storm-client/src/jvm/org/apache/storm/spout/ISpout.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.storm.spout;

import java.io.Serializable;
import java.util.Map;
import org.apache.storm.task.TopologyContext;

/**
 * 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 org.apache.storm.Config}), Storm will send a fail message to the spout for the message.
 *
 * <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>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.
 */
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:
     *
     * @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<String, Object> 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.
     */
    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);
}