntentOperation;
 import net.onrc.onos.intent.IntentMap;
+//import net.onrc.onos.intent.Intent.IntentState;
 import net.onrc.onos.ofcontroller.networkgraph.INetworkGraphService;
 import net.onrc.onos.ofcontroller.networkgraph.NetworkGraph;
 import net.onrc.onos.registry.controller.IControllerRegistryService;
@@ -28,6 +29,8 @@
 import java.util.LinkedList;
 import java.util.Map;
 import org.codehaus.jackson.node.ArrayNode;
+import org.codehaus.jackson.node.ObjectNode;
+import org.restlet.resource.Get;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -43,11 +46,11 @@
 
     private class IntentStatus {
         String intentId;
-        boolean status;
+        String status;
         
         public IntentStatus() {}
         
-        public IntentStatus(String intentId, boolean status) {
+        public IntentStatus(String intentId, String status) {
             this.intentId = intentId;
             this.status = status;
         }
@@ -60,33 +63,28 @@
             this.intentId = intentId;
         }
         
-        public boolean getStatus() {
+        public String getStatus() {
             return status;
         }
         
-        public void setStatus(boolean status) {
+        public void setStatus(String status) {
             this.status = status;
         }
     }
     
     @Post("json")
-    public String store(String jsonFlowIntent) throws IOException {
+    public String store(String jsonIntent) throws IOException {
 	IDatagridService datagridService = (IDatagridService) getContext()
 		.getAttributes().get(IDatagridService.class.getCanonicalName());
 	if (datagridService == null) {
 	    log.debug("FlowIntentResource ONOS Datagrid Service not found");
 	    return "";
 	}
-	INetworkGraphService networkGraphService = (INetworkGraphService) getContext()
-		.getAttributes().get(
-			INetworkGraphService.class.getCanonicalName());
-	NetworkGraph graph = networkGraphService.getNetworkGraph();
         String reply = "";
 	ObjectMapper mapper = new ObjectMapper();
 	JsonNode jNode = null;
 	try {
-	    System.out.println("json string " + jsonFlowIntent);
-	    jNode = mapper.readValue(jsonFlowIntent, JsonNode.class);
+	    jNode = mapper.readValue(jsonIntent, JsonNode.class);
 	} catch (JsonGenerationException ex) {
 	    log.error("JsonGeneration exception ", ex);
 	} catch (JsonMappingException ex) {
@@ -98,11 +96,15 @@
 	if (jNode != null) {
 	    Kryo kryo = new Kryo();
 	    reply = parseJsonNode(kryo, jNode.getElements(), datagridService);
-	    // datagridService.registerIntent(intents);
 	}
         return reply;
     }
 
+    @Get("json")
+    public String retrieve() {
+        return "123";
+    }
+    
     private String parseJsonNode(Kryo kryo, Iterator<JsonNode> nodes,
 	    IDatagridService datagridService) throws IOException {
         LinkedList<IntentOperation> operations = new LinkedList<>();
@@ -119,8 +121,7 @@
 		    data = node.get(fieldName);
                     parseFields(data, fieldName, fields);
 		}
-                System.out.println("recv fields " + fields);
-                boolean status = processIntent(fields, operations);
+                String status = processIntent(fields, operations);
                 appendIntentStatus(status, (String)fields.get("intent_id"), mapper, arrayNode);
 		// datagridService.registerIntent(Long.toString(uuid),
 		// sb.toString().getBytes());
@@ -132,22 +133,25 @@
         
     }
 
-    private void appendIntentStatus(boolean status, final String applnIntentId, 
+    private void appendIntentStatus(String status, final String applnIntentId, 
             ObjectMapper mapper, ArrayNode arrayNode) throws IOException {
+        System.out.println("status " + status);
         String intentId = applnIntentId.split(":")[1];
-        String boolStr = Boolean.TRUE.toString();
-        if (status == false) {
-            boolStr = Boolean.FALSE.toString();
-        }
-        String jsonString = "{\"intent_id\":" + intentId + "," + "\"status\":" + boolStr + "}";
-        JsonNode parsedNode = mapper.readValue(jsonString, JsonNode.class);
-        arrayNode.add(parsedNode);
+        ObjectNode node = mapper.createObjectNode();
+        node.put("intent_id", intentId);
+        node.put("status", status);
+        arrayNode.add(node);
     }
     
-    private boolean processIntent(Map<String, Object> fields, LinkedList<IntentOperation> operations) {
+    private String processIntent(Map<String, Object> fields, LinkedList<IntentOperation> operations) {
         String intentType = (String)fields.get("intent_type");
-        boolean status = false;
+        String intentOp = (String)fields.get("intent_op");
+        String status = null;
         
+        IntentOperation.Operator operation = IntentOperation.Operator.ADD;
+        if ((intentOp.equals("remove"))) {
+            operation = IntentOperation.Operator.REMOVE;
+        }
         if (intentType.equals("shortest_intent_type")) {
             ShortestPathIntent spi = new ShortestPathIntent((String) fields.get("intent_id"),
                     Long.decode((String) fields.get("srcSwitch")),
@@ -156,8 +160,9 @@
                     Long.decode((String) fields.get("dstSwitch")),
                     (long) fields.get("dstPort"),
                     MACAddress.valueOf((String) fields.get("dstMac")).toLong());
-            operations.add(new IntentOperation(IntentOperation.Operator.ADD, spi));
-            status = true;
+            operations.add(new IntentOperation(operation, spi));
+            System.out.println("intent operation " + operation.toString());
+            status = (spi.getState()).toString();
         } else {
             ConstrainedShortestPathIntent cspi = new ConstrainedShortestPathIntent((String) fields.get("intent_id"),
                     Long.decode((String) fields.get("srcSwitch")),
@@ -167,24 +172,20 @@
                     (long) fields.get("dstPort"),
                     MACAddress.valueOf((String) fields.get("dstMac")).toLong(),
                     (double) fields.get("bandwidth"));
-            operations.add(new IntentOperation(IntentOperation.Operator.ADD, cspi));
-            status = true;
+            operations.add(new IntentOperation(operation, cspi));
+            status = (cspi.getState()).toString();
         }
         return status;
     }
 
     private void parseFields(JsonNode node, String fieldName, Map<String, Object> fields) {
         if ((node.isTextual())) {
-            System.out.println("textual fieldname = " + fieldName);
             fields.put(fieldName, node.getTextValue());
         } else if ((node.isInt())) {
-            System.out.println("int fieldname = " + fieldName);
             fields.put(fieldName, (long)node.getIntValue());
         } else if (node.isDouble()) {
-            System.out.println("double fieldname = " + fieldName);
             fields.put(fieldName, node.getDoubleValue());
         } else if ((node.isLong())) {
-            System.out.println("long fieldname = " + fieldName);
             fields.put(fieldName, node.getLongValue());
         }
     }
diff --git a/src/main/java/net/onrc/onos/intent/Intent.java b/src/main/java/net/onrc/onos/intent/Intent.java
index de960f0..3429d1f 100644
--- a/src/main/java/net/onrc/onos/intent/Intent.java
+++ b/src/main/java/net/onrc/onos/intent/Intent.java
@@ -4,7 +4,7 @@
  * @author Toshio Koide (t-koide@onlab.us)
  */
 public class Intent {	
-	enum IntentState {
+	public enum IntentState {
 		CREATED,
 		INST_REQ,
 		INST_NACK,
diff --git a/src/main/java/net/onrc/onos/ofcontroller/flowmanager/PerformanceMonitor.java b/src/main/java/net/onrc/onos/ofcontroller/flowmanager/PerformanceMonitor.java
index 9e23555..3721ceb 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/flowmanager/PerformanceMonitor.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/flowmanager/PerformanceMonitor.java
@@ -1,8 +1,5 @@
 package net.onrc.onos.ofcontroller.flowmanager;
 
-import java.util.ArrayList;
-import java.util.List;
-import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Queue;
 import java.util.concurrent.ConcurrentHashMap;
@@ -15,7 +12,7 @@
  * Class for collecting performance measurements
  */
 public class PerformanceMonitor {
-    private final static Map<String, Queue<Measurement>> map = new ConcurrentHashMap<>();;
+    private final static ConcurrentHashMap<String, Queue<Measurement>> map = new ConcurrentHashMap<>();;
     private final static Logger log = LoggerFactory.getLogger(PerformanceMonitor.class);
     private static long overhead;
     private static long experimentStart = Long.MAX_VALUE;
@@ -23,10 +20,10 @@
 
     /**
      * Start a performance measurement, identified by a tag
-     * 
+     *
      * Note: Only a single measurement can use the same tag at a time.
      * ..... not true anymore.
-     * 
+     *
      * @param tag for performance measurement
      */
     public static Measurement start(String tag) {
@@ -37,7 +34,11 @@
 	Queue<Measurement> list = map.get(tag);
 	if(list == null) {
 	    list = new ConcurrentLinkedQueue<Measurement>();
-	    map.put(tag, list);
+	    Queue<Measurement> existing_list = map.putIfAbsent(tag, list);
+	    if (existing_list != null) {
+		// someone concurrently added, using theirs
+		list = existing_list;
+	    }
 	}
 	Measurement m = new Measurement();
 	list.add(m);
@@ -45,12 +46,12 @@
 	overhead += System.nanoTime() - start;
 	return m;
     }
-    
+
     /**
-     * Stop a performance measurement. 
-     * 
+     * Stop a performance measurement.
+     *
      * You must have already started a measurement with tag.
-     * 
+     *
      * @param tag for performance measurement
      */
     public static void stop(String tag) {
@@ -65,7 +66,7 @@
 	}
 	overhead += System.nanoTime() - time;
     }
-        
+
     /**
      * Clear all performance measurements.
      */
@@ -74,7 +75,7 @@
 	overhead = 0;
 	experimentStart = Long.MAX_VALUE;
     }
-    
+
     /**
      * Write all performance measurements to the log
      */
@@ -104,7 +105,7 @@
 		    if(stop > experimentEnd) {
 			experimentEnd = stop;
 		    }
-		}		
+		}
 		// Collect statistics for average
 		total += m.elapsed();
 		count++;
@@ -113,10 +114,10 @@
 	    // Normalize start/stop
 	    start -= experimentStart;
 	    stop -= experimentStart;
-	    result += key + '=' + 
-		    (avg / normalization) + '/' + 
-		    (start / normalization) + '/' + 
-		    (stop / normalization) + '/' + 
+	    result += key + '=' +
+		    (avg / normalization) + '/' +
+		    (start / normalization) + '/' +
+		    (stop / normalization) + '/' +
 		    count + '\n';
 	}
 	double overheadMs = overhead / normalization;
@@ -152,8 +153,8 @@
     public static class Measurement {
 	long start;
 	long stop = -1;
-	
-	/** 
+
+	/**
 	 * Start the measurement
 	 */
 	public void start() {
@@ -161,7 +162,7 @@
 		start = System.nanoTime();
 	    }
 	}
-	
+
 	/**
 	 * Stop the measurement
 	 */
@@ -169,7 +170,7 @@
 	    long now = System.nanoTime();
 	    stop(now);
 	}
-	
+
 	/**
 	 * Stop the measurement at a specific time
 	 * @param time to stop
@@ -179,10 +180,10 @@
 		stop = time;
 	    }
 	}
-	
+
 	/**
 	 * Compute the elapsed time of the measurement in nanoseconds
-	 * 
+	 *
 	 * @return the measurement time in nanoseconds, or -1 if the measurement is stil running.
 	 */
 	public long elapsed() {
@@ -193,19 +194,20 @@
 		return stop - start;
 	    }
 	}
-		
+
 	/**
 	 * Returns the number of milliseconds for the measurement as a String.
 	 */
+	@Override
 	public String toString() {
 	    double milli = elapsed() / normalization;
 	    double startMs = start / normalization;
 	    double stopMs = stop / normalization;
-	    
+
 	    return milli + "ms/" + startMs + '/' + stopMs;
 	}
     }
-    
+
     public static void main(String args[]){
 	// test the measurement overhead
 	String tag;
diff --git a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/AbstractNetworkGraph.java b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/AbstractNetworkGraph.java
deleted file mode 100644
index b2f9247..0000000
--- a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/AbstractNetworkGraph.java
+++ /dev/null
@@ -1,109 +0,0 @@
-package net.onrc.onos.ofcontroller.networkgraph;
-
-import java.net.InetAddress;
-import java.util.Collection;
-import java.util.Collections;
-import java.util.LinkedList;
-import java.util.List;
-import java.util.Set;
-import java.util.concurrent.ConcurrentHashMap;
-import java.util.concurrent.ConcurrentMap;
-
-import net.floodlightcontroller.util.MACAddress;
-
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
-
-public class AbstractNetworkGraph implements NetworkGraph {
-	@SuppressWarnings("unused")
-	private static final Logger log = LoggerFactory.getLogger(AbstractNetworkGraph.class);
-
-	// DPID -> Switch
-	protected ConcurrentMap<Long, Switch> switches;
-
-	protected ConcurrentMap<InetAddress, Set<Device>> addr2Device;
-	protected ConcurrentMap<MACAddress, Device> mac2Device;
-
-	public AbstractNetworkGraph() {
-		// TODO: Does these object need to be stored in Concurrent Collection?
-		switches = new ConcurrentHashMap<>();
-		addr2Device = new ConcurrentHashMap<>();
-		mac2Device = new ConcurrentHashMap<>();
-	}
-
-	@Override
-	public Switch getSwitch(Long dpid) {
-		// TODO Check if it is safe to directly return this Object.
-		return switches.get(dpid);
-	}
-
-	@Override
-	public Iterable<Switch> getSwitches() {
-		// TODO Check if it is safe to directly return this Object.
-		return Collections.unmodifiableCollection(switches.values());
-	}
-
-	@Override
-	public Iterable<Link> getLinks() {
-		List<Link> linklist = new LinkedList<>();
-
-		for (Switch sw : switches.values()) {
-			Iterable<Link> links = sw.getOutgoingLinks();
-			for (Link l : links) {
-				linklist.add(l);
-			}
-		}
-		return linklist;
-	}
-
-	@Override
-	public Iterable<Link> getOutgoingLinksFromSwitch(Long dpid) {
-		Switch sw = getSwitch(dpid);
-		if (sw == null) {
-			return Collections.emptyList();
-		}
-		Iterable<Link> links = sw.getOutgoingLinks();
-		if (links instanceof Collection) {
-			return Collections.unmodifiableCollection((Collection<Link>) links);
-		} else {
-			List<Link> linklist = new LinkedList<>();
-			for (Link l : links) {
-				linklist.add(l);
-			}
-			return linklist;
-		}
-	}
-
-	@Override
-	public Iterable<Link> getIncomingLinksFromSwitch(Long dpid) {
-		Switch sw = getSwitch(dpid);
-		if (sw == null) {
-			return Collections.emptyList();
-		}
-		Iterable<Link> links = sw.getIncomingLinks();
-		if (links instanceof Collection) {
-			return Collections.unmodifiableCollection((Collection<Link>) links);
-		} else {
-			List<Link> linklist = new LinkedList<>();
-			for (Link l : links) {
-				linklist.add(l);
-			}
-			return linklist;
-		}
-	}
-
-
-	@Override
-	public Iterable<Device> getDeviceByIp(InetAddress ipAddress) {
-		Set<Device> devices = addr2Device.get(ipAddress);
-		if (devices == null) {
-			return Collections.emptySet();
-		}
-		return Collections.unmodifiableCollection(devices);
-	}
-
-	@Override
-	public Device getDeviceByMac(MACAddress address) {
-		return mac2Device.get(address);
-	}
-}
diff --git a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/DeviceImpl.java b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/DeviceImpl.java
index 808d4a6..4a79edd 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/DeviceImpl.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/DeviceImpl.java
@@ -53,7 +53,7 @@
     }
 
     /**
-     * Only {@link NetworkGraphImpl} should use this method
+     * Only {@link TopologyManager} should use this method
      * @param p
      */
     void addAttachmentPoint(Port p) {
@@ -62,7 +62,7 @@
     }
 
     /**
-     * Only {@link NetworkGraphImpl} should use this method
+     * Only {@link TopologyManager} should use this method
      * @param p
      */
     boolean removeAttachmentPoint(Port p) {
@@ -70,7 +70,7 @@
     }
 
     /**
-     * Only {@link NetworkGraphImpl} should use this method
+     * Only {@link TopologyManager} should use this method
      * @param p
      */
     boolean addIpAddress(InetAddress addr) {
@@ -78,7 +78,7 @@
     }
 
     /**
-     * Only {@link NetworkGraphImpl} should use this method
+     * Only {@link TopologyManager} should use this method
      * @param p
      */
     boolean removeIpAddress(InetAddress addr) {
diff --git a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/INetworkGraphListener.java b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/INetworkGraphListener.java
new file mode 100644
index 0000000..88806ef
--- /dev/null
+++ b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/INetworkGraphListener.java
@@ -0,0 +1,22 @@
+package net.onrc.onos.ofcontroller.networkgraph;
+
+/**
+ * Interface which needs to be implemented to receive Topology events from
+ * NetworkGraph
+ *
+ * TODO Should these interface hand over Event object or Object in NetworkGraph.
+ */
+public interface INetworkGraphListener {
+    public void putSwitchEvent(SwitchEvent switchEvent);
+    public void removeSwitchEvent(SwitchEvent switchEvent);
+
+    public void putPortEvent(PortEvent portEvent);
+    public void removePortEvent(PortEvent portEvent);
+
+    public void putLinkEvent(LinkEvent linkEvent);
+    public void removeLinkEvent(LinkEvent linkEvent);
+
+    public void putDeviceEvent(DeviceEvent deviceEvent);
+    public void removeDeviceEvent(DeviceEvent deviceEvent);
+
+}
diff --git a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/INetworkGraphService.java b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/INetworkGraphService.java
index 318c0f7..5d94f65 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/INetworkGraphService.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/INetworkGraphService.java
@@ -11,11 +11,14 @@
 	 * @return
 	 */
     public NetworkGraph getNetworkGraph();
-    
+
+    public void registerNetworkGraphListener(INetworkGraphListener listener);
+    public void deregisterNetworkGraphListener(INetworkGraphListener listener);
+
     /**
      * Allows a module to get a reference to the southbound interface to
      * the network graph.
-     * TODO Figure out how to hide the southbound interface from 
+     * TODO Figure out how to hide the southbound interface from
      * applications/modules that shouldn't touch it
      * @return
      */
diff --git a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphDatastore.java b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphDatastore.java
index d34ac46..d5dd9e0 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphDatastore.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphDatastore.java
@@ -37,9 +37,9 @@
 
 	private static final int NUM_RETRIES = 10;
 
-	private final NetworkGraphImpl graph;
+	private final TopologyManager graph;
 
-	public NetworkGraphDatastore(NetworkGraphImpl graph) {
+	public NetworkGraphDatastore(TopologyManager graph) {
 		this.graph = graph;
 	}
 
@@ -164,7 +164,7 @@
 
 	public void addLink(LinkEvent linkEvent) {
 		log.debug("Adding link {}", linkEvent);
-		
+
 		RCLink rcLink = new RCLink(linkEvent.getSrc().getDpid(), linkEvent.getSrc().getNumber(),
 				linkEvent.getDst().getDpid(), linkEvent.getDst().getNumber());
 
@@ -219,6 +219,7 @@
 				//rcDstPort.read();
 				rcLink.read();
 			} catch (ObjectDoesntExistException e) {
+			    // XXX Note: This error might be harmless, if triggered by out-dated remove Link event
 				log.error("Remove link failed {}", linkEvent, e);
 				return;
 			}
diff --git a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphImpl.java b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphImpl.java
index feb0f57..88cedac 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphImpl.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphImpl.java
@@ -1,1076 +1,109 @@
 package net.onrc.onos.ofcontroller.networkgraph;
 
 import java.net.InetAddress;
-import java.util.ArrayList;
 import java.util.Collection;
-import java.util.HashSet;
+import java.util.Collections;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Set;
-import java.util.concurrent.BlockingQueue;
-import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
 
-import net.onrc.onos.datagrid.IDatagridService;
-import net.onrc.onos.datagrid.IEventChannel;
-import net.onrc.onos.datagrid.IEventChannelListener;
-import net.onrc.onos.datastore.topology.RCLink;
-import net.onrc.onos.datastore.topology.RCPort;
-import net.onrc.onos.datastore.topology.RCSwitch;
-import net.onrc.onos.ofcontroller.networkgraph.PortEvent.SwitchPort;
-import net.onrc.onos.ofcontroller.util.EventEntry;
-import net.onrc.onos.ofcontroller.util.Dpid;
+import net.floodlightcontroller.util.MACAddress;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-/**
- * The "NB" read-only Network Map.
- *
- * - Maintain Invariant/Relationships between Topology Objects.
- *
- * TODO To be synchronized based on TopologyEvent Notification.
- *
- * TODO TBD: Caller is expected to maintain parent/child calling order. Parent
- * Object must exist before adding sub component(Add Switch -> Port).
- *
- * TODO TBD: This class may delay the requested change to handle event
- * re-ordering. e.g.) Link Add came in, but Switch was not there.
- *
- */
-public class NetworkGraphImpl extends AbstractNetworkGraph implements
-	NetworkGraphDiscoveryInterface, NetworkGraphReplicationInterface {
+public class NetworkGraphImpl implements NetworkGraph {
+	@SuppressWarnings("unused")
+	private static final Logger log = LoggerFactory.getLogger(NetworkGraphImpl.class);
 
-    private static final Logger log = LoggerFactory
-	    .getLogger(NetworkGraphImpl.class);
+	// DPID -> Switch
+	protected ConcurrentMap<Long, Switch> switches;
 
-    private IEventChannel<byte[], TopologyEvent> eventChannel;
-    private static final String EVENT_CHANNEL_NAME = "onos.topology";
-    private EventHandler eventHandler = new EventHandler();
+	protected ConcurrentMap<InetAddress, Set<Device>> addr2Device;
+	protected ConcurrentMap<MACAddress, Device> mac2Device;
 
-    private final NetworkGraphDatastore datastore;
-
-    public NetworkGraphImpl() {
-	super();
-	datastore = new NetworkGraphDatastore(this);
-    }
-
-    /**
-     * Event handler class.
-     */
-    private class EventHandler extends Thread implements
-	IEventChannelListener<byte[], TopologyEvent> {
-	private BlockingQueue<EventEntry<TopologyEvent>> topologyEvents =
-	    new LinkedBlockingQueue<EventEntry<TopologyEvent>>();
-
-	/**
-	 * Startup processing.
-	 */
-	private void startup() {
-	    //
-	    // TODO: Read all state from the database
-	    // For now, as a shortcut we read it from the datagrid
-	    //
-	    Collection<TopologyEvent> topologyEvents =
-		eventChannel.getAllEntries();
-	    Collection<EventEntry<TopologyEvent>> collection =
-		new LinkedList<EventEntry<TopologyEvent>>();
-
-	    for (TopologyEvent topologyEvent : topologyEvents) {
-		EventEntry<TopologyEvent> eventEntry =
-		    new EventEntry<TopologyEvent>(EventEntry.Type.ENTRY_ADD,
-						  topologyEvent);
-		collection.add(eventEntry);
-	    }
-	    processEvents(collection);
-	}
-
-	/**
-	 * Run the thread.
-	 */
-	public void run() {
-	    Collection<EventEntry<TopologyEvent>> collection =
-		new LinkedList<EventEntry<TopologyEvent>>();
-
-	    this.setName("NetworkGraphImpl.EventHandler " + this.getId());
-	    startup();
-
-	    //
-	    // The main loop
-	    //
-	    try {
-		while (true) {
-		    EventEntry<TopologyEvent> eventEntry = topologyEvents.take();
-		    collection.add(eventEntry);
-		    topologyEvents.drainTo(collection);
-
-		    processEvents(collection);
-		    collection.clear();
-		}
-	    } catch (Exception exception) {
-		log.debug("Exception processing Topology Events: ", exception);
-	    }
-	}
-
-	/**
-	 * Process all topology events.
-	 *
-	 * @param events the events to process.
-	 */
-	private void processEvents(Collection<EventEntry<TopologyEvent>> events) {
-	    for (EventEntry<TopologyEvent> event : events) {
-		TopologyEvent topologyEvent = event.eventData();
-		switch (event.eventType()) {
-		case ENTRY_ADD:
-		    log.debug("Topology event ENTRY_ADD: {}", topologyEvent);
-		    if (topologyEvent.switchEvent != null)
-			putSwitchReplicationEvent(topologyEvent.switchEvent);
-		    if (topologyEvent.portEvent != null)
-			putPortReplicationEvent(topologyEvent.portEvent);
-		    if (topologyEvent.linkEvent != null)
-			putLinkReplicationEvent(topologyEvent.linkEvent);
-		    if (topologyEvent.deviceEvent != null)
-			putDeviceReplicationEvent(topologyEvent.deviceEvent);
-		    break;
-		case ENTRY_REMOVE:
-		    log.debug("Topology event ENTRY_REMOVE: {}", topologyEvent);
-		    if (topologyEvent.switchEvent != null)
-			removeSwitchReplicationEvent(topologyEvent.switchEvent);
-		    if (topologyEvent.portEvent != null)
-			removePortReplicationEvent(topologyEvent.portEvent);
-		    if (topologyEvent.linkEvent != null)
-			removeLinkReplicationEvent(topologyEvent.linkEvent);
-		    if (topologyEvent.deviceEvent != null)
-			removeDeviceReplicationEvent(topologyEvent.deviceEvent);
-		    break;
-		}
-	    }
-	}
-
-	/**
-	 * Receive a notification that an entry is added.
-	 *
-	 * @param value the value for the entry.
-	 */
-	@Override
-	public void entryAdded(TopologyEvent value) {
-	    EventEntry<TopologyEvent> eventEntry =
-		new EventEntry<TopologyEvent>(EventEntry.Type.ENTRY_ADD,
-					      value);
-	    topologyEvents.add(eventEntry);
-	}
-
-	/**
-	 * Receive a notification that an entry is removed.
-	 *
-	 * @param value the value for the entry.
-	 */
-	@Override
-	public void entryRemoved(TopologyEvent value) {
-	    EventEntry<TopologyEvent> eventEntry =
-		new EventEntry<TopologyEvent>(EventEntry.Type.ENTRY_REMOVE,
-					      value);
-	    topologyEvents.add(eventEntry);
-	}
-
-	/**
-	 * Receive a notification that an entry is updated.
-	 *
-	 * @param value the value for the entry.
-	 */
-	@Override
-	public void entryUpdated(TopologyEvent value) {
-	    // NOTE: The ADD and UPDATE events are processed in same way
-	    entryAdded(value);
-	}
-    }
-
-    /**
-     * Startup processing.
-     *
-     * @param datagridService the datagrid service to use.
-     */
-    void startup(IDatagridService datagridService) {
-	eventChannel = datagridService.addListener(EVENT_CHANNEL_NAME,
-						   eventHandler,
-						   byte[].class,
-						   TopologyEvent.class);
-	eventHandler.start();
-    }
-
-    /**
-     * Exception to be thrown when Modification to the Network Graph cannot be continued due to broken invariant.
-     *
-     * XXX Should this be checked exception or RuntimeException
-     */
-    public static class BrokenInvariantException extends RuntimeException {
-	private static final long serialVersionUID = 1L;
-
-	public BrokenInvariantException() {
-	    super();
-	}
-
-	public BrokenInvariantException(String message) {
-	    super(message);
-	}
-    }
-
-    /* ******************************
-     * NetworkGraphDiscoveryInterface methods
-     * ******************************/
-
-	@Override
-	public void putSwitchEvent(SwitchEvent switchEvent) {
-		if (prepareForAddSwitchEvent(switchEvent)) {
-			datastore.addSwitch(switchEvent);
-			putSwitch(switchEvent);
-			// Send out notification
-			TopologyEvent topologyEvent =
-			    new TopologyEvent(switchEvent);
-			eventChannel.addEntry(topologyEvent.getID(),
-					      topologyEvent);
-		}
-		// TODO handle invariant violation
+	public NetworkGraphImpl() {
+		// TODO: Does these object need to be stored in Concurrent Collection?
+		switches = new ConcurrentHashMap<>();
+		addr2Device = new ConcurrentHashMap<>();
+		mac2Device = new ConcurrentHashMap<>();
 	}
 
 	@Override
-	public void removeSwitchEvent(SwitchEvent switchEvent) {
-		if (prepareForRemoveSwitchEvent(switchEvent)) {
-			datastore.deactivateSwitch(switchEvent);
-			removeSwitch(switchEvent);
-			// Send out notification
-			eventChannel.removeEntry(switchEvent.getID());
-		}
-		// TODO handle invariant violation
+	public Switch getSwitch(Long dpid) {
+		// TODO Check if it is safe to directly return this Object.
+		return switches.get(dpid);
 	}
 
 	@Override
-	public void putPortEvent(PortEvent portEvent) {
-		if (prepareForAddPortEvent(portEvent)) {
-			datastore.addPort(portEvent);
-			putPort(portEvent);
-			// Send out notification
-			TopologyEvent topologyEvent =
-			    new TopologyEvent(portEvent);
-			eventChannel.addEntry(topologyEvent.getID(),
-					      topologyEvent);
-		}
-		// TODO handle invariant violation
+	public Iterable<Switch> getSwitches() {
+		// TODO Check if it is safe to directly return this Object.
+		return Collections.unmodifiableCollection(switches.values());
 	}
 
 	@Override
-	public void removePortEvent(PortEvent portEvent) {
-		if (prepareForRemovePortEvent(portEvent)) {
-			datastore.deactivatePort(portEvent);
-			removePort(portEvent);
-			// Send out notification
-			eventChannel.removeEntry(portEvent.getID());
-		}
-		// TODO handle invariant violation
-	}
+	public Iterable<Link> getLinks() {
+		List<Link> linklist = new LinkedList<>();
 
-	@Override
-	public void putLinkEvent(LinkEvent linkEvent) {
-		if (prepareForAddLinkEvent(linkEvent)) {
-			datastore.addLink(linkEvent);
-			putLink(linkEvent);
-			// Send out notification
-			TopologyEvent topologyEvent =
-			    new TopologyEvent(linkEvent);
-			eventChannel.addEntry(topologyEvent.getID(),
-					      topologyEvent);
-		}
-		// TODO handle invariant violation
-	}
-
-	@Override
-	public void removeLinkEvent(LinkEvent linkEvent) {
-		if (prepareForRemoveLinkEvent(linkEvent)) {
-			datastore.removeLink(linkEvent);
-			removeLink(linkEvent);
-			// Send out notification
-			eventChannel.removeEntry(linkEvent.getID());
-		}
-		// TODO handle invariant violation
-	}
-
-	@Override
-	public void putDeviceEvent(DeviceEvent deviceEvent) {
-		if (prepareForAddDeviceEvent(deviceEvent)) {
-//			datastore.addDevice(deviceEvent);
-			// Send out notification
-			TopologyEvent topologyEvent =
-			    new TopologyEvent(deviceEvent);
-			eventChannel.addEntry(topologyEvent.getID(),
-					      topologyEvent);
-		}
-		// TODO handle invariant violation
-		// XXX if prepareFor~ method returned false, event should be dropped
-	}
-
-	@Override
-	public void removeDeviceEvent(DeviceEvent deviceEvent) {
-		if (prepareForRemoveDeviceEvent(deviceEvent)) {
-//			datastore.removeDevice(deviceEvent);
-			// Send out notification
-			eventChannel.removeEntry(deviceEvent.getID());
-		}
-		// TODO handle invariant violation
-		// XXX if prepareFor~ method returned false, event should be dropped
-	}
-
-	/* *****************
-	 * Internal methods to maintain invariants of the network graph
-	 * *****************/
-
-	/**
-	 *
-	 * @param swEvt
-	 * @return true if ready to accept event.
-	 */
-	private boolean prepareForAddSwitchEvent(SwitchEvent swEvt) {
-	    // No show stopping precondition
-	    // Prep: remove(deactivate) Ports on Switch, which is not on event
-	    removePortsNotOnEvent(swEvt);
-	    return true;
-	}
-
-	private boolean prepareForRemoveSwitchEvent(SwitchEvent swEvt) {
-	    // No show stopping precondition
-	    // Prep: remove(deactivate) Ports on Switch, which is not on event
-	    // XXX may be remove switch should imply wipe all ports
-	    removePortsNotOnEvent(swEvt);
-	    return true;
-	}
-
-	private void removePortsNotOnEvent(SwitchEvent swEvt) {
-	    Switch sw = switches.get( swEvt.getDpid() );
-	    if ( sw != null ) {
-		Set<Long> port_noOnEvent = new HashSet<>();
-		for( PortEvent portEvent : swEvt.getPorts()) {
-		    port_noOnEvent.add(portEvent.getNumber());
-		}
-		// Existing ports not on event should be removed.
-		// TODO Should batch eventually for performance?
-		List<Port> portsToRemove = new ArrayList<Port>();
-		for( Port p : sw.getPorts() ) {
-		    if ( !port_noOnEvent.contains(p.getNumber()) ) {
-			//PortEvent rmEvent = new PortEvent(p.getSwitch().getDpid(), p.getNumber());
-			// calling Discovery removePort() API to wipe from DB, etc.
-			//removePortEvent(rmEvent);
-		    
-		    // We can't remove ports here because this will trigger a remove
-		    // from the switch's port list, which we are currently iterating
-		    // over.
-		    portsToRemove.add(p);
-		    }
-		}
-		for (Port p : portsToRemove) {
-			PortEvent rmEvent = new PortEvent(p.getSwitch().getDpid(), p.getNumber());
-			// calling Discovery removePort() API to wipe from DB, etc.
-			removePortEvent(rmEvent);
-		}
-	    }
-	}
-
-	private boolean prepareForAddPortEvent(PortEvent portEvt) {
-		// Parent Switch must exist
-		if ( getSwitch(portEvt.getDpid()) == null) {
-		    return false;
-		}
-		// Prep: None
-		return true;
-	}
-
-	private boolean prepareForRemovePortEvent(PortEvent portEvt) {
-		// Parent Switch must exist
-		Switch sw = getSwitch(portEvt.getDpid());
-		if ( sw ==  null ) {
-		    log.debug("Switch already removed? {}", portEvt);
-		    return false;
-		}
-		Port port = sw.getPort(portEvt.getNumber());
-		if ( port == null ) {
-		    log.debug("Port already removed? {}", portEvt);
-		    // let it pass
-		    return true;
-		}
-
-		// Prep: Remove Link and Device Attachment
-		for (Device device : port.getDevices()) {
-		    log.debug("Removing Device {} on Port {}", device, portEvt);
-		    DeviceEvent devEvt = new DeviceEvent(device.getMacAddress());
-		    devEvt.addAttachmentPoint(new SwitchPort(port.getSwitch().getDpid(), port.getNumber()));
-		    // calling Discovery API to wipe from DB, etc.
-		    removeDeviceEvent(devEvt);
-		}
-		Set<Link> links = new HashSet<>();
-		links.add(port.getOutgoingLink());
-		links.add(port.getIncomingLink());
-		for ( Link link : links) {
-		    if (link == null ) {
-			continue;
-		    }
-		    log.debug("Removing Link {} on Port {}", link, portEvt);
-		    LinkEvent linkEvent = new LinkEvent(link.getSourceSwitchDpid(), link.getSourcePortNumber(), link.getDestinationSwitchDpid(), link.getDestinationPortNumber());
-		    // calling Discovery API to wipe from DB, etc.
-		    removeLinkEvent(linkEvent);
-		}
-		return true;
-	}
-
-	private boolean prepareForAddLinkEvent(LinkEvent linkEvt) {
-	    // Src/Dst Switch must exist
-	    Switch srcSw = getSwitch(linkEvt.getSrc().dpid);
-	    Switch dstSw = getSwitch(linkEvt.getDst().dpid);
-	    if ( srcSw == null || dstSw == null ) {
-		return false;
-	    }
-	    // Src/Dst Port must exist
-	    Port srcPort = srcSw.getPort(linkEvt.getSrc().number);
-	    Port dstPort = dstSw.getPort(linkEvt.getDst().number);
-	    if ( srcPort == null || dstPort == null ) {
-		return false;
-	    }
-
-	    // Prep: remove Device attachment on both Ports
-	    for (Device device : srcPort.getDevices()) {
-		DeviceEvent devEvt = new DeviceEvent(device.getMacAddress());
-		devEvt.addAttachmentPoint(new SwitchPort(srcPort.getSwitch().getDpid(), srcPort.getNumber()));
-		// calling Discovery API to wipe from DB, etc.
-		removeDeviceEvent(devEvt);
-	    }
-	    for (Device device : dstPort.getDevices()) {
-		DeviceEvent devEvt = new DeviceEvent(device.getMacAddress());
-		devEvt.addAttachmentPoint(new SwitchPort(dstPort.getSwitch().getDpid(), dstPort.getNumber()));
-		// calling Discovery API to wipe from DB, etc.
-		removeDeviceEvent(devEvt);
-	    }
-
-	    return true;
-	}
-
-	private boolean prepareForRemoveLinkEvent(LinkEvent linkEvt) {
-	    // Src/Dst Switch must exist
-	    Switch srcSw = getSwitch(linkEvt.getSrc().dpid);
-	    Switch dstSw = getSwitch(linkEvt.getDst().dpid);
-	    if ( srcSw == null || dstSw == null ) {
-		log.warn("Rejecting removeLink {} because switch doesn't exist", linkEvt);
-		return false;
-	    }
-	    // Src/Dst Port must exist
-	    Port srcPort = srcSw.getPort(linkEvt.getSrc().number);
-	    Port dstPort = dstSw.getPort(linkEvt.getDst().number);
-	    if ( srcPort == null || dstPort == null ) {
-		log.warn("Rejecting removeLink {} because port doesn't exist", linkEvt);
-		return false;
-	    }
-
-	    Link link = srcPort.getOutgoingLink();
-
-	    // Link is already gone, or different Link exist in memory
-	    // XXX Check if we should reject or just accept these cases.
-	    // it should be harmless to remove the Link on event from DB anyways
-	    if (link == null ||
-		    !link.getDestinationPortNumber().equals(linkEvt.getDst().number)
-		    || !link.getDestinationSwitchDpid().equals(linkEvt.getDst().dpid)) {
-		log.warn("Rejecting removeLink {} because link doesn't exist", linkEvt);
-		return false;
-	    }
-	    // Prep: None
-	    return true;
-	}
-
-	/**
-	 *
-	 * @param deviceEvt Event will be modified to remove inapplicable attachemntPoints/ipAddress
-	 * @return false if this event should be dropped.
-	 */
-	private boolean prepareForAddDeviceEvent(DeviceEvent deviceEvt) {
-	    boolean preconditionBroken = false;
-	    ArrayList<PortEvent.SwitchPort> failedSwitchPort = new ArrayList<>();
-	    for ( PortEvent.SwitchPort swp : deviceEvt.getAttachmentPoints() ) {
-		// Attached Ports' Parent Switch must exist
-		Switch sw = getSwitch(swp.dpid);
-		if ( sw ==  null ) {
-		    preconditionBroken = true;
-		    failedSwitchPort.add(swp);
-		    continue;
-		}
-		// Attached Ports must exist
-		Port port = sw.getPort(swp.number);
-		if ( port == null ) {
-		    preconditionBroken = true;
-		    failedSwitchPort.add(swp);
-		    continue;
-		}
-		// Attached Ports must not have Link
-		if ( port.getOutgoingLink() != null || port.getIncomingLink() != null ) {
-		    preconditionBroken = true;
-		    failedSwitchPort.add(swp);
-		    continue;
-		}
-	    }
-
-	    // Rewriting event to exclude failed attachmentPoint
-	    // XXX Assumption behind this is that inapplicable device event should
-	    // be dropped, not deferred. If we decide to defer Device event,
-	    // rewriting can become a problem
-	    List<SwitchPort>  attachmentPoints = deviceEvt.getAttachmentPoints();
-	    attachmentPoints.removeAll(failedSwitchPort);
-	    deviceEvt.setAttachmentPoints(attachmentPoints);
-
-	    if ( deviceEvt.getAttachmentPoints().isEmpty() && deviceEvt.getIpAddresses().isEmpty() ) {
-		// return false to represent: Nothing left to do for this event. Caller should drop event
-		return false;
-	    }
-
-	// Should we return false to tell caller that the event was trimmed?
-	// if ( preconditionBroken ) {
-	//     return false;
-	// }
-
-	    return true;
-	}
-
-	private boolean prepareForRemoveDeviceEvent(DeviceEvent deviceEvt) {
-		// No show stopping precondition?
-		// Prep: none
-		return true;
-	}
-
-	/* ******************************
-	 * NetworkGraphReplicationInterface methods
-	 * ******************************/
-
-	@Override
-	public void putSwitchReplicationEvent(SwitchEvent switchEvent) {
-	    if (prepareForAddSwitchEvent(switchEvent)) {
-		putSwitch(switchEvent);
-	    }
-	    // TODO handle invariant violation
-	}
-
-	@Override
-	public void removeSwitchReplicationEvent(SwitchEvent switchEvent) {
-	    if (prepareForRemoveSwitchEvent(switchEvent)) {
-		removeSwitch(switchEvent);
-	    }
-	    // TODO handle invariant violation
-	}
-
-	@Override
-	public void putPortReplicationEvent(PortEvent portEvent) {
-	    if (prepareForAddPortEvent(portEvent)) {
-		putPort(portEvent);
-	    }
-	    // TODO handle invariant violation
-	}
-
-	@Override
-	public void removePortReplicationEvent(PortEvent portEvent) {
-	    if (prepareForRemovePortEvent(portEvent)) {
-		removePort(portEvent);
-	    }
-	    // TODO handle invariant violation
-	}
-
-	@Override
-	public void putLinkReplicationEvent(LinkEvent linkEvent) {
-	    if (prepareForAddLinkEvent(linkEvent)) {
-		putLink(linkEvent);
-	    }
-	    // TODO handle invariant violation
-	}
-
-	@Override
-	public void removeLinkReplicationEvent(LinkEvent linkEvent) {
-	    if (prepareForRemoveLinkEvent(linkEvent)) {
-		removeLink(linkEvent);
-	    }
-	    // TODO handle invariant violation
-	}
-
-	@Override
-	public void putDeviceReplicationEvent(DeviceEvent deviceEvent) {
-	    if (prepareForAddDeviceEvent(deviceEvent)) {
-		putDevice(deviceEvent);
-	    }
-	    // TODO handle invariant violation
-	}
-
-	@Override
-	public void removeDeviceReplicationEvent(DeviceEvent deviceEvent) {
-	    if (prepareForRemoveDeviceEvent(deviceEvent)) {
-		removeDevice(deviceEvent);
-	    }
-	    // TODO handle invariant violation
-	}
-
-	/* ************************************************
-	 * Internal In-memory object mutation methods.
-	 * ************************************************/
-
-	void putSwitch(SwitchEvent swEvt) {
-	    if (swEvt == null) {
-		throw new IllegalArgumentException("Switch cannot be null");
-	    }
-
-	    Switch sw = switches.get(swEvt.getDpid());
-
-	    if (sw == null) {
-		sw = new SwitchImpl(this, swEvt.getDpid());
-		Switch existing = switches.putIfAbsent(swEvt.getDpid(), sw);
-		if (existing != null) {
-		    log.warn(
-			    "Concurrent putSwitch not expected. Continuing updating {}",
-			    existing);
-		    sw = existing;
-		}
-	    }
-
-	    // Update when more attributes are added to Event object
-	    // no attribute to update for now
-
-	    // TODO handle child Port event properly for performance
-	    for (PortEvent portEvt : swEvt.getPorts() ) {
-		putPort(portEvt);
-	    }
-
-	}
-
-	void removeSwitch(SwitchEvent swEvt) {
-	    if (swEvt == null) {
-		throw new IllegalArgumentException("Switch cannot be null");
-	    }
-
-	    // TODO handle child Port event properly for performance
-	    for (PortEvent portEvt : swEvt.getPorts() ) {
-		removePort(portEvt);
-	    }
-
-	    Switch sw = switches.get(swEvt.getDpid());
-
-	    if (sw == null) {
-		log.warn("Switch {} already removed, ignoring", swEvt);
-		return;
-	    }
-
-	    // Sanity check
-	    if (!sw.getPorts().isEmpty()) {
-		log.warn(
-			"Ports on Switch {} should be removed prior to removing Switch. Removing Switch anyways",
-			swEvt);
-		// XXX Should we remove Port?
-	    }
-	    if (!sw.getDevices().isEmpty()) {
-		log.warn(
-			"Devices on Switch {} should be removed prior to removing Switch. Removing Switch anyways",
-			swEvt);
-		// XXX Should we remove Device to Switch relation?
-	    }
-	    if (!sw.getIncomingLinks().iterator().hasNext()) {
-		log.warn(
-			"IncomingLinks on Switch {} should be removed prior to removing Switch. Removing Switch anyways",
-			swEvt);
-		// XXX Should we remove Link?
-	    }
-	    if (!sw.getOutgoingLinks().iterator().hasNext()) {
-		log.warn(
-			"OutgoingLinks on Switch {} should be removed prior to removing Switch. Removing Switch anyways",
-			swEvt);
-		// XXX Should we remove Link?
-	    }
-
-	    boolean removed = switches.remove(swEvt.getDpid(), sw);
-	    if (removed) {
-		log.warn(
-			"Switch instance was replaced concurrently while removing {}. Something is not right.",
-			sw);
-	    }
-	}
-
-	void putPort(PortEvent portEvt) {
-	    if (portEvt == null) {
-		throw new IllegalArgumentException("Port cannot be null");
-	    }
-	    Switch sw = switches.get(portEvt.getDpid());
-	    if (sw == null) {
-		throw new BrokenInvariantException(String.format(
-			"Switch with dpid %s did not exist.",
-			new Dpid(portEvt.getDpid())));
-	    }
-	    Port p = sw.getPort(portEvt.getNumber());
-	    PortImpl port = null;
-	    if (p != null) {
-		port = getPortImpl(p);
-	    }
-
-	    if (port == null) {
-		port = new PortImpl(this, sw, portEvt.getNumber());
-	    }
-
-	    // TODO update attributes
-
-	    SwitchImpl s = getSwitchImpl(sw);
-	    s.addPort(port);
-	}
-
-	void removePort(PortEvent portEvt) {
-	    if (portEvt == null) {
-		throw new IllegalArgumentException("Port cannot be null");
-	    }
-
-	    Switch sw = switches.get(portEvt.getDpid());
-	    if (sw == null) {
-		log.warn("Parent Switch for Port {} already removed, ignoring", portEvt);
-		return;
-	    }
-
-	    Port p = sw.getPort(portEvt.getNumber());
-	    if (p == null) {
-		log.warn("Port {} already removed, ignoring", portEvt);
-		return;
-	    }
-
-	    // check if there is something referring to this Port
-
-	    if (!p.getDevices().iterator().hasNext()) {
-		log.warn(
-			"Devices on Port {} should be removed prior to removing Port. Removing Port anyways",
-			portEvt);
-		// XXX Should we remove Device to Port relation?
-	    }
-	    if (p.getIncomingLink() != null) {
-		log.warn(
-			"IncomingLinks on Port {} should be removed prior to removing Port. Removing Port anyways",
-			portEvt);
-		// XXX Should we remove Link?
-	    }
-	    if (p.getOutgoingLink() != null) {
-		log.warn(
-			"OutgoingLinks on Port {} should be removed prior to removing Port. Removing Port anyways",
-			portEvt);
-		// XXX Should we remove Link?
-	    }
-
-	    // remove Port from Switch
-	    SwitchImpl s = getSwitchImpl(sw);
-	    s.removePort(p);
-	}
-
-	void putLink(LinkEvent linkEvt) {
-	    if (linkEvt == null) {
-		throw new IllegalArgumentException("Link cannot be null");
-	    }
-
-	    Switch srcSw = switches.get(linkEvt.getSrc().dpid);
-	    if (srcSw == null) {
-		throw new BrokenInvariantException(
-			String.format(
-				"Switch with dpid %s did not exist.",
-				new Dpid(linkEvt.getSrc().dpid)));
-	    }
-
-	    Switch dstSw = switches.get(linkEvt.getDst().dpid);
-	    if (dstSw == null) {
-		throw new BrokenInvariantException(
-			String.format(
-				"Switch with dpid %s did not exist.",
-				new Dpid(linkEvt.getDst().dpid)));
-	    }
-
-	    Port srcPort = srcSw.getPort(linkEvt.getSrc().number);
-	    if (srcPort == null) {
-		throw new BrokenInvariantException(
-			String.format(
-				"Src Port %s of a Link did not exist.",
-				linkEvt.getSrc() ));
-	    }
-
-	    Port dstPort = dstSw.getPort(linkEvt.getDst().number);
-	    if (dstPort == null) {
-		throw new BrokenInvariantException(
-			String.format(
-				"Dst Port %s of a Link did not exist.",
-				linkEvt.getDst() ));
-	    }
-
-	    // getting Link instance from destination port incoming Link
-	    Link l = dstPort.getIncomingLink();
-	    LinkImpl link = null;
-	    assert( l == srcPort.getOutgoingLink() );
-	    if (l != null) {
-		link = getLinkImpl(l);
-	    }
-
-	    if (link == null) {
-		link = new LinkImpl(this, srcPort, dstPort);
-	    }
-
-
-	    PortImpl dstPortMem = getPortImpl(dstPort);
-	    PortImpl srcPortMem = getPortImpl(srcPort);
-
-	    // Add Link first to avoid further Device addition
-
-	    // add Link to Port
-	    dstPortMem.setIncomingLink(link);
-	    srcPortMem.setOutgoingLink(link);
-
-	    // remove Device Pointing to Port if any
-	    for(Device d : dstPortMem.getDevices() ) {
-		log.error("Device {} on Port {} should have been removed prior to adding Link {}", d, dstPort, linkEvt);
-		DeviceImpl dev = getDeviceImpl(d);
-		dev.removeAttachmentPoint(dstPort);
-		// This implies that change is made to Device Object.
-		// sending Device attachment point removed event
-		DeviceEvent rmEvent = new DeviceEvent(d.getMacAddress());
-		rmEvent.addAttachmentPoint(new SwitchPort(dstPort.getDpid(), dstPort.getNumber()));
-		removeDeviceEvent(rmEvent);
-	    }
-	    dstPortMem.removeAllDevice();
-	    for(Device d : srcPortMem.getDevices() ) {
-		log.error("Device {} on Port {} should have been removed prior to adding Link {}", d, srcPort, linkEvt);
-		DeviceImpl dev = getDeviceImpl(d);
-		dev.removeAttachmentPoint(srcPort);
-		// This implies that change is made to Device Object.
-		// sending Device attachment point removed event
-		DeviceEvent rmEvent = new DeviceEvent(d.getMacAddress());
-		rmEvent.addAttachmentPoint(new SwitchPort(dstPort.getDpid(), dstPort.getNumber()));
-		removeDeviceEvent(rmEvent);
-	    }
-	    srcPortMem.removeAllDevice();
-
-	}
-
-	void removeLink(LinkEvent linkEvt) {
-	    if (linkEvt == null) {
-		throw new IllegalArgumentException("Link cannot be null");
-	    }
-
-	    Switch srcSw = switches.get(linkEvt.getSrc().dpid);
-	    if (srcSw == null) {
-		log.warn("Src Switch for Link {} already removed, ignoring", linkEvt);
-		return;
-	    }
-
-	    Switch dstSw = switches.get(linkEvt.getDst().dpid);
-	    if (dstSw == null) {
-		log.warn("Dst Switch for Link {} already removed, ignoring", linkEvt);
-		return;
-	    }
-
-	    Port srcPort = srcSw.getPort(linkEvt.getSrc().number);
-	    if (srcPort == null) {
-		log.warn("Src Port for Link {} already removed, ignoring", linkEvt);
-		return;
-	    }
-
-	    Port dstPort = dstSw.getPort(linkEvt.getDst().number);
-	    if (dstPort == null) {
-		log.warn("Dst Port for Link {} already removed, ignoring", linkEvt);
-		return;
-	    }
-
-	    Link l = dstPort.getIncomingLink();
-	    if (  l == null ) {
-		log.warn("Link {} already removed on destination Port", linkEvt);
-	    }
-	    l = srcPort.getOutgoingLink();
-	    if (  l == null ) {
-		log.warn("Link {} already removed on src Port", linkEvt);
-	    }
-
-	    getPortImpl(dstPort).setIncomingLink(null);
-	    getPortImpl(srcPort).setOutgoingLink(null);
-	}
-
-	// XXX Need to rework Device related
-	void putDevice(DeviceEvent deviceEvt) {
-	    if (deviceEvt == null) {
-		throw new IllegalArgumentException("Device cannot be null");
-	    }
-
-	    Device device = getDeviceByMac(deviceEvt.getMac());
-	    if ( device == null ) {
-		device = new DeviceImpl(this, deviceEvt.getMac());
-		Device existing = mac2Device.putIfAbsent(deviceEvt.getMac(), device);
-		if (existing != null) {
-		    log.warn(
-			    "Concurrent putDevice seems to be in action. Continuing updating {}",
-			    existing);
-		    device = existing;
-		}
-	    }
-	    DeviceImpl memDevice = getDeviceImpl(device);
-
-	    // for each attachment point
-	    for (SwitchPort swp : deviceEvt.getAttachmentPoints() ) {
-		// Attached Ports' Parent Switch must exist
-		Switch sw = getSwitch(swp.dpid);
-		if ( sw ==  null ) {
-		    log.warn("Switch for the attachment point {} did not exist. skipping mutation", swp);
-		    continue;
-		}
-		// Attached Ports must exist
-		Port port = sw.getPort(swp.number);
-		if ( port == null ) {
-		    log.warn("Port for the attachment point {} did not exist. skipping mutation", swp);
-		    continue;
-		}
-		// Attached Ports must not have Link
-		if ( port.getOutgoingLink() != null || port.getIncomingLink() != null ) {
-		    log.warn("Link (Out:{},In:{}) exist on the attachment point, skipping mutation.", port.getOutgoingLink(), port.getIncomingLink());
-		    continue;
-		}
-
-		// finally add Device <-> Port on In-memory structure
-		PortImpl memPort = getPortImpl(port);
-		memPort.addDevice(device);
-		memDevice.addAttachmentPoint(port);
-	    }
-
-	    // for each IP address
-	    for( InetAddress ipAddr : deviceEvt.getIpAddresses() ) {
-		// Add Device -> IP
-		memDevice.addIpAddress(ipAddr);
-
-		// Add IP -> Set<Device>
-		boolean updated = false;
-		do {
-		    Set<Device> devices = this.addr2Device.get(ipAddr);
-		    if ( devices == null ) {
-			devices = new HashSet<>();
-			Set<Device> existing = this.addr2Device.putIfAbsent(ipAddr, devices);
-			if ( existing == null ) {
-			    // success
-			    updated = true;
+		for (Switch sw : switches.values()) {
+			Iterable<Link> links = sw.getOutgoingLinks();
+			for (Link l : links) {
+				linklist.add(l);
 			}
-		    } else {
-			Set<Device> updateDevices = new HashSet<>(devices);
-			updateDevices.add(device);
-			updated = this.addr2Device.replace(ipAddr, devices, updateDevices);
-		    }
-		    if (!updated) {
-			log.debug("Collision detected, updating IP to Device mapping retrying.");
-		    }
-		} while( !updated );
-	    }
-	}
-
-	void removeDevice(DeviceEvent deviceEvt) {
-	    if (deviceEvt == null) {
-		throw new IllegalArgumentException("Device cannot be null");
-	    }
-
-	    Device device = getDeviceByMac(deviceEvt.getMac());
-	    if ( device == null ) {
-		log.warn("Device {} already removed, ignoring", deviceEvt);
-		return;
-	    }
-	    DeviceImpl memDevice = getDeviceImpl(device);
-
-	    // for each attachment point
-	    for (SwitchPort swp : deviceEvt.getAttachmentPoints() ) {
-		// Attached Ports' Parent Switch must exist
-		Switch sw = getSwitch(swp.dpid);
-		if ( sw ==  null ) {
-		    log.warn("Switch for the attachment point {} did not exist. skipping attachment point mutation", swp);
-		    continue;
 		}
-		// Attached Ports must exist
-		Port port = sw.getPort(swp.number);
-		if ( port == null ) {
-		    log.warn("Port for the attachment point {} did not exist. skipping attachment point mutation", swp);
-		    continue;
+		return linklist;
+	}
+
+	@Override
+	public Iterable<Link> getOutgoingLinksFromSwitch(Long dpid) {
+		Switch sw = getSwitch(dpid);
+		if (sw == null) {
+			return Collections.emptyList();
 		}
-
-		// finally remove Device <-> Port on In-memory structure
-		PortImpl memPort = getPortImpl(port);
-		memPort.removeDevice(device);
-		memDevice.removeAttachmentPoint(port);
-	    }
-
-	    // for each IP address
-	    for( InetAddress ipAddr : deviceEvt.getIpAddresses() ) {
-		// Remove Device -> IP
-		memDevice.removeIpAddress(ipAddr);
-
-		// Remove IP -> Set<Device>
-		boolean updated = false;
-		do {
-		    Set<Device> devices = this.addr2Device.get(ipAddr);
-		    if ( devices == null ) {
-			// already empty set, nothing to do
-			updated = true;
-		    } else {
-			Set<Device> updateDevices = new HashSet<>(devices);
-			updateDevices.remove(device);
-			updated = this.addr2Device.replace(ipAddr, devices, updateDevices);
-		    }
-		    if (!updated) {
-			log.debug("Collision detected, updating IP to Device mapping retrying.");
-		    }
-		} while( !updated );
-	    }
-	}
-
-	private SwitchImpl getSwitchImpl(Switch sw) {
-	    if (sw instanceof SwitchImpl) {
-		return (SwitchImpl) sw;
-	    }
-	    throw new ClassCastException("SwitchImpl expected, but found: " + sw);
-	}
-
-	private PortImpl getPortImpl(Port p) {
-	    if (p instanceof PortImpl) {
-		return (PortImpl) p;
-	    }
-	    throw new ClassCastException("PortImpl expected, but found: " + p);
-	}
-
-	private LinkImpl getLinkImpl(Link l) {
-	    if (l instanceof LinkImpl) {
-		return (LinkImpl) l;
-	    }
-	    throw new ClassCastException("LinkImpl expected, but found: " + l);
-	}
-
-	private DeviceImpl getDeviceImpl(Device d) {
-	    if (d instanceof DeviceImpl) {
-		return (DeviceImpl) d;
-	    }
-	    throw new ClassCastException("DeviceImpl expected, but found: " + d);
-	}
-
-	@Deprecated
-	public void loadWholeTopologyFromDB() {
-	    // XXX clear everything first?
-
-	    for (RCSwitch sw : RCSwitch.getAllSwitches()) {
-		if ( sw.getStatus() != RCSwitch.STATUS.ACTIVE ) {
-		    continue;
+		Iterable<Link> links = sw.getOutgoingLinks();
+		if (links instanceof Collection) {
+			return Collections.unmodifiableCollection((Collection<Link>) links);
+		} else {
+			List<Link> linklist = new LinkedList<>();
+			for (Link l : links) {
+				linklist.add(l);
+			}
+			return linklist;
 		}
-		putSwitchReplicationEvent(new SwitchEvent(sw.getDpid()));
-	    }
+	}
 
-	    for (RCPort p : RCPort.getAllPorts()) {
-		if (p.getStatus() != RCPort.STATUS.ACTIVE) {
-		    continue;
+	@Override
+	public Iterable<Link> getIncomingLinksFromSwitch(Long dpid) {
+		Switch sw = getSwitch(dpid);
+		if (sw == null) {
+			return Collections.emptyList();
 		}
-		putPortReplicationEvent(new PortEvent(p.getDpid(), p.getNumber() ));
-	    }
+		Iterable<Link> links = sw.getIncomingLinks();
+		if (links instanceof Collection) {
+			return Collections.unmodifiableCollection((Collection<Link>) links);
+		} else {
+			List<Link> linklist = new LinkedList<>();
+			for (Link l : links) {
+				linklist.add(l);
+			}
+			return linklist;
+		}
+	}
 
-	    // TODO Is Device going to be in DB? If so, read from DB.
-	    //	for (RCDevice d : RCDevice.getAllDevices()) {
-	    //	    DeviceEvent devEvent = new DeviceEvent( MACAddress.valueOf(d.getMac()) );
-	    //	    for (byte[] portId : d.getAllPortIds() ) {
-	    //		devEvent.addAttachmentPoint( new SwitchPort( RCPort.getDpidFromKey(portId), RCPort.getNumberFromKey(portId) ));
-	    //	    }
-	    //	}
 
-	    for (RCLink l : RCLink.getAllLinks()) {
-		putLinkReplicationEvent( new LinkEvent(l.getSrc().dpid, l.getSrc().number, l.getDst().dpid, l.getDst().number));
-	    }
+	@Override
+	public Iterable<Device> getDeviceByIp(InetAddress ipAddress) {
+		Set<Device> devices = addr2Device.get(ipAddress);
+		if (devices == null) {
+			return Collections.emptySet();
+		}
+		return Collections.unmodifiableCollection(devices);
+	}
+
+	@Override
+	public Device getDeviceByMac(MACAddress address) {
+		return mac2Device.get(address);
 	}
 }
diff --git a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphModule.java b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphModule.java
index 3edf5c2..4b2ef34 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphModule.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/NetworkGraphModule.java
@@ -5,6 +5,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.CopyOnWriteArrayList;
 
 import net.floodlightcontroller.core.module.FloodlightModuleContext;
 import net.floodlightcontroller.core.module.FloodlightModuleException;
@@ -13,30 +14,34 @@
 import net.floodlightcontroller.restserver.IRestApiService;
 import net.onrc.onos.datagrid.IDatagridService;
 import net.onrc.onos.ofcontroller.networkgraph.web.NetworkGraphWebRoutable;
+import net.onrc.onos.registry.controller.IControllerRegistryService;
 
 public class NetworkGraphModule implements IFloodlightModule, INetworkGraphService {
 
 	// This is initialized as a module for now
 	// private RCNetworkGraphPublisher eventListener;
-	
-	private NetworkGraphImpl networkGraph;
+
+	private TopologyManager networkGraph;
 	//private NetworkGraphDatastore southboundNetworkGraph;
 	private IDatagridService datagridService;
+	private IControllerRegistryService registryService;
+
+	private CopyOnWriteArrayList<INetworkGraphListener> networkGraphListeners;
 
 	private IRestApiService restApi;
 
 	@Override
 	public Collection<Class<? extends IFloodlightService>> getModuleServices() {
-		List<Class<? extends IFloodlightService>> services = 
+		List<Class<? extends IFloodlightService>> services =
 				new ArrayList<Class<? extends IFloodlightService>>();
 		services.add(INetworkGraphService.class);
 		return services;
 	}
 
 	@Override
-	public Map<Class<? extends IFloodlightService>, IFloodlightService> 
+	public Map<Class<? extends IFloodlightService>, IFloodlightService>
 			getServiceImpls() {
-		Map<Class<? extends IFloodlightService>, IFloodlightService> impls = 
+		Map<Class<? extends IFloodlightService>, IFloodlightService> impls =
 				new HashMap<Class<? extends IFloodlightService>, IFloodlightService>();
 		impls.put(INetworkGraphService.class, this);
 		return impls;
@@ -44,20 +49,22 @@
 
 	@Override
 	public Collection<Class<? extends IFloodlightService>> getModuleDependencies() {
-		List<Class<? extends IFloodlightService>> dependencies = 
+		List<Class<? extends IFloodlightService>> dependencies =
 				new ArrayList<Class<? extends IFloodlightService>>();
 		dependencies.add(IDatagridService.class);
 		dependencies.add(IRestApiService.class);
 		return dependencies;
 	}
-	
+
 	@Override
 	public void init(FloodlightModuleContext context)
 			throws FloodlightModuleException {
 		restApi = context.getServiceImpl(IRestApiService.class);
 		datagridService = context.getServiceImpl(IDatagridService.class);
-		
-		networkGraph = new NetworkGraphImpl();
+		registryService = context.getServiceImpl(IControllerRegistryService.class);
+
+		networkGraphListeners = new CopyOnWriteArrayList<>();
+		networkGraph = new TopologyManager(registryService, networkGraphListeners);
 		//southboundNetworkGraph = new NetworkGraphDatastore(networkGraph);
 	}
 
@@ -77,4 +84,14 @@
 		return networkGraph;
 	}
 
+	@Override
+	public void registerNetworkGraphListener(INetworkGraphListener listener) {
+	    networkGraphListeners.addIfAbsent(listener);
+	}
+
+	@Override
+	public void deregisterNetworkGraphListener(INetworkGraphListener listener) {
+	    networkGraphListeners.remove(listener);
+	}
+
 }
diff --git a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/TopologyEvent.java b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/TopologyEvent.java
index 927d266..2892e0c 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/TopologyEvent.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/TopologyEvent.java
@@ -8,6 +8,8 @@
  * in a single transaction.
  */
 public class TopologyEvent {
+    public static final String NOBODY = "";
+    String originID = NOBODY;
     SwitchEvent switchEvent = null;		// Set for Switch event
     PortEvent portEvent = null;			// Set for Port event
     LinkEvent linkEvent = null;			// Set for Link event
@@ -24,8 +26,9 @@
      *
      * @param switchEvent the Switch event to use.
      */
-    TopologyEvent(SwitchEvent switchEvent) {
+    TopologyEvent(SwitchEvent switchEvent, String originID) {
 	this.switchEvent = switchEvent;
+	setOriginID(originID);
     }
 
     /**
@@ -33,8 +36,9 @@
      *
      * @param portEvent the Port event to use.
      */
-    TopologyEvent(PortEvent portEvent) {
+    TopologyEvent(PortEvent portEvent, String originID) {
 	this.portEvent = portEvent;
+	setOriginID(originID);
     }
 
     /**
@@ -42,8 +46,9 @@
      *
      * @param linkEvent the Link event to use.
      */
-    TopologyEvent(LinkEvent linkEvent) {
+    TopologyEvent(LinkEvent linkEvent, String originID) {
 	this.linkEvent = linkEvent;
+	setOriginID(originID);
     }
 
     /**
@@ -51,8 +56,9 @@
      *
      * @param deviceEvent the Device event to use.
      */
-    TopologyEvent(DeviceEvent deviceEvent) {
+    TopologyEvent(DeviceEvent deviceEvent, String originID) {
 	this.deviceEvent = deviceEvent;
+	setOriginID(originID);
     }
 
     /**
@@ -70,7 +76,7 @@
 	    return linkEvent.toString();
 	if (deviceEvent != null)
 	    return deviceEvent.toString();
-	return null;
+	return "[Empty TopologyEvent]";
     }
 
     /**
@@ -89,4 +95,16 @@
 	    return deviceEvent.getID();
 	return null;
     }
+
+    public String getOriginID() {
+	return originID;
+    }
+
+    void setOriginID(String originID) {
+	if (originID != null) {
+	    this.originID = originID;
+	} else {
+	    this.originID = NOBODY;
+	}
+    }
 }
diff --git a/src/main/java/net/onrc/onos/ofcontroller/networkgraph/TopologyManager.java b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/TopologyManager.java
new file mode 100644
index 0000000..d0e215f
--- /dev/null
+++ b/src/main/java/net/onrc/onos/ofcontroller/networkgraph/TopologyManager.java
@@ -0,0 +1,1144 @@
+package net.onrc.onos.ofcontroller.networkgraph;
+
+import java.net.InetAddress;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.HashSet;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.concurrent.LinkedBlockingQueue;
+
+import net.onrc.onos.datagrid.IDatagridService;
+import net.onrc.onos.datagrid.IEventChannel;
+import net.onrc.onos.datagrid.IEventChannelListener;
+import net.onrc.onos.datastore.topology.RCLink;
+import net.onrc.onos.datastore.topology.RCPort;
+import net.onrc.onos.datastore.topology.RCSwitch;
+import net.onrc.onos.ofcontroller.networkgraph.PortEvent.SwitchPort;
+import net.onrc.onos.ofcontroller.util.EventEntry;
+import net.onrc.onos.ofcontroller.util.Dpid;
+import net.onrc.onos.registry.controller.IControllerRegistryService;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The "NB" read-only Network Map.
+ *
+ * - Maintain Invariant/Relationships between Topology Objects.
+ *
+ * TODO To be synchronized based on TopologyEvent Notification.
+ *
+ * TODO TBD: Caller is expected to maintain parent/child calling order. Parent
+ * Object must exist before adding sub component(Add Switch -> Port).
+ *
+ * TODO TBD: This class may delay the requested change to handle event
+ * re-ordering. e.g.) Link Add came in, but Switch was not there.
+ *
+ */
+public class TopologyManager extends NetworkGraphImpl implements
+	NetworkGraphDiscoveryInterface, NetworkGraphReplicationInterface {
+
+    private static final Logger log = LoggerFactory
+	    .getLogger(TopologyManager.class);
+
+    private IEventChannel<byte[], TopologyEvent> eventChannel;
+    private static final String EVENT_CHANNEL_NAME = "onos.topology";
+    private EventHandler eventHandler = new EventHandler();
+
+    private final NetworkGraphDatastore datastore;
+    private final IControllerRegistryService registryService;
+    private CopyOnWriteArrayList<INetworkGraphListener> networkGraphListeners;
+
+    public TopologyManager(IControllerRegistryService registryService, CopyOnWriteArrayList<INetworkGraphListener> networkGraphListeners) {
+	super();
+	datastore = new NetworkGraphDatastore(this);
+	this.registryService = registryService;
+	this.networkGraphListeners = networkGraphListeners;
+    }
+
+    /**
+     * Event handler class.
+     */
+    private class EventHandler extends Thread implements
+	IEventChannelListener<byte[], TopologyEvent> {
+	private BlockingQueue<EventEntry<TopologyEvent>> topologyEvents =
+	    new LinkedBlockingQueue<EventEntry<TopologyEvent>>();
+
+	/**
+	 * Startup processing.
+	 */
+	private void startup() {
+	    //
+	    // TODO: Read all state from the database
+	    // For now, as a shortcut we read it from the datagrid
+	    //
+	    Collection<TopologyEvent> topologyEvents =
+		eventChannel.getAllEntries();
+	    Collection<EventEntry<TopologyEvent>> collection =
+		new LinkedList<EventEntry<TopologyEvent>>();
+
+	    for (TopologyEvent topologyEvent : topologyEvents) {
+		EventEntry<TopologyEvent> eventEntry =
+		    new EventEntry<TopologyEvent>(EventEntry.Type.ENTRY_ADD,
+						  topologyEvent);
+		collection.add(eventEntry);
+	    }
+	    processEvents(collection);
+	}
+
+	/**
+	 * Run the thread.
+	 */
+	@Override
+	public void run() {
+	    Collection<EventEntry<TopologyEvent>> collection =
+		new LinkedList<EventEntry<TopologyEvent>>();
+
+	    this.setName("TopologyManager.EventHandler " + this.getId());
+	    startup();
+
+	    //
+	    // The main loop
+	    //
+	    try {
+		while (true) {
+		    EventEntry<TopologyEvent> eventEntry = topologyEvents.take();
+		    collection.add(eventEntry);
+		    topologyEvents.drainTo(collection);
+
+		    processEvents(collection);
+		    collection.clear();
+		}
+	    } catch (Exception exception) {
+		log.debug("Exception processing Topology Events: ", exception);
+	    }
+	}
+
+	/**
+	 * Process all topology events.
+	 *
+	 * @param events the events to process.
+	 */
+	private void processEvents(Collection<EventEntry<TopologyEvent>> events) {
+	    for (EventEntry<TopologyEvent> event : events) {
+		if (event.eventData().getOriginID().equals(registryService.getControllerId())) {
+		    // ignore event triggered by myself
+		    continue;
+		}
+		TopologyEvent topologyEvent = event.eventData();
+		switch (event.eventType()) {
+		case ENTRY_ADD:
+		    log.debug("Topology event ENTRY_ADD: {}", topologyEvent);
+		    if (topologyEvent.switchEvent != null)
+			putSwitchReplicationEvent(topologyEvent.switchEvent);
+		    if (topologyEvent.portEvent != null)
+			putPortReplicationEvent(topologyEvent.portEvent);
+		    if (topologyEvent.linkEvent != null)
+			putLinkReplicationEvent(topologyEvent.linkEvent);
+		    if (topologyEvent.deviceEvent != null)
+			putDeviceReplicationEvent(topologyEvent.deviceEvent);
+		    break;
+		case ENTRY_REMOVE:
+		    log.debug("Topology event ENTRY_REMOVE: {}", topologyEvent);
+		    if (topologyEvent.switchEvent != null)
+			removeSwitchReplicationEvent(topologyEvent.switchEvent);
+		    if (topologyEvent.portEvent != null)
+			removePortReplicationEvent(topologyEvent.portEvent);
+		    if (topologyEvent.linkEvent != null)
+			removeLinkReplicationEvent(topologyEvent.linkEvent);
+		    if (topologyEvent.deviceEvent != null)
+			removeDeviceReplicationEvent(topologyEvent.deviceEvent);
+		    break;
+		}
+	    }
+	}
+
+	/**
+	 * Receive a notification that an entry is added.
+	 *
+	 * @param value the value for the entry.
+	 */
+	@Override
+	public void entryAdded(TopologyEvent value) {
+	    EventEntry<TopologyEvent> eventEntry =
+		new EventEntry<TopologyEvent>(EventEntry.Type.ENTRY_ADD,
+					      value);
+	    topologyEvents.add(eventEntry);
+	}
+
+	/**
+	 * Receive a notification that an entry is removed.
+	 *
+	 * @param value the value for the entry.
+	 */
+	@Override
+	public void entryRemoved(TopologyEvent value) {
+	    EventEntry<TopologyEvent> eventEntry =
+		new EventEntry<TopologyEvent>(EventEntry.Type.ENTRY_REMOVE,
+					      value);
+	    topologyEvents.add(eventEntry);
+	}
+
+	/**
+	 * Receive a notification that an entry is updated.
+	 *
+	 * @param value the value for the entry.
+	 */
+	@Override
+	public void entryUpdated(TopologyEvent value) {
+	    // NOTE: The ADD and UPDATE events are processed in same way
+	    entryAdded(value);
+	}
+    }
+
+    /**
+     * Startup processing.
+     *
+     * @param datagridService the datagrid service to use.
+     */
+    void startup(IDatagridService datagridService) {
+	eventChannel = datagridService.addListener(EVENT_CHANNEL_NAME,
+						   eventHandler,
+						   byte[].class,
+						   TopologyEvent.class);
+	eventHandler.start();
+    }
+
+    /**
+     * Exception to be thrown when Modification to the Network Graph cannot be continued due to broken invariant.
+     *
+     * XXX Should this be checked exception or RuntimeException
+     */
+    public static class BrokenInvariantException extends RuntimeException {
+	private static final long serialVersionUID = 1L;
+
+	public BrokenInvariantException() {
+	    super();
+	}
+
+	public BrokenInvariantException(String message) {
+	    super(message);
+	}
+    }
+
+    /* ******************************
+     * NetworkGraphDiscoveryInterface methods
+     * ******************************/
+
+	@Override
+	public void putSwitchEvent(SwitchEvent switchEvent) {
+		if (prepareForAddSwitchEvent(switchEvent)) {
+			datastore.addSwitch(switchEvent);
+			putSwitch(switchEvent);
+			// Send out notification
+			TopologyEvent topologyEvent =
+			    new TopologyEvent(switchEvent, registryService.getControllerId());
+			eventChannel.addEntry(topologyEvent.getID(),
+					      topologyEvent);
+		}
+		// TODO handle invariant violation
+	}
+
+	@Override
+	public void removeSwitchEvent(SwitchEvent switchEvent) {
+		if (prepareForRemoveSwitchEvent(switchEvent)) {
+			datastore.deactivateSwitch(switchEvent);
+			removeSwitch(switchEvent);
+			// Send out notification
+			eventChannel.removeEntry(switchEvent.getID());
+		}
+		// TODO handle invariant violation
+	}
+
+	@Override
+	public void putPortEvent(PortEvent portEvent) {
+		if (prepareForAddPortEvent(portEvent)) {
+			datastore.addPort(portEvent);
+			putPort(portEvent);
+			// Send out notification
+			TopologyEvent topologyEvent =
+			    new TopologyEvent(portEvent, registryService.getControllerId());
+			eventChannel.addEntry(topologyEvent.getID(),
+					      topologyEvent);
+		}
+		// TODO handle invariant violation
+	}
+
+	@Override
+	public void removePortEvent(PortEvent portEvent) {
+		if (prepareForRemovePortEvent(portEvent)) {
+			datastore.deactivatePort(portEvent);
+			removePort(portEvent);
+			// Send out notification
+			eventChannel.removeEntry(portEvent.getID());
+		}
+		// TODO handle invariant violation
+	}
+
+	@Override
+	public void putLinkEvent(LinkEvent linkEvent) {
+		if (prepareForAddLinkEvent(linkEvent)) {
+			datastore.addLink(linkEvent);
+			putLink(linkEvent);
+			// Send out notification
+			TopologyEvent topologyEvent =
+			    new TopologyEvent(linkEvent, registryService.getControllerId());
+			eventChannel.addEntry(topologyEvent.getID(),
+					      topologyEvent);
+		}
+		// TODO handle invariant violation
+	}
+
+	@Override
+	public void removeLinkEvent(LinkEvent linkEvent) {
+	    removeLinkEvent(linkEvent, false);
+
+	}
+
+	private void removeLinkEvent(LinkEvent linkEvent, boolean dstCheckBeforeDBmodify) {
+		if (prepareForRemoveLinkEvent(linkEvent)) {
+			if (dstCheckBeforeDBmodify) {
+			    // write to DB only if it is owner of the dst dpid
+			    if (registryService.hasControl(linkEvent.getDst().dpid)) {
+				datastore.removeLink(linkEvent);
+			    }
+			} else {
+			    datastore.removeLink(linkEvent);
+			}
+			removeLink(linkEvent);
+			// Send out notification
+			eventChannel.removeEntry(linkEvent.getID());
+		}
+		// TODO handle invariant violation
+	}
+
+	@Override
+	public void putDeviceEvent(DeviceEvent deviceEvent) {
+		if (prepareForAddDeviceEvent(deviceEvent)) {
+//			datastore.addDevice(deviceEvent);
+//			putDevice(deviceEvent);
+			// Send out notification
+			TopologyEvent topologyEvent =
+			    new TopologyEvent(deviceEvent, registryService.getControllerId());
+			eventChannel.addEntry(topologyEvent.getID(),
+					      topologyEvent);
+		}
+		// TODO handle invariant violation
+		// XXX if prepareFor~ method returned false, event should be dropped
+	}
+
+	@Override
+	public void removeDeviceEvent(DeviceEvent deviceEvent) {
+		if (prepareForRemoveDeviceEvent(deviceEvent)) {
+//			datastore.removeDevice(deviceEvent);
+//			removeDevice(deviceEvent);
+			// Send out notification
+			eventChannel.removeEntry(deviceEvent.getID());
+		}
+		// TODO handle invariant violation
+		// XXX if prepareFor~ method returned false, event should be dropped
+	}
+
+	/* *****************
+	 * Internal methods to maintain invariants of the network graph
+	 * *****************/
+
+	/**
+	 *
+	 * @param swEvt
+	 * @return true if ready to accept event.
+	 */
+	private boolean prepareForAddSwitchEvent(SwitchEvent swEvt) {
+	    // No show stopping precondition
+	    // Prep: remove(deactivate) Ports on Switch, which is not on event
+	    removePortsNotOnEvent(swEvt);
+	    return true;
+	}
+
+	private boolean prepareForRemoveSwitchEvent(SwitchEvent swEvt) {
+	    // No show stopping precondition
+	    // Prep: remove(deactivate) Ports on Switch, which is not on event
+	    // XXX may be remove switch should imply wipe all ports
+	    removePortsNotOnEvent(swEvt);
+	    return true;
+	}
+
+	private void removePortsNotOnEvent(SwitchEvent swEvt) {
+	    Switch sw = switches.get( swEvt.getDpid() );
+	    if ( sw != null ) {
+		Set<Long> port_noOnEvent = new HashSet<>();
+		for( PortEvent portEvent : swEvt.getPorts()) {
+		    port_noOnEvent.add(portEvent.getNumber());
+		}
+		// Existing ports not on event should be removed.
+		// TODO Should batch eventually for performance?
+		List<Port> portsToRemove = new ArrayList<Port>();
+		for( Port p : sw.getPorts() ) {
+		    if ( !port_noOnEvent.contains(p.getNumber()) ) {
+			//PortEvent rmEvent = new PortEvent(p.getSwitch().getDpid(), p.getNumber());
+			// calling Discovery removePort() API to wipe from DB, etc.
+			//removePortEvent(rmEvent);
+
+		    // We can't remove ports here because this will trigger a remove
+		    // from the switch's port list, which we are currently iterating
+		    // over.
+		    portsToRemove.add(p);
+		    }
+		}
+		for (Port p : portsToRemove) {
+			PortEvent rmEvent = new PortEvent(p.getSwitch().getDpid(), p.getNumber());
+			// calling Discovery removePort() API to wipe from DB, etc.
+			removePortEvent(rmEvent);
+		}
+	    }
+	}
+
+	private boolean prepareForAddPortEvent(PortEvent portEvt) {
+		// Parent Switch must exist
+		if ( getSwitch(portEvt.getDpid()) == null) {
+			log.warn("Dropping add port event because switch doesn't exist: {}",
+					portEvt);
+		    return false;
+		}
+		// Prep: None
+		return true;
+	}
+
+	private boolean prepareForRemovePortEvent(PortEvent portEvt) {
+		Port port = getPort(portEvt.getDpid(), portEvt.getNumber());
+		if ( port == null ) {
+		    log.debug("Port already removed? {}", portEvt);
+		    // let it pass
+		    return true;
+		}
+
+		// Prep: Remove Link and Device Attachment
+		ArrayList<DeviceEvent> deviceEvts = new ArrayList<>();
+		for (Device device : port.getDevices()) {
+		    log.debug("Removing Device {} on Port {}", device, portEvt);
+		    DeviceEvent devEvt = new DeviceEvent(device.getMacAddress());
+		    devEvt.addAttachmentPoint(new SwitchPort(port.getSwitch().getDpid(), port.getNumber()));
+		    deviceEvts.add(devEvt);
+		}
+		for (DeviceEvent devEvt : deviceEvts) {
+		    // calling Discovery API to wipe from DB, etc.
+		    removeDeviceEvent(devEvt);
+		}
+
+		Set<Link> links = new HashSet<>();
+		links.add(port.getOutgoingLink());
+		links.add(port.getIncomingLink());
+		for ( Link link : links) {
+		    if (link == null ) {
+			continue;
+		    }
+		    log.debug("Removing Link {} on Port {}", link, portEvt);
+		    LinkEvent linkEvent = new LinkEvent(link.getSourceSwitchDpid(), link.getSourcePortNumber(), link.getDestinationSwitchDpid(), link.getDestinationPortNumber());
+		    // calling Discovery API to wipe from DB, etc.
+
+		    // Call internal remove Link, which will check
+		    // ownership of DST dpid and modify DB only if it is the owner
+		    removeLinkEvent(linkEvent, true);
+		}
+		return true;
+	}
+
+	private boolean prepareForAddLinkEvent(LinkEvent linkEvt) {
+	    // Src/Dst Port must exist
+	    Port srcPort = getPort(linkEvt.getSrc().dpid, linkEvt.getSrc().number);
+	    Port dstPort = getPort(linkEvt.getDst().dpid, linkEvt.getDst().number);
+	    if ( srcPort == null || dstPort == null ) {
+	    log.warn("Dropping add link event because port doesn't exist: {}",
+	    		linkEvt);
+		return false;
+	    }
+
+	    // Prep: remove Device attachment on both Ports
+	    ArrayList<DeviceEvent> deviceEvents = new ArrayList<>();
+	    for (Device device : srcPort.getDevices()) {
+		DeviceEvent devEvt = new DeviceEvent(device.getMacAddress());
+		devEvt.addAttachmentPoint(new SwitchPort(srcPort.getSwitch().getDpid(), srcPort.getNumber()));
+		deviceEvents.add(devEvt);
+	    }
+	    for (Device device : dstPort.getDevices()) {
+		DeviceEvent devEvt = new DeviceEvent(device.getMacAddress());
+		devEvt.addAttachmentPoint(new SwitchPort(dstPort.getSwitch().getDpid(), dstPort.getNumber()));
+		deviceEvents.add(devEvt);
+	    }
+	    for (DeviceEvent devEvt : deviceEvents) {
+		// calling Discovery API to wipe from DB, etc.
+		removeDeviceEvent(devEvt);
+	    }
+
+	    return true;
+	}
+
+	private boolean prepareForRemoveLinkEvent(LinkEvent linkEvt) {
+	    // Src/Dst Port must exist
+	    Port srcPort = getPort(linkEvt.getSrc().dpid, linkEvt.getSrc().number);
+	    Port dstPort = getPort(linkEvt.getDst().dpid, linkEvt.getDst().number);
+	    if ( srcPort == null || dstPort == null ) {
+		log.warn("Dropping remove link event because port doesn't exist {}", linkEvt);
+		return false;
+	    }
+
+	    Link link = srcPort.getOutgoingLink();
+
+	    // Link is already gone, or different Link exist in memory
+	    // XXX Check if we should reject or just accept these cases.
+	    // it should be harmless to remove the Link on event from DB anyways
+	    if (link == null ||
+		    !link.getDestinationPortNumber().equals(linkEvt.getDst().number)
+		    || !link.getDestinationSwitchDpid().equals(linkEvt.getDst().dpid)) {
+		log.warn("Dropping remove link event because link doesn't exist: {}", linkEvt);
+		return false;
+	    }
+	    // Prep: None
+	    return true;
+	}
+
+	/**
+	 *
+	 * @param deviceEvt Event will be modified to remove inapplicable attachemntPoints/ipAddress
+	 * @return false if this event should be dropped.
+	 */
+	private boolean prepareForAddDeviceEvent(DeviceEvent deviceEvt) {
+	    boolean preconditionBroken = false;
+	    ArrayList<PortEvent.SwitchPort> failedSwitchPort = new ArrayList<>();
+	    for ( PortEvent.SwitchPort swp : deviceEvt.getAttachmentPoints() ) {
+		// Attached Ports must exist
+		Port port = getPort(swp.dpid, swp.number);
+		if ( port == null ) {
+		    preconditionBroken = true;
+		    failedSwitchPort.add(swp);
+		    continue;
+		}
+		// Attached Ports must not have Link
+		if ( port.getOutgoingLink() != null || port.getIncomingLink() != null ) {
+		    preconditionBroken = true;
+		    failedSwitchPort.add(swp);
+		    continue;
+		}
+	    }
+
+	    // Rewriting event to exclude failed attachmentPoint
+	    // XXX Assumption behind this is that inapplicable device event should
+	    // be dropped, not deferred. If we decide to defer Device event,
+	    // rewriting can become a problem
+	    List<SwitchPort>  attachmentPoints = deviceEvt.getAttachmentPoints();
+	    attachmentPoints.removeAll(failedSwitchPort);
+	    deviceEvt.setAttachmentPoints(attachmentPoints);
+
+	    if ( deviceEvt.getAttachmentPoints().isEmpty() && deviceEvt.getIpAddresses().isEmpty() ) {
+		// return false to represent: Nothing left to do for this event. Caller should drop event
+		return false;
+	    }
+
+	// Should we return false to tell caller that the event was trimmed?
+	// if ( preconditionBroken ) {
+	//     return false;
+	// }
+
+	    return true;
+	}
+
+	private boolean prepareForRemoveDeviceEvent(DeviceEvent deviceEvt) {
+		// No show stopping precondition?
+		// Prep: none
+		return true;
+	}
+
+	/* ******************************
+	 * NetworkGraphReplicationInterface methods
+	 * ******************************/
+
+	@Override
+	public void putSwitchReplicationEvent(SwitchEvent switchEvent) {
+	    if (prepareForAddSwitchEvent(switchEvent)) {
+		putSwitch(switchEvent);
+	    }
+	    // TODO handle invariant violation
+	    // trigger instance local topology event handler
+	    dispatchPutSwitchEvent(switchEvent);
+	}
+
+	@Override
+	public void removeSwitchReplicationEvent(SwitchEvent switchEvent) {
+	    if (prepareForRemoveSwitchEvent(switchEvent)) {
+		removeSwitch(switchEvent);
+	    }
+	    // TODO handle invariant violation
+	    // trigger instance local topology event handler
+	    dispatchRemoveSwitchEvent(switchEvent);
+	}
+
+	@Override
+	public void putPortReplicationEvent(PortEvent portEvent) {
+	    if (prepareForAddPortEvent(portEvent)) {
+		putPort(portEvent);
+	    }
+	    // TODO handle invariant violation
+	    // trigger instance local topology event handler
+	    dispatchPutPortEvent(portEvent);
+	}
+
+	@Override
+	public void removePortReplicationEvent(PortEvent portEvent) {
+	    if (prepareForRemovePortEvent(portEvent)) {
+		removePort(portEvent);
+	    }
+	    // TODO handle invariant violation
+	    // trigger instance local topology event handler
+	    dispatchRemovePortEvent(portEvent);
+	}
+
+	@Override
+	public void putLinkReplicationEvent(LinkEvent linkEvent) {
+	    if (prepareForAddLinkEvent(linkEvent)) {
+		putLink(linkEvent);
+	    }
+	    // TODO handle invariant violation
+	    // trigger instance local topology event handler
+	    dispatchPutLinkEvent(linkEvent);
+	}
+
+	@Override
+	public void removeLinkReplicationEvent(LinkEvent linkEvent) {
+	    if (prepareForRemoveLinkEvent(linkEvent)) {
+		removeLink(linkEvent);
+	    }
+	    // TODO handle invariant violation
+	    // trigger instance local topology event handler
+	    dispatchRemoveLinkEvent(linkEvent);
+	}
+
+	@Override
+	public void putDeviceReplicationEvent(DeviceEvent deviceEvent) {
+	    if (prepareForAddDeviceEvent(deviceEvent)) {
+		putDevice(deviceEvent);
+	    }
+	    // TODO handle invariant violation
+	    // trigger instance local topology event handler
+	    dispatchPutDeviceEvent(deviceEvent);
+	}
+
+	@Override
+	public void removeDeviceReplicationEvent(DeviceEvent deviceEvent) {
+	    if (prepareForRemoveDeviceEvent(deviceEvent)) {
+		removeDevice(deviceEvent);
+	    }
+	    // TODO handle invariant violation
+	    // trigger instance local topology event handler
+	    dispatchRemoveDeviceEvent(deviceEvent);
+	}
+
+	/* ************************************************
+	 * Internal In-memory object mutation methods.
+	 * ************************************************/
+
+	void putSwitch(SwitchEvent swEvt) {
+	    if (swEvt == null) {
+		throw new IllegalArgumentException("Switch cannot be null");
+	    }
+
+	    Switch sw = switches.get(swEvt.getDpid());
+
+	    if (sw == null) {
+		sw = new SwitchImpl(this, swEvt.getDpid());
+		Switch existing = switches.putIfAbsent(swEvt.getDpid(), sw);
+		if (existing != null) {
+		    log.warn(
+			    "Concurrent putSwitch not expected. Continuing updating {}",
+			    existing);
+		    sw = existing;
+		}
+	    }
+
+	    // Update when more attributes are added to Event object
+	    // no attribute to update for now
+
+	    // TODO handle child Port event properly for performance
+	    for (PortEvent portEvt : swEvt.getPorts() ) {
+		putPort(portEvt);
+	    }
+
+	}
+
+	void removeSwitch(SwitchEvent swEvt) {
+	    if (swEvt == null) {
+		throw new IllegalArgumentException("Switch cannot be null");
+	    }
+
+	    // TODO handle child Port event properly for performance
+	    for (PortEvent portEvt : swEvt.getPorts() ) {
+		removePort(portEvt);
+	    }
+
+	    Switch sw = switches.get(swEvt.getDpid());
+
+	    if (sw == null) {
+		log.warn("Switch {} already removed, ignoring", swEvt);
+		return;
+	    }
+
+	    // remove all ports if there still exist
+	    ArrayList<PortEvent> portsToRemove = new ArrayList<>();
+	    for (Port port : sw.getPorts()) {
+		log.warn(
+			"Port {} on Switch {} should be removed prior to removing Switch. Removing Port now",
+			port, swEvt);
+		PortEvent portEvt = new PortEvent(port.getDpid(), port.getNumber());
+		portsToRemove.add(portEvt);
+	    }
+	    for (PortEvent portEvt : portsToRemove) {
+		// XXX calling removePortEvent() may trigger duplicate event, once at prepare phase, second time here
+		// If event can be squashed, ignored etc. at receiver side it shouldn't be a problem, but if not
+		// need to re-visit this issue.
+
+		// Note: removePortEvent() implies removal of attached Device, etc.
+		// if we decide not to call removePortEvent(), Device needs to be handled properly
+		removePortEvent(portEvt);
+	    }
+
+	    boolean removed = switches.remove(swEvt.getDpid(), sw);
+	    if (removed) {
+		log.warn(
+			"Switch instance was replaced concurrently while removing {}. Something is not right.",
+			sw);
+	    }
+	}
+
+	void putPort(PortEvent portEvt) {
+	    if (portEvt == null) {
+		throw new IllegalArgumentException("Port cannot be null");
+	    }
+	    Switch sw = switches.get(portEvt.getDpid());
+	    if (sw == null) {
+		throw new BrokenInvariantException(String.format(
+			"Switch with dpid %s did not exist.",
+			new Dpid(portEvt.getDpid())));
+	    }
+	    Port p = sw.getPort(portEvt.getNumber());
+	    PortImpl port = null;
+	    if (p != null) {
+		port = getPortImpl(p);
+	    }
+
+	    if (port == null) {
+		port = new PortImpl(this, sw, portEvt.getNumber());
+	    }
+
+	    // TODO update attributes
+
+	    SwitchImpl s = getSwitchImpl(sw);
+	    s.addPort(port);
+	}
+
+	void removePort(PortEvent portEvt) {
+	    if (portEvt == null) {
+		throw new IllegalArgumentException("Port cannot be null");
+	    }
+
+	    Switch sw = switches.get(portEvt.getDpid());
+	    if (sw == null) {
+		log.warn("Parent Switch for Port {} already removed, ignoring", portEvt);
+		return;
+	    }
+
+	    Port p = sw.getPort(portEvt.getNumber());
+	    if (p == null) {
+		log.warn("Port {} already removed, ignoring", portEvt);
+		return;
+	    }
+
+	    // Remove Link and Device Attachment
+	    for (Device device : p.getDevices()) {
+		log.debug("Removing Device {} on Port {}", device, portEvt);
+		DeviceEvent devEvt = new DeviceEvent(device.getMacAddress());
+		devEvt.addAttachmentPoint(new SwitchPort(p.getSwitch().getDpid(), p.getNumber()));
+
+		// XXX calling removeDeviceEvent() may trigger duplicate event, once at prepare phase, second time here
+		// If event can be squashed, ignored etc. at receiver side it shouldn't be a problem, but if not
+		// need to re-visit
+
+		// calling Discovery API to wipe from DB, etc.
+		removeDeviceEvent(devEvt);
+	    }
+	    Set<Link> links = new HashSet<>();
+	    links.add(p.getOutgoingLink());
+	    links.add(p.getIncomingLink());
+	    ArrayList<LinkEvent> linksToRemove = new ArrayList<>();
+	    for (Link link : links) {
+		if (link == null) {
+		    continue;
+		}
+		log.debug("Removing Link {} on Port {}", link, portEvt);
+		LinkEvent linkEvent = new LinkEvent(link.getSourceSwitchDpid(), link.getSourcePortNumber(), link.getDestinationSwitchDpid(), link.getDestinationPortNumber());
+		linksToRemove.add(linkEvent);
+	    }
+	    for (LinkEvent linkEvent : linksToRemove) {
+		// XXX calling removeLinkEvent() may trigger duplicate event, once at prepare phase, second time here
+		// If event can be squashed, ignored etc. at receiver side it shouldn't be a problem, but if not
+		// need to re-visit
+
+		// calling Discovery API to wipe from DB, etc.
+		removeLinkEvent(linkEvent);
+	    }
+
+	    // remove Port from Switch
+	    SwitchImpl s = getSwitchImpl(sw);
+	    s.removePort(p);
+	}
+
+	void putLink(LinkEvent linkEvt) {
+	    if (linkEvt == null) {
+		throw new IllegalArgumentException("Link cannot be null");
+	    }
+
+	    Port srcPort = getPort(linkEvt.getSrc().dpid, linkEvt.getSrc().number);
+	    if (srcPort == null) {
+		throw new BrokenInvariantException(
+			String.format(
+				"Src Port %s of a Link did not exist.",
+				linkEvt.getSrc() ));
+	    }
+
+	    Port dstPort = getPort(linkEvt.getDst().dpid, linkEvt.getDst().number);
+	    if (dstPort == null) {
+		throw new BrokenInvariantException(
+			String.format(
+				"Dst Port %s of a Link did not exist.",
+				linkEvt.getDst() ));
+	    }
+
+	    // getting Link instance from destination port incoming Link
+	    Link l = dstPort.getIncomingLink();
+	    LinkImpl link = null;
+	    assert( l == srcPort.getOutgoingLink() );
+	    if (l != null) {
+		link = getLinkImpl(l);
+	    }
+
+	    if (link == null) {
+		link = new LinkImpl(this, srcPort, dstPort);
+	    }
+
+
+	    PortImpl dstPortMem = getPortImpl(dstPort);
+	    PortImpl srcPortMem = getPortImpl(srcPort);
+
+	    // Add Link first to avoid further Device addition
+
+	    // add Link to Port
+	    dstPortMem.setIncomingLink(link);
+	    srcPortMem.setOutgoingLink(link);
+
+	    // remove Device Pointing to Port if any
+	    for(Device d : dstPortMem.getDevices() ) {
+		log.error("Device {} on Port {} should have been removed prior to adding Link {}", d, dstPort, linkEvt);
+		DeviceImpl dev = getDeviceImpl(d);
+		dev.removeAttachmentPoint(dstPort);
+		// This implies that change is made to Device Object.
+		// sending Device attachment point removed event
+		DeviceEvent rmEvent = new DeviceEvent(d.getMacAddress());
+		rmEvent.addAttachmentPoint(new SwitchPort(dstPort.getDpid(), dstPort.getNumber()));
+		removeDeviceEvent(rmEvent);
+	    }
+	    dstPortMem.removeAllDevice();
+	    for(Device d : srcPortMem.getDevices() ) {
+		log.error("Device {} on Port {} should have been removed prior to adding Link {}", d, srcPort, linkEvt);
+		DeviceImpl dev = getDeviceImpl(d);
+		dev.removeAttachmentPoint(srcPort);
+		// This implies that change is made to Device Object.
+		// sending Device attachment point removed event
+		DeviceEvent rmEvent = new DeviceEvent(d.getMacAddress());
+		rmEvent.addAttachmentPoint(new SwitchPort(dstPort.getDpid(), dstPort.getNumber()));
+		removeDeviceEvent(rmEvent);
+	    }
+	    srcPortMem.removeAllDevice();
+
+	}
+
+	void removeLink(LinkEvent linkEvt) {
+	    if (linkEvt == null) {
+		throw new IllegalArgumentException("Link cannot be null");
+	    }
+
+	    Port srcPort = getPort(linkEvt.getSrc().dpid, linkEvt.getSrc().number);
+	    if (srcPort == null) {
+		log.warn("Src Port for Link {} already removed, ignoring", linkEvt);
+		return;
+	    }
+
+	    Port dstPort = getPort(linkEvt.getDst().dpid, linkEvt.getDst().number);
+	    if (dstPort == null) {
+		log.warn("Dst Port for Link {} already removed, ignoring", linkEvt);
+		return;
+	    }
+
+	    Link l = dstPort.getIncomingLink();
+	    if (  l == null ) {
+		log.warn("Link {} already removed on destination Port", linkEvt);
+	    }
+	    l = srcPort.getOutgoingLink();
+	    if (  l == null ) {
+		log.warn("Link {} already removed on src Port", linkEvt);
+	    }
+
+	    getPortImpl(dstPort).setIncomingLink(null);
+	    getPortImpl(srcPort).setOutgoingLink(null);
+	}
+
+	// XXX Need to rework Device related
+	void putDevice(DeviceEvent deviceEvt) {
+	    if (deviceEvt == null) {
+		throw new IllegalArgumentException("Device cannot be null");
+	    }
+
+	    Device device = getDeviceByMac(deviceEvt.getMac());
+	    if ( device == null ) {
+		device = new DeviceImpl(this, deviceEvt.getMac());
+		Device existing = mac2Device.putIfAbsent(deviceEvt.getMac(), device);
+		if (existing != null) {
+		    log.warn(
+			    "Concurrent putDevice seems to be in action. Continuing updating {}",
+			    existing);
+		    device = existing;
+		}
+	    }
+	    DeviceImpl memDevice = getDeviceImpl(device);
+
+	    // for each attachment point
+	    for (SwitchPort swp : deviceEvt.getAttachmentPoints() ) {
+		// Attached Ports must exist
+		Port port = getPort(swp.dpid, swp.number);
+		if ( port == null ) {
+		    log.warn("Port for the attachment point {} did not exist. skipping mutation", swp);
+		    continue;
+		}
+		// Attached Ports must not have Link
+		if ( port.getOutgoingLink() != null || port.getIncomingLink() != null ) {
+		    log.warn("Link (Out:{},In:{}) exist on the attachment point, skipping mutation.", port.getOutgoingLink(), port.getIncomingLink());
+		    continue;
+		}
+
+		// finally add Device <-> Port on In-memory structure
+		PortImpl memPort = getPortImpl(port);
+		memPort.addDevice(device);
+		memDevice.addAttachmentPoint(port);
+	    }
+
+	    // for each IP address
+	    for( InetAddress ipAddr : deviceEvt.getIpAddresses() ) {
+		// Add Device -> IP
+		memDevice.addIpAddress(ipAddr);
+
+		// Add IP -> Set<Device>
+		boolean updated = false;
+		do {
+		    Set<Device> devices = this.addr2Device.get(ipAddr);
+		    if ( devices == null ) {
+			devices = new HashSet<>();
+			Set<Device> existing = this.addr2Device.putIfAbsent(ipAddr, devices);
+			if ( existing == null ) {
+			    // success
+			    updated = true;
+			}
+		    } else {
+			Set<Device> updateDevices = new HashSet<>(devices);
+			updateDevices.add(device);
+			updated = this.addr2Device.replace(ipAddr, devices, updateDevices);
+		    }
+		    if (!updated) {
+			log.debug("Collision detected, updating IP to Device mapping retrying.");
+		    }
+		} while( !updated );
+	    }
+	}
+
+	void removeDevice(DeviceEvent deviceEvt) {
+	    if (deviceEvt == null) {
+		throw new IllegalArgumentException("Device cannot be null");
+	    }
+
+	    Device device = getDeviceByMac(deviceEvt.getMac());
+	    if ( device == null ) {
+		log.warn("Device {} already removed, ignoring", deviceEvt);
+		return;
+	    }
+	    DeviceImpl memDevice = getDeviceImpl(device);
+
+	    // for each attachment point
+	    for (SwitchPort swp : deviceEvt.getAttachmentPoints() ) {
+		// Attached Ports must exist
+		Port port = getPort(swp.dpid, swp.number);
+		if ( port == null ) {
+		    log.warn("Port for the attachment point {} did not exist. skipping attachment point mutation", swp);
+		    continue;
+		}
+
+		// finally remove Device <-> Port on In-memory structure
+		PortImpl memPort = getPortImpl(port);
+		memPort.removeDevice(device);
+		memDevice.removeAttachmentPoint(port);
+	    }
+
+	    // for each IP address
+	    for( InetAddress ipAddr : deviceEvt.getIpAddresses() ) {
+		// Remove Device -> IP
+		memDevice.removeIpAddress(ipAddr);
+
+		// Remove IP -> Set<Device>
+		boolean updated = false;
+		do {
+		    Set<Device> devices = this.addr2Device.get(ipAddr);
+		    if ( devices == null ) {
+			// already empty set, nothing to do
+			updated = true;
+		    } else {
+			Set<Device> updateDevices = new HashSet<>(devices);
+			updateDevices.remove(device);
+			updated = this.addr2Device.replace(ipAddr, devices, updateDevices);
+		    }
+		    if (!updated) {
+			log.debug("Collision detected, updating IP to Device mapping retrying.");
+		    }
+		} while( !updated );
+	    }
+	}
+
+	private void dispatchPutSwitchEvent(SwitchEvent switchEvent) {
+	    for (INetworkGraphListener listener : this.networkGraphListeners) {
+		// TODO Should copy before handing them over to listener
+		listener.putSwitchEvent(switchEvent);
+	    }
+	}
+
+	private void dispatchRemoveSwitchEvent(SwitchEvent switchEvent) {
+	    for (INetworkGraphListener listener : this.networkGraphListeners) {
+		// TODO Should copy before handing them over to listener
+		listener.removeSwitchEvent(switchEvent);
+	    }
+	}
+
+	private void dispatchPutPortEvent(PortEvent portEvent) {
+	    for (INetworkGraphListener listener : this.networkGraphListeners) {
+		// TODO Should copy before handing them over to listener
+		listener.putPortEvent(portEvent);
+	    }
+	}
+
+	private void dispatchRemovePortEvent(PortEvent portEvent) {
+	    for (INetworkGraphListener listener : this.networkGraphListeners) {
+		// TODO Should copy before handing them over to listener
+		listener.removePortEvent(portEvent);
+	    }
+	}
+
+	private void dispatchPutLinkEvent(LinkEvent linkEvent) {
+	    for (INetworkGraphListener listener : this.networkGraphListeners) {
+		// TODO Should copy before handing them over to listener
+		listener.putLinkEvent(linkEvent);
+	    }
+	}
+
+	private void dispatchRemoveLinkEvent(LinkEvent linkEvent) {
+	    for (INetworkGraphListener listener : this.networkGraphListeners) {
+		// TODO Should copy before handing them over to listener
+		listener.removeLinkEvent(linkEvent);
+	    }
+	}
+
+	private void dispatchPutDeviceEvent(DeviceEvent deviceEvent) {
+	    for (INetworkGraphListener listener : this.networkGraphListeners) {
+		// TODO Should copy before handing them over to listener
+		listener.putDeviceEvent(deviceEvent);;
+	    }
+	}
+
+	private void dispatchRemoveDeviceEvent(DeviceEvent deviceEvent) {
+	    for (INetworkGraphListener listener : this.networkGraphListeners) {
+		// TODO Should copy before handing them over to listener
+		listener.removeDeviceEvent(deviceEvent);
+	    }
+	}
+
+	// we might want to include this in NetworkGraph interface
+	private Port getPort(Long dpid, Long number) {
+	    Switch sw = getSwitch(dpid);
+	    if (sw != null) {
+		return sw.getPort(number);
+	    }
+	    return null;
+	}
+
+	private SwitchImpl getSwitchImpl(Switch sw) {
+	    if (sw instanceof SwitchImpl) {
+		return (SwitchImpl) sw;
+	    }
+	    throw new ClassCastException("SwitchImpl expected, but found: " + sw);
+	}
+
+	private PortImpl getPortImpl(Port p) {
+	    if (p instanceof PortImpl) {
+		return (PortImpl) p;
+	    }
+	    throw new ClassCastException("PortImpl expected, but found: " + p);
+	}
+
+	private LinkImpl getLinkImpl(Link l) {
+	    if (l instanceof LinkImpl) {
+		return (LinkImpl) l;
+	    }
+	    throw new ClassCastException("LinkImpl expected, but found: " + l);
+	}
+
+	private DeviceImpl getDeviceImpl(Device d) {
+	    if (d instanceof DeviceImpl) {
+		return (DeviceImpl) d;
+	    }
+	    throw new ClassCastException("DeviceImpl expected, but found: " + d);
+	}
+
+	@Deprecated
+	public void loadWholeTopologyFromDB() {
+	    // XXX May need to clear whole topology first, depending on
+	    // how we initially subscribe to replication events
+
+	    for (RCSwitch sw : RCSwitch.getAllSwitches()) {
+		if ( sw.getStatus() != RCSwitch.STATUS.ACTIVE ) {
+		    continue;
+		}
+		putSwitchReplicationEvent(new SwitchEvent(sw.getDpid()));
+	    }
+
+	    for (RCPort p : RCPort.getAllPorts()) {
+		if (p.getStatus() != RCPort.STATUS.ACTIVE) {
+		    continue;
+		}
+		putPortReplicationEvent(new PortEvent(p.getDpid(), p.getNumber() ));
+	    }
+
+	    // TODO Is Device going to be in DB? If so, read from DB.
+	    //	for (RCDevice d : RCDevice.getAllDevices()) {
+	    //	    DeviceEvent devEvent = new DeviceEvent( MACAddress.valueOf(d.getMac()) );
+	    //	    for (byte[] portId : d.getAllPortIds() ) {
+	    //		devEvent.addAttachmentPoint( new SwitchPort( RCPort.getDpidFromKey(portId), RCPort.getNumberFromKey(portId) ));
+	    //	    }
+	    //	}
+
+	    for (RCLink l : RCLink.getAllLinks()) {
+		// check if src/dst switch/port exist before triggering event
+		Port srcPort = getPort(l.getSrc().dpid, l.getSrc().number);
+		Port dstPort = getPort(l.getDst().dpid, l.getDst().number);
+		if ( srcPort == null || dstPort == null ) {
+		    continue;
+		}
+		putLinkReplicationEvent( new LinkEvent(l.getSrc().dpid, l.getSrc().number, l.getDst().dpid, l.getDst().number));
+	    }
+	}
+}
diff --git a/src/test/java/net/onrc/onos/intent/MockNetworkGraph.java b/src/test/java/net/onrc/onos/intent/MockNetworkGraph.java
index ec0d54f..cde98f0 100644
--- a/src/test/java/net/onrc/onos/intent/MockNetworkGraph.java
+++ b/src/test/java/net/onrc/onos/intent/MockNetworkGraph.java
@@ -1,12 +1,12 @@
 package net.onrc.onos.intent;
 
-import net.onrc.onos.ofcontroller.networkgraph.AbstractNetworkGraph;
+import net.onrc.onos.ofcontroller.networkgraph.NetworkGraphImpl;
 import net.onrc.onos.ofcontroller.networkgraph.Link;
 import net.onrc.onos.ofcontroller.networkgraph.LinkImpl;
 import net.onrc.onos.ofcontroller.networkgraph.Switch;
 import net.onrc.onos.ofcontroller.networkgraph.SwitchImpl;
 
-public class MockNetworkGraph extends AbstractNetworkGraph {
+public class MockNetworkGraph extends NetworkGraphImpl {
 	public Switch addSwitch(Long switchId) {
 		SwitchImpl sw = new SwitchImpl(this, switchId);
 		switches.put(sw.getDpid(), sw);
diff --git a/web/add-intent.rb b/web/add-intent.rb
deleted file mode 100644
index b1d8dfd..0000000
--- a/web/add-intent.rb
+++ /dev/null
@@ -1,122 +0,0 @@
-require "rest-client"
-require "optparse"
-
-options = { :intent_id => 123, :intent_type => "shortest_intent_type", :max_switches => 4 }
-
-parser = OptionParser.new do |opts|
-  opts.banner = "Usage add-intent [options]"
-  opts.on('-t', '--max_intents max_intents', 'max. number of intents') do |max_intents|
-    options[:max_intents] = max_intents
-  end
-  opts.on('-a', '--application application_id', 'set application id') do |appl_id|
-    options[:application_id] = appl_id.to_i
-  end
-  opts.on('-i', '--intent_id intent_id', 'global intent id') do |id|
-    options[:intent_id] = id.to_i
-  end
-  opts.on('-s', '--shortest', 'create a shortest path intent') do
-    options[:intent_type] = "shortest_intent_type"
-  end
-  opts.on('-c', '--constrained', 'create a constrained shortest path intent') do
-    options[:intent_type] = "constrained_shortest_intent_type"
-  end
-  opts.on('-m', '--max_switches max_switches', 'max. number of switches') do |max_switches|
-    options[:max_switches] = max_switches.to_i
-  end
-  opts.on('-h', '--help', 'Display help') do
-    puts opts
-    exit
-  end
-end
-parser.parse!
-
-puts options.inspect
-server = options[:server]
-server ||= "127.0.0.1"
-port = options[:port]
-port ||= 8080
-
-def rand_mac
-  mac = `openssl rand -hex 6`
-  mac.scan(/(..)/).join(":")
-end
-
-def rand_switch
-  switch = `openssl rand -hex 5`.chomp
-end
-
-class Intent
-  attr_reader :switches
-  attr_reader :ports
-  attr_reader :intent_id
-  attr_reader :application_id
-  attr_reader :intent_type
-
-  def initialize options
-    parse_options options
-  end
-
-  def create_intent 
-    json_intents = []
-    @switches.each do |sw|
-      rest = switches - [sw]
-      json_intents = _create_intent sw, rest, json_intents
-    end
-    json_intents
-  end
-
-  def parse_options options
-    max_switches = options[:max_switches].to_i || 4
-    @switches = (1..max_switches).to_a
-    @ports = (1..(max_switches - 1)).to_a
-    @intent_id = options[:intent_id]
-    @intent_id ||= 1
-    @application_id = options[:application_id]
-    @application_id ||= 1
-    @intent_type = options[:intent_type]
-  end
-
-
-  def _create_intent src_switch, iterable_switches, json_intents
-    network_id = 1
-    iterable_switches.each_index do |sw_i|
-      dst_switch = iterable_switches[sw_i]
-      sw_set = @switches - [dst_switch]
-      dst_port = sw_set.index(src_switch)
-      dst_port = dst_port + 1
-      intent = {
-        :intent_id => "#{@application_id}:#{@intent_id}",
-        :intent_type => @intent_type,
-        :srcSwitch => src_switch.to_s,
-        :srcPort => @ports[sw_i],
-        :srcMac => "00:00:c0:a8:#{mac_format(src_switch)}",
-        :dstSwitch => iterable_switches[sw_i].to_s,
-        :dstPort => dst_port,
-        :dstMac => "00:00:c0:a8:#{mac_format(iterable_switches[sw_i].to_i)}"
-      }
-puts intent
-      @intent_id = @intent_id + 1
-      json_intents << intent
-puts
-    end
-    #sha256 = Digest::SHA256.new
-    #sha256.update intent_hash.to_s
-    #puts sha256.hexdigest
-    #puts "intent hash = #{intent_hash}"
-    json_intents
-  end
-
-  def mac_format number
-    if number > 255
-      divisor = number / 256 
-      remainder = number % 256
-      return sprintf("%02x:%02x",divisor ,remainder)
-    end
-    "00:%02x" % number
-  end
-end
-
-intent = Intent.new options
-json_data = intent.create_intent
-response = RestClient.post "http://#{server}:#{port}/wm/onos/datagrid/add/intent/json", json_data.to_json, :content_type => :json, :accept => :json
-puts response.inspect
diff --git a/web/rest-intent/add-get-intent.rb b/web/rest-intent/add-get-intent.rb
new file mode 100644
index 0000000..1627aea
--- /dev/null
+++ b/web/rest-intent/add-get-intent.rb
@@ -0,0 +1,207 @@
+require "rest-client"
+require "optparse"
+
+options = { 
+  :rest_op => "add",
+  :intent_id => 123, 
+  :intent_type => "shortest_intent_type", 
+  :max_switches => 4,
+  :intent_op => "add"
+}
+
+parser = OptionParser.new do |opts|
+  opts.banner = "Usage add-get-intent [options]"
+  opts.on('-g', '--get_intents', 'get intents state') do
+    options[:rest_op] = "get"
+  end
+  opts.on('-t', '--max_intents max_intents', 'max. number of intents') do |max_intents|
+    options[:max_intents] = max_intents
+  end
+  opts.on('-l', '--application application_id', 'set application id') do |appl_id|
+    options[:application_id] = appl_id.to_i
+  end
+  opts.on('-i', '--intent_id intent_id', 'global intent id') do |id|
+    options[:intent_id] = id.to_i
+  end
+  # optional argument
+  opts.on('-s', '--shortest', 'create a shortest path intent') do
+    options[:intent_type] = "shortest_intent_type"
+  end
+  # optional argument
+  opts.on('-c', '--constrained', 'create a constrained shortest path intent') do
+    options[:intent_type] = "constrained_shortest_intent_type"
+  end
+  # optional argument
+  opts.on('-r', '--random_intent', 'create minimum no. of random intents') do
+    options[:random_intent] = true
+  end
+  opts.on('-m', '--max_switches max_switches', 'max. number of switches') do |max_switches|
+    options[:max_switches] = max_switches.to_i
+  end
+  opts.on('-o', '--intent_op add|remove', 'an operation to post an intent') do |operation|
+    options[:intent_op] = operation
+  end
+  opts.on('-w', '--server server', 'server to post intents') do |server|
+    options[:server] = server
+  end
+  opts.on('-p','--port port', 'server port') do |port|
+    options[:port] = port
+  end
+  opts.on('b', '--bulk_limit bulk_limit', 'bulk request upto this limit') do |bulk_limit|
+    options[:bulk_limit] = bulk_limit
+  end
+  opts.on('-h', '--help', 'Display help') do
+    puts opts
+    exit
+  end
+end
+parser.parse!
+
+def rand_mac
+  mac = `openssl rand -hex 6`
+  mac.scan(/(..)/).join(":")
+end
+
+def rand_switch
+  switch = `openssl rand -hex 5`.chomp
+end
+
+class Intent
+  attr_reader :switches, :ports, :intent_id
+  attr_reader :application_id, :intent_type, :intent_op
+  attr_reader :random_intent, :server, :port
+  attr_reader :bulk_limit
+
+  def initialize options
+    parse_options options
+  end
+
+  def post_intent 
+    create_specific_intent
+  end
+
+  def get_intent
+    request = RestClient.get "http://#{@server}:#{@port}/wm/onos/datagrid/get/intents/json"
+    puts request
+  end
+
+  private 
+
+  def create_specific_intent
+    if @random_intent == true
+      create_random_intent
+    else
+      create_many_intents
+    end
+  end
+
+  # create as many intents as the number of switches
+  def create_many_intents
+    intents = []
+    @switches.each do |sw|
+      rest = @switches - [sw]
+      intents = _create_intent sw, rest, intents
+puts intents.size
+      post_slice intents
+    end
+    post_slice intents, true
+  end
+
+  # pick a random src switch and create intents to all other switches
+  def create_random_intent
+    intents = []
+    sw = @switches.shuffle[0]
+    rest = @switches - [sw]
+    intents = _create_intent sw, rest, intents
+    post_slice intents, true
+  end
+
+  def post_slice intents, last=false
+    @bulk_limit = @bulk_limit.to_i
+    if intents.size >= @bulk_limit
+      post intents.slice!(0..(@bulk_limit - 1))
+    end
+    if last == true
+      loop do
+        new_bulk_limit = intents.size > @bulk_limit ? @bulk_limit : intents.size
+        post intents.slice!(0..(new_bulk_limit - 1))
+        break if new_bulk_limit < @bulk_limit
+      end
+    end
+  end
+
+  def post intents
+    json_data = intents.to_json
+    response = RestClient.post "http://#{@server}:#{@port}/wm/onos/datagrid/#{intent_op}/intent/json", json_data, :content_type => :json, :accept => :json
+    puts response
+  end
+
+  def parse_options options
+    max_switches = options[:max_switches].to_i || 4
+    @switches = (1..max_switches).to_a
+    @ports = (1..(max_switches - 1)).to_a
+    @intent_id = options[:intent_id]
+    @intent_id ||= 1
+    @application_id = options[:application_id]
+    @application_id ||= 1
+    @intent_type = options[:intent_type]
+    @intent_op = options[:intent_op]
+    @intent_op ||= "add"
+    @random_intent = options[:random_intent]
+    @random_intent ||= false
+    @server = options[:server]
+    @server ||= "127.0.0.1"
+    @port = options[:port]
+    @port ||= 8080
+    @bulk_limit = options[:bulk_limit]
+    @bulk_limit ||= 10000
+  end
+
+
+  def _create_intent src_switch, iterable_switches, json_intents
+    network_id = 1
+    iterable_switches.each_index do |sw_i|
+      dst_switch = iterable_switches[sw_i]
+      sw_set = @switches - [dst_switch]
+      dst_port = sw_set.index(src_switch)
+      dst_port = dst_port + 1
+      intent = {
+        :intent_id => "#{@application_id}:#{@intent_id}",
+        :intent_type => @intent_type,
+        :intent_op => @intent_op,
+        :srcSwitch => src_switch.to_s,
+        :srcPort => @ports[sw_i],
+        :srcMac => "00:00:c0:a8:#{mac_format(src_switch)}",
+        :dstSwitch => iterable_switches[sw_i].to_s,
+        :dstPort => dst_port,
+        :dstMac => "00:00:c0:a8:#{mac_format(iterable_switches[sw_i].to_i)}"
+      }
+puts intent.inspect
+      @intent_id = @intent_id + 1
+      json_intents << intent
+puts
+    end
+    #sha256 = Digest::SHA256.new
+    #sha256.update intent_hash.to_s
+    #puts sha256.hexdigest
+    #puts "intent hash = #{intent_hash}"
+    json_intents
+  end
+
+  def mac_format number
+    if number > 255
+      divisor = number / 256 
+      remainder = number % 256
+      return sprintf("%02x:%02x",divisor ,remainder)
+    end
+    "00:%02x" % number
+  end
+end
+
+intent = Intent.new options
+if options[:rest_op] == "get"
+  intent.get_intent
+else
+  json_data = intent.post_intent
+end
+