Added LockService interface.
diff --git a/core/api/src/main/java/org/onlab/onos/store/service/LockEventListener.java b/core/api/src/main/java/org/onlab/onos/store/service/LockEventListener.java
new file mode 100644
index 0000000..73f78fe
--- /dev/null
+++ b/core/api/src/main/java/org/onlab/onos/store/service/LockEventListener.java
@@ -0,0 +1,13 @@
+package org.onlab.onos.store.service;
+
+/**
+ * Listener for lock events.
+ */
+public interface LockEventListener {
+
+ /**
+ * Notifies the listener of a lock's lease expiration event.
+ * @param lock lock whose lease has expired.
+ */
+ void leaseExpired(Lock lock);
+}