AntiEntropy sketch + required Serializer work

Change-Id: Ibac5f4eede6b420202683114c3262e01b7264eff
diff --git a/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/ImmutableMapSerializer.java b/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/ImmutableMapSerializer.java
new file mode 100644
index 0000000..244cc57
--- /dev/null
+++ b/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/ImmutableMapSerializer.java
@@ -0,0 +1,49 @@
+package org.onlab.onos.store.serializers;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+
+import org.onlab.util.KryoPool.FamilySerializer;
+
+import com.esotericsoftware.kryo.Kryo;
+import com.esotericsoftware.kryo.io.Input;
+import com.esotericsoftware.kryo.io.Output;
+import com.esotericsoftware.kryo.serializers.MapSerializer;
+import com.google.common.collect.ImmutableMap;
+
+/**
+* Kryo Serializer for {@link ImmutableMap}.
+*/
+public class ImmutableMapSerializer extends FamilySerializer<ImmutableMap<?, ?>> {
+
+    private final MapSerializer mapSerializer = new MapSerializer();
+
+    public ImmutableMapSerializer() {
+        // non-null, immutable
+        super(false, true);
+    }
+
+    @Override
+    public void write(Kryo kryo, Output output, ImmutableMap<?, ?> object) {
+        // wrapping with unmodifiableMap proxy
+        // to avoid Kryo from writing only the reference marker of this instance,
+        // which will be embedded right before this method call.
+        kryo.writeObject(output, Collections.unmodifiableMap(object), mapSerializer);
+    }
+
+    @Override
+    public ImmutableMap<?, ?> read(Kryo kryo, Input input,
+                                    Class<ImmutableMap<?, ?>> type) {
+        Map<?, ?> map = kryo.readObject(input, HashMap.class, mapSerializer);
+        return ImmutableMap.copyOf(map);
+    }
+
+    @Override
+    public void registerFamilies(Kryo kryo) {
+        kryo.register(ImmutableMap.of().getClass(), this);
+        kryo.register(ImmutableMap.of(1, 2).getClass(), this);
+        kryo.register(ImmutableMap.of(1, 2, 3, 4).getClass(), this);
+        // TODO register required ImmutableMap variants
+    }
+}