[Emu] MutexExecutionService for running tasks that need to run only on a single instance.

Change-Id: Idf9fedbbf15c014e97c77db25aa608cd1db53b27

+/*
+ * Copyright 2015 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.onosproject.store.service;
+
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Executor;
+
+/**
+ * Service for mutually exclusive job execution.
+ */
+public interface MutexExecutionService {
+
+    /**
+     * Runs the specified task in a mutually exclusive fashion.
+     * @param task task to run
+     * @param exclusionPath path on which different instances synchronize
+     * @param executor executor to use for running the task
+     * @return future that is completed when the task execution completes.
+     */
+    CompletableFuture<Void> execute(MutexTask task, String exclusionPath, Executor executor);
+}
\ No newline at end of file

+/*
+ * Copyright 2015 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.onosproject.store.service;
+
+/**
+ * The MutexTask interface should be implemented by any class whose
+ * instances distributed across controllers are intended to be executed
+ * in a mutually exclusive fashion.
+ */
+public interface MutexTask {
+
+    /**
+     * Begins the execution of a mutually exclusive task.
+     * The start method will be called once the "lock" is acquired.
+     * After the start method returns the lock is released and some other
+     * instance can take over execution.
+     */
+    void start();
+
+    /**
+     * This method will be called when exclusivity of task execution
+     * can no longer be guaranteed. The implementation should take necessary steps
+     * to halt task execution in order to ensure correctness.
+     */
+    void stop();
+}