slider-core/src/main/java/org/apache/slider/server/services/workflow/WorkflowCompositeService.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.slider.server.services.workflow;

import com.google.common.base.Preconditions;
import org.apache.hadoop.service.CompositeService;
import org.apache.hadoop.service.Service;
import org.apache.hadoop.service.ServiceStateChangeListener;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.List;

/**
 * An extended composite service which stops itself if any child service
 * fails, or when all its children have successfully stopped without failure.
 *
 * Lifecycle
 * <ol>
 *   <li>If any child exits with a failure: this service stops, propagating
 *   the exception.</li>
 *   <li>When all child services has stopped, this service stops itself</li>
 * </ol>
 *
 */
public class WorkflowCompositeService extends CompositeService
    implements ServiceParent, ServiceStateChangeListener {

  private static final Logger LOG =
    LoggerFactory.getLogger(WorkflowCompositeService.class);

  /**
   * Construct an instance
   * @param name name of this service instance
   */
  public WorkflowCompositeService(String name) {
    super(name);
  }


  /**
   * Construct an instance with the default name.
   */
  public WorkflowCompositeService() {
    this("WorkflowCompositeService");
  }

  /**
   * Varargs constructor
   * @param name name of this service instance
   * @param children children
   */
  public WorkflowCompositeService(String name, Service... children) {
    this(name);
    for (Service child : children) {
      addService(child);
    }
  }

  /**
   * Construct with a list of children
   * @param name name of this service instance
   * @param children children to add
   */
  public WorkflowCompositeService(String name, List<Service> children) {
    this(name);
    for (Service child : children) {
      addService(child);
    }
  }

  /**
   * Add a service, and register it
   * @param service the {@link Service} to be added.
   * Important: do not add a service to a parent during your own serviceInit/start,
   * in Hadoop 2.2; you will trigger a ConcurrentModificationException.
   */
  @Override
  public synchronized void addService(Service service) {
    Preconditions.checkArgument(service != null, "null service argument");
    service.registerServiceListener(this);
    super.addService(service);
  }

  /**
   * When this service is started, any service stopping with a failure
   * exception is converted immediately into a failure of this service, 
   * storing the failure and stopping ourselves.
   * @param child the service that has changed.
   */
  @Override
  public void stateChanged(Service child) {
    //if that child stopped while we are running:
    if (isInState(STATE.STARTED) && child.isInState(STATE.STOPPED)) {
      // a child service has stopped
      //did the child fail? if so: propagate
      Throwable failureCause = child.getFailureCause();
      if (failureCause != null) {
        LOG.info("Child service " + child + " failed", failureCause);
        //failure. Convert to an exception
        Exception e = (failureCause instanceof Exception) ?
            (Exception) failureCause : new Exception(failureCause);
        //flip ourselves into the failed state
        noteFailure(e);
        stop();
      } else {
        LOG.info("Child service completed {}", child);
        if (areAllChildrenStopped()) {
          LOG.info("All children are halted: stopping");
          stop();
        }
      }
    }
  }

  /**
   * Probe to query if all children are stopped -simply
   * by taking a snapshot of the child service list and enumerating
   * their state. 
   * The state of the children may change during this operation -that will
   * not get picked up.
   * @return true if all the children are stopped.
   */
  private boolean areAllChildrenStopped() {
    List<Service> children = getServices();
    boolean stopped = true;
    for (Service child : children) {
      if (!child.isInState(STATE.STOPPED)) {
        stopped = false;
        break;
      }
    }
    return stopped;
  }
}