    001/*
    002 * Copyright (c) 2007-2016 Concurrent, Inc. All Rights Reserved.
    003 *
    004 * Project and contact information: http://www.cascading.org/
    005 *
    006 * This file is part of the Cascading project.
    007 *
    008 * Licensed under the Apache License, Version 2.0 (the "License");
    009 * you may not use this file except in compliance with the License.
    010 * You may obtain a copy of the License at
    011 *
    012 *     http://www.apache.org/licenses/LICENSE-2.0
    013 *
    014 * Unless required by applicable law or agreed to in writing, software
    015 * distributed under the License is distributed on an "AS IS" BASIS,
    016 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    017 * See the License for the specific language governing permissions and
    018 * limitations under the License.
    019 */
    020
    021package cascading.pipe;
    022
    023/**
    024 * The Checkpoint pipe, if supported by the current planner, will force data to be persisted at the point in
    025 * the tuple stream an instance of Checkpoint is inserted into the pipe assembly.
    026 * <p/>
    027 * If a checkpoint {@link cascading.tap.Tap} is added to the {@link cascading.flow.FlowDef} via the
    028 * {@link cascading.flow.FlowDef#addCheckpoint(Checkpoint, cascading.tap.Tap)} method, that Tap instance
    029 * will be used to capture the intermediate result sets.
    030 * <p/>
    031 * It is required that any Scheme used as a checkpoint must source {@link cascading.tuple.Fields#UNKNOWN} and
    032 * sink {@link cascading.tuple.Fields#ALL}.
    033 * <p/>
    034 * If used with a {@link cascading.scheme.hadoop.TextDelimited} {@link cascading.scheme.Scheme} class and
    035 * the {@code hasHeader} value is {@code true}, a header with the resolved field names will be written to the file.
    036 * <p/>
    037 * This is especially useful for debugging complex flows.
    038 * <p/>
    039 * For the {@link cascading.flow.hadoop.HadoopFlowConnector} and Hadoop platform, a Checkpoint will force a new
    040 * MapReduce job ({@link cascading.flow.hadoop.HadoopFlowStep} into the {@link cascading.flow.Flow} plan.
    041 * <p/>
    042 * This can be important when used in conjunction with a {@link HashJoin} where all the operations upstream
    043 * from the HashJoin significantly filter out data allowing it to fit in memory.
    044 */
    045public class Checkpoint extends Pipe
    046  {
    047  /**
    048   * Constructor Checkpoint creates a new Checkpoint pipe which inherits the name of its previous pipe.
    049   *
    050   * @param previous of type Pipe
    051   */
    052  public Checkpoint( Pipe previous )
    053    {
    054    super( previous );
    055    }
    056
    057  /**
    058   * Constructor Checkpoint creates a new Checkpoint pipe with the given name.
    059   *
    060   * @param previous of type Pipe
    061   */
    062  public Checkpoint( String name, Pipe previous )
    063    {
    064    super( name, previous );
    065    }
    066  }