content: /**
   * 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 backtype.storm.spout;
  
  import backtype.storm.task.TopologyContext;
  import java.util.Map;
  import java.io.Serializable;
  
  /**
   * ISpout is the core interface for implementing spouts. A Spout is responsible for feeding messages into the topology for processing. For every tuple emitted
   * by a spout, Storm will track the (potentially very large) DAG of tuples generated based on a tuple emitted by the spout. When Storm detects that every tuple
   * in that DAG has been successfully processed, it will send an ack message to the Spout.
   * 
   * <p>
   * If a tuple fails to be fully processed within the configured timeout for the topology (see {@link backtype.storm.Config}), Storm will send a fail message to
   * the spout for the message.
   * </p>
   * 
   * <p>
   * When a Spout emits a tuple, it can tag the tuple with a message id. The message id can be any type. When Storm acks or fails a message, it will pass back to
   * the spout the same message id to identify which tuple it's referring to. If the spout leaves out the message id, or sets it to null, then Storm will not
   * track the message and the spout will not receive any ack or fail callbacks for the message.
   * </p>
   * 
   * <p>
   * Storm executes ack, fail, and nextTuple all on the same thread. This means that an implementor of an ISpout does not need to worry about concurrency issues
   * between those methods. However, it also means that an implementor must ensure that nextTuple is non-blocking: otherwise the method could block acks and fails
   * that are pending to be processed.
   * </p>
   */
  public interface ISpout extends Serializable {
      /**
       * Called when a task for this component is initialized within a worker on the cluster. It provides the spout with the environment in which the spout
       * executes.
       * 
       * <p>
       * This includes the:
       * </p>
       * 
       * @param conf The Storm configuration for this spout. This is the configuration provided to the topology merged in with cluster configuration on this
       *            machine.
       * @param context This object can be used to get information about this task's place within the topology, including the task id and component id of this
       *            task, input and output information, etc.
       * @param collector The collector is used to emit tuples from this spout. Tuples can be emitted at any time, including the open and close methods. The
       *            collector is thread-safe and should be saved as an instance variable of this spout object.
       */
      void open(Map conf, TopologyContext context, SpoutOutputCollector collector);
  
      /**
       * Called when an ISpout is going to be shutdown. There is no guarentee that close will be called, because the supervisor kill -9's worker processes on the
       * cluster.
       * 
       * <p>
       * The one context where close is guaranteed to be called is a topology is killed when running Storm in local mode.
       * </p>
       */
      void close();
  
      /**
       * Called when a spout has been activated out of a deactivated mode. nextTuple will be called on this spout soon. A spout can become activated after having
       * been deactivated when the topology is manipulated using the `storm` client.
       */
      void activate();
  
      /**
       * Called when a spout has been deactivated. nextTuple will not be called while a spout is deactivated. The spout may or may not be reactivated in the
       * future.
       */
      void deactivate();
  
      /**
       * When this method is called, Storm is requesting that the Spout emit tuples to the output collector. This method should be non-blocking, so if the Spout
       * has no tuples to emit, this method should return. nextTuple, ack, and fail are all called in a tight loop in a single thread in the spout task. When
       * there are no tuples to emit, it is courteous to have nextTuple sleep for a short amount of time (like a single millisecond) so as not to waste too much
       * CPU.
       */
      void nextTuple();
  
      /**
       * Storm has determined that the tuple emitted by this spout with the msgId identifier has been fully processed. Typically, an implementation of this method
       * will take that message off the queue and prevent it from being replayed.
       */
      void ack(Object msgId);
  
      /**
       * The tuple emitted by this spout with the msgId identifier has failed to be fully processed. Typically, an implementation of this method will put that
       * message back on the queue to be replayed at a later time.
       */
      void fail(Object msgId);
  }