storm-client/src/jvm/org/apache/storm/trident/operation/Function.java

/**
 * 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.storm.trident.operation;

import java.util.Map;
import org.apache.storm.trident.tuple.TridentTuple;

/**
 * A function takes in a set of input fields and emits zero or more tuples as output. The fields of the output tuple are appended to the
 * original input tuple in the stream. If a function emits no tuples, the original input tuple is filtered out. Otherwise, the input tuple
 * is duplicated for each output tuple.
 *
 * <p>For example, if you have the following function:
 *
 * <p>```java public class MyFunction extends BaseFunction { public void execute(TridentTuple tuple, TridentCollector collector)
 * { for(int i=0; i < tuple.getInteger(0); i++) { collector.emit(new Values(i)); } } }
 * ```
 *
 * <p>Now suppose you have a stream in the variable `mystream` with the fields `["a", "b", "c"]` with the following tuples:
 *
 * <p>``` [1, 2, 3] [4, 1, 6] [3, 0, 8] ``` If you had the following code in your topology definition:
 *
 * <p>```java mystream.each(new Fields("b"), new MyFunction(), new Fields("d"))) ```
 *
 * <p>The resulting tuples would have the fields `["a", "b", "c", "d"]` and look like this:
 *
 * <p>``` [1, 2, 3, 0] [1, 2, 3, 1] [4, 1, 6, 0] ```
 *
 * <p>In this case, the parameter `new Fields("b")` tells Trident that you would like to select the field "b" as input to the function, and
 * that will be the only field in the Tuple passed to the `execute()` method. The value of "b" in the first tuple (2) causes the for loop to
 * execute twice, so 2 tuples are emitted. similarly the second tuple causes one tuple to be emitted. For the third tuple, the value of 0
 * causes the `for` loop to be skipped, so nothing is emitted and the incoming tuple is filtered out of the stream.
 *
 * <p>### Configuration If your `Function` implementation has configuration requirements, you will typically want to extend {@link
 * org.apache.storm.trident.operation.BaseFunction} and override the {@link org.apache.storm.trident.operation.Operation#prepare(Map,
 * TridentOperationContext)} method to perform your custom initialization.
 *
 * <p>### Performance Considerations Because Trident Functions perform logic on individual tuples -- as opposed to
 * batches -- it is advisable to avoid expensive operations such as database operations in a Function, if possible. For
 * data store interactions it is better to use a {@link org.apache.storm.trident.state.State} or
 * {@link org.apache.storm.trident.state.QueryFunction} implementation since Trident states operate on batch partitions
 * and can perform bulk updates to a database.
 */
public interface Function extends EachOperation {
    /**
     * Performs the function logic on an individual tuple and emits 0 or more tuples.
     *
     * @param tuple     The incoming tuple
     * @param collector A collector instance that can be used to emit tuples
     */
    void execute(TridentTuple tuple, TridentCollector collector);
}