LifecycleAware.java

/*
 * 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.flume.lifecycle;

import org.apache.flume.annotations.InterfaceAudience;
import org.apache.flume.annotations.InterfaceStability;


/**
 * <p>
 * An interface implemented by any class that has a defined, stateful,
 * lifecycle.
 * </p>
 * <p>
 * Implementations of {@link LifecycleAware} conform to a standard method of
 * starting, stopping, and reporting their current state. Additionally, this
 * interface creates a standard method of communicating failure to perform a
 * lifecycle operation to the caller (i.e. via {@link LifecycleException}). It
 * is never considered valid to call {@link #start()} or
 * {@link #stop()} more than once or to call them in the wrong order.
 * While this is not strictly enforced, it may be in the future.
 * </p>
 * <p>
 * Example services may include Flume nodes and the master, but also lower level
 * components that can be controlled in a similar manner.
 * </p>
 * <p>
 * Example usage
 * </p>
 * <code>
 *  public class MyService implements LifecycleAware {
 *
 *    private LifecycleState lifecycleState;
 *
 *    public MyService() {
 *      lifecycleState = LifecycleState.IDLE;
 *    }
 *
 *    @Override
 *    public void start(Context context) throws LifecycleException,
 *      InterruptedException {
 *
 *      ...your code does something.
 *
 *      lifecycleState = LifecycleState.START;
 *    }
 *
 *    @Override
 *    public void stop(Context context) throws LifecycleException,
 *      InterruptedException {
 *
 *      try {
 *        ...you stop services here.
 *      } catch (SomethingException) {
 *        lifecycleState = LifecycleState.ERROR;
 *      }
 *
 *      lifecycleState = LifecycleState.STOP;
 *    }
 *
 *    @Override
 *    public LifecycleState getLifecycleState() {
 *      return lifecycleState;
 *    }
 *
 *  }
 * </code>
 */
@InterfaceAudience.Public
@InterfaceStability.Stable
public interface LifecycleAware {

  /**
   * <p>
   * Starts a service or component.
   * </p>
   * <p>
   * Implementations should determine the result of any start logic and effect
   * the return value of {@link #getLifecycleState()} accordingly.
   * </p>
   *
   * @throws LifecycleException
   * @throws InterruptedException
   */
  public void start();

  /**
   * <p>
   * Stops a service or component.
   * </p>
   * <p>
   * Implementations should determine the result of any stop logic and effect
   * the return value of {@link #getLifecycleState()} accordingly.
   * </p>
   *
   * @throws LifecycleException
   * @throws InterruptedException
   */
  public void stop();

  /**
   * <p>
   * Return the current state of the service or component.
   * </p>
   */
  public LifecycleState getLifecycleState();

}