001 /* 002 * Copyright (c) 2007-2014 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 */ 020 021 package cascading.operation; 022 023 import cascading.flow.FlowProcess; 024 import 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 */ 048 public 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 }