/*
 * 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.flink.api.common.functions;

import java.io.Serializable;

import org.apache.flink.annotation.Public;
import org.apache.flink.util.Collector;

/**
 * The interface for CoGroup functions. CoGroup functions combine two data sets by first grouping each data set
 * after a key and then "joining" the groups by calling this function with the two sets for each key. 
 * If a key is present in only one of the two inputs, it may be that one of the groups is empty.
 * <p>
 * The basic syntax for using CoGoup on two data sets is as follows:
 * <pre>{@code
 * DataSet<X> set1 = ...;
 * DataSet<Y> set2 = ...;
 * 
 * set1.coGroup(set2).where(<key-definition>).equalTo(<key-definition>).with(new MyCoGroupFunction());
 * }</pre>
 * <p>
 * {@code set1} is here considered the first input, {@code set2} the second input.
 * <p>
 * Some keys may only be contained in one of the two original data sets. In that case, the CoGroup function is invoked
 * with in empty input for the side of the data set that did not contain elements with that specific key.
 * 
 * @param <IN1> The data type of the first input data set.
 * @param <IN2> The data type of the second input data set.
 * @param <O> The data type of the returned elements.
 */
@Public
public interface CoGroupFunction<IN1, IN2, O> extends Function, Serializable {
	
	/**
	 * This method must be implemented to provide a user implementation of a
	 * coGroup. It is called for each pair of element groups where the elements share the
	 * same key.
	 * 
	 * @param first The records from the first input.
	 * @param second The records from the second.
	 * @param out A collector to return elements.
	 * 
	 * @throws Exception The function may throw Exceptions, which will cause the program to cancel,
	 *                   and may trigger the recovery logic.
	 */
	public void coGroup(Iterable<IN1> first, Iterable<IN2> second, Collector<O> out) throws Exception;
}