alluxio-master-2.x/core/server/master/src/main/java/alluxio/master/meta/MetaMasterSync.java
/*
 * The Alluxio Open Foundation licenses this work under the Apache License, version 2.0
 * (the "License"). You may not use this work except in compliance with the License, which is
 * available at www.apache.org/licenses/LICENSE-2.0
 *
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
 * either express or implied, as more fully set forth in the License.
 *
 * See the NOTICE file distributed with this work for information regarding copyright ownership.
 */

package alluxio.master.meta;

import alluxio.conf.Configuration;
import alluxio.grpc.MetaCommand;
import alluxio.grpc.Scope;
import alluxio.heartbeat.HeartbeatExecutor;
import alluxio.wire.Address;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.IOException;
import java.util.concurrent.atomic.AtomicReference;
import javax.annotation.concurrent.NotThreadSafe;

/**
 * If a master is detected as a standby master. It will set up its MetaMasterSync and manage its own
 * {@link RetryHandlingMetaMasterMasterClient} which helps communicate with the leader master.
 *
 * When running, the standby master will send its heartbeat to the leader master. The leader master
 * may respond to the heartbeat with a command which will be executed. After which, the task will
 * wait for the elapsed time since its last heartbeat has reached the heartbeat interval. Then the
 * cycle will continue.
 */
@NotThreadSafe
public final class MetaMasterSync implements HeartbeatExecutor {
  private static final Logger LOG = LoggerFactory.getLogger(MetaMasterSync.class);
  private static final long UNINITIALIZED_MASTER_ID = -1L;

  /** The address of this standby master. */
  private final Address mMasterAddress;

  /** Client for communication with the leader master. */
  private final RetryHandlingMetaMasterMasterClient mMasterClient;

  /** The ID of this standby master. */
  private final AtomicReference<Long> mMasterId = new AtomicReference<>(UNINITIALIZED_MASTER_ID);

  /**
   * Creates a new instance of {@link MetaMasterSync}.
   *
   * @param masterAddress the master address
   * @param masterClient the meta master client
   */
  public MetaMasterSync(Address masterAddress, RetryHandlingMetaMasterMasterClient masterClient) {
    mMasterAddress = masterAddress;
    mMasterClient = masterClient;
  }

  /**
   * Heartbeats to the leader master node.
   */
  @Override
  public void heartbeat(long timeLimitMs) {
    MetaCommand command = null;
    try {
      if (mMasterId.get() == UNINITIALIZED_MASTER_ID) {
        setIdAndRegister();
      }
      LOG.debug("Standby master: {} send a heartbeat request to the leader master.",
          mMasterId.get());
      command = mMasterClient.heartbeat(mMasterId.get());
      handleCommand(command);
    } catch (IOException e) {
      // An error occurred, log and ignore it or error if heartbeat timeout is reached
      if (command == null) {
        LOG.error("Failed to receive leader master heartbeat command.", e);
      } else {
        LOG.error("Failed to execute leader master heartbeat command: {}", command, e);
      }
      mMasterClient.disconnect();
    }
  }

  /**
   * Handles a leader master command.
   *
   * @param cmd the command to execute
   */
  private void handleCommand(MetaCommand cmd) throws IOException {
    if (cmd == null) {
      return;
    }
    switch (cmd) {
      case MetaCommand_Nothing:
        break;
      // Leader master requests re-registration
      case MetaCommand_Register:
        setIdAndRegister();
        break;
      // Unknown request
      case MetaCommand_Unknown:
        LOG.error("Master heartbeat sends unknown command {}", cmd);
        break;
      default:
        throw new RuntimeException("Un-recognized command from leader master " + cmd);
    }
  }

  /**
   * Sets the master id and registers with the Alluxio leader master.
   */
  private void setIdAndRegister() throws IOException {
    mMasterId.set(mMasterClient.getId(mMasterAddress));
    mMasterClient.register(mMasterId.get(),
        Configuration.getConfiguration(Scope.MASTER));
  }

  @Override
  public void close() {
    mMasterClient.close();
  }
}
