content: /**
   * 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.apache.aurora.scheduler.app;
  
  import java.io.Closeable;
  import java.util.function.Supplier;
  
  import com.google.common.collect.ImmutableSet;
  
  import org.apache.aurora.scheduler.discovery.ServiceInstance;
  
  /**
   * Monitors a service group's membership and supplies a live view of the most recent set.
   */
  public interface ServiceGroupMonitor extends Supplier<ImmutableSet<ServiceInstance>>, Closeable {
  
    /**
     * Indicates a problem initiating monitoring of a service group.
     */
    class MonitorException extends Exception {
      public MonitorException(String message, Throwable cause) {
        super(message, cause);
      }
    }
  
    /**
     * Starts monitoring the service group.
     *
     * When the service group membership no longer needs to be maintained, this monitor should be
     * {@link #close() closed}.
     *
     * @throws MonitorException if there is a problem initiating monitoring of the service group.
     */
    void start() throws MonitorException;
  }
  