/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.apache.flink.runtime.leaderretrieval;

import org.apache.flink.runtime.highavailability.HighAvailabilityServices;

import java.util.UUID;

import static org.apache.flink.util.Preconditions.checkNotNull;
import static org.apache.flink.util.Preconditions.checkState;

/**
 * Standalone implementation of the {@link LeaderRetrievalService}. This implementation
 * assumes that there is only a single contender for leadership
 * (e.g., a single JobManager or ResourceManager process) and that this process is
 * reachable under a constant address.
 * 
 * <p>As soon as this service is started, it immediately notifies the leader listener
 * of the leader contender with the pre-configured address.
 */
public class StandaloneLeaderRetrievalService implements LeaderRetrievalService {

	private final Object startStopLock = new Object();
	
	/** The fix address of the leader */
	private final String leaderAddress;

	/** The fix leader ID (leader lock fencing token) */
	private final UUID leaderId;

	/** Flag whether this service is started */
	private boolean started;

	/**
	 * Creates a StandaloneLeaderRetrievalService with the given leader address.
	 * The leaderId will be null.
	 *
	 * @param leaderAddress The leader's pre-configured address
	 * @deprecated Use {@link #StandaloneLeaderRetrievalService(String, UUID)} instead
	 */
	@Deprecated
	public StandaloneLeaderRetrievalService(String leaderAddress) {
		this.leaderAddress = checkNotNull(leaderAddress);
		this.leaderId = HighAvailabilityServices.DEFAULT_LEADER_ID;
	}

	/**
	 * Creates a StandaloneLeaderRetrievalService with the given leader address.
	 *
	 * @param leaderAddress The leader's pre-configured address
	 * @param leaderId      The constant leaderId.
	 */
	public StandaloneLeaderRetrievalService(String leaderAddress, UUID leaderId) {
		this.leaderAddress = checkNotNull(leaderAddress);
		this.leaderId = checkNotNull(leaderId);
	}

	// ------------------------------------------------------------------------

	@Override
	public void start(LeaderRetrievalListener listener) {
		checkNotNull(listener, "Listener must not be null.");

		synchronized (startStopLock) {
			checkState(!started, "StandaloneLeaderRetrievalService can only be started once.");
			started = true;

			// directly notify the listener, because we already know the leading JobManager's address
			listener.notifyLeaderAddress(leaderAddress, leaderId);
		}
	}

	@Override
	public void stop() {
		synchronized (startStopLock) {
			started = false;
		}
	}
}