Added API for the (new) Leadership Service. For now there is a single

Leader elected for all purposes.
In the future the leader could be per topic.

Change-Id: I6d4f997b40bfdaa6fa9a5547e7641bf62c9d3ddb

+/*
+ * Copyright 2014 Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.onlab.onos.cluster;
+
+import org.onlab.onos.event.AbstractEvent;
+
+/**
+ * Describes leadership-related event.
+ */
+public class LeadershipEvent extends AbstractEvent<LeadershipEvent.Type, ControllerNode> {
+
+    /**
+     * Type of leadership-related events.
+     */
+    public enum Type {
+        /**
+         * Signifies that the leader has changed. The event subject is the
+         * new leader.
+         */
+        LEADER_CHANGED
+    }
+
+    /**
+     * Creates an event of a given type and for the specified instance and the
+     * current time.
+     *
+     * @param type     leadership event type
+     * @param instance cluster device subject
+     */
+    public LeadershipEvent(Type type, ControllerNode instance) {
+        super(type, instance);
+    }
+
+    /**
+     * Creates an event of a given type and for the specified device and time.
+     *
+     * @param type     device event type
+     * @param instance event device subject
+     * @param time     occurrence time
+     */
+    public LeadershipEvent(Type type, ControllerNode instance, long time) {
+        super(type, instance, time);
+    }
+
+}

+/*
+ * Copyright 2014 Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.onlab.onos.cluster;
+
+import org.onlab.onos.event.EventListener;
+
+/**
+ * Entity capable of receiving device leadership-related events.
+ */
+public interface LeadershipEventListener  extends EventListener<LeadershipEvent> {
+}

+/*
+ * Copyright 2014 Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.onlab.onos.cluster;
+
+/**
+ * Service for obtaining information about the leader election.
+ */
+public interface LeadershipService {
+
+    /**
+     * Returns the current leader controller node.
+     *
+     * @return current leader controller node
+     */
+    ControllerNode getLeader();
+
+    /**
+     * Adds the specified leadership event listener.
+     *
+     * @param listener the leadership listener
+     */
+    void addListener(LeadershipEventListener listener);
+
+    /**
+     * Removes the specified leadership event listener.
+     *
+     * @param listener the leadership listener
+     */
+    void removeListener(LeadershipEventListener listener);
+
+}