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.operation;
022
023import cascading.flow.FlowProcess;
024import cascading.tuple.TupleEntry;
025
026/**
027 * An Aggregator takes the set of all values associated with a unique grouping and returns
028 * zero or more values. {@link cascading.operation.aggregator.MaxValue}, {@link cascading.operation.aggregator.MinValue},
029 * {@link cascading.operation.aggregator.Count}, and {@link cascading.operation.aggregator.Average} are good examples.
030 * <p/>
031 * Aggregator implementations should be reentrant. There is no guarantee an Aggregator instance will be executed in a
032 * unique vm, or by a single thread. The {@link #start(cascading.flow.FlowProcess, AggregatorCall)}
033 * method provides a mechanism for maintaining a 'context' object to hold intermediate values.
034 * <p/>
035 * Note {@link TupleEntry} instances are reused internally so should not be stored. Instead use the TupleEntry or Tuple
036 * copy constructors to make safe copies.
037 * <p/>
038 * Since Aggregators can be chained, and Cascading pipelines all operation results, any Aggregators
039 * coming ahead of the current Aggregator must return a value before the {@link #complete(cascading.flow.FlowProcess, AggregatorCall)}
040 * method on this Aggregator is called. Subsequently, if any previous Aggregators return more than one Tuple result,
041 * this complete() method will be called for each Tuple emitted.
042 * <p/>
043 * Thus it is a best practice to implement a {@link Buffer} when emitting more than one, or zero Tuple results.
044 *
045 * @see AggregatorCall
046 * @see OperationCall
047 */
048public interface Aggregator<Context> extends Operation<Context>
049  {
050  /**
051   * Method start initializes the aggregation procedure and is called for every unique grouping.
052   * <p/>
053   * The AggregatorCall context should be initialized here if necessary.
054   * <p/>
055   * The first time this method is called for a given 'process', the AggregatorCall context will be null. This method should
056   * set a new instance of the user defined context object. When the AggregatorCall context is not null, it is up to
057   * the developer to create a new instance, or 'recycle' the given instance. If recycled, it must be re-initialized to
058   * remove any previous state/values.
059   * <p/>
060   * For example, if a Map is used to hold the intermediate data for each subsequent
061   * {@link #aggregate(cascading.flow.FlowProcess, AggregatorCall)} call,
062   * new HashMap() should be set on the AggregatorCall instance when {@link cascading.operation.AggregatorCall#getContext()} is null.
063   * On the next grouping, start() will be called again, but this time with the old Map instance. In this case,
064   * map.clear() should be invoked before returning.
065   *
066   * @param flowProcess    of type FlowProcess
067   * @param aggregatorCall of type AggregatorCall
068   */
069  void start( FlowProcess flowProcess, AggregatorCall<Context> aggregatorCall );
070
071  /**
072   * Method aggregate is called for each {@link TupleEntry} value in the current grouping.
073   * <p/>
074   * TupleEntry entry, or entry.getTuple() should not be stored directly in the context. A copy of the tuple
075   * should be made via the {@code new Tuple( entry.getTuple() )} copy constructor.
076   *
077   * @param flowProcess    of type FlowProcess
078   * @param aggregatorCall of type AggregatorCall
079   */
080  void aggregate( FlowProcess flowProcess, AggregatorCall<Context> aggregatorCall );
081
082  /**
083   * Method complete will be issued last after every {@link TupleEntry} has been passed to the
084   * {@link #aggregate(cascading.flow.FlowProcess, AggregatorCall)}
085   * method.  Any final calculation should be completed here and passed to the outputCollector.
086   *
087   * @param flowProcess    of type FlowProcess
088   * @param aggregatorCall of type AggregatorCall
089   */
090  void complete( FlowProcess flowProcess, AggregatorCall<Context> aggregatorCall );
091  }