001/*
002 * Copyright (c) 2007-2017 Xplenty, 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  }