58e88fd3809d7a3e76e643f3df3f7386d6d32b78
[samza.git] / samza-api / src / main / java / org / apache / samza / operators / functions / FoldLeftFunction.java
1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one
3 * or more contributor license agreements. See the NOTICE file
4 * distributed with this work for additional information
5 * regarding copyright ownership. The ASF licenses this file
6 * to you under the Apache License, Version 2.0 (the
7 * "License"); you may not use this file except in compliance
8 * with the License. You may obtain a copy of the License at
9 *
10 * http://www.apache.org/licenses/LICENSE-2.0
11 *
12 * Unless required by applicable law or agreed to in writing,
13 * software distributed under the License is distributed on an
14 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
15 * KIND, either express or implied. See the License for the
16 * specific language governing permissions and limitations
17 * under the License.
18 */
19
20 package org.apache.samza.operators.functions;
21
22 /**
23 * A fold function that incrementally combines and aggregates values for a window.
24 */
25 public interface FoldLeftFunction<M, WV> extends InitableFunction {
26
27 /**
28 * Incrementally combine and aggregate values for the window. Guaranteed to be invoked for every
29 * message added to the window.
30 *
31 * @param message the incoming message that is added to the window. This object should not be mutated.
32 * @param oldValue the previous value
33 * @return the new value
34 */
35 WV apply(M message, WV oldValue);
36 }