Matching a simple pattern using Flink's CEP library

CEPPatternAB.java

/*
 * Copyright 2021 Ververica GmbH
 *
 * Licensed 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 com.ververica.flink.example;

import org.apache.flink.api.common.eventtime.WatermarkStrategy;
import org.apache.flink.cep.CEP;
import org.apache.flink.cep.PatternStream;
import org.apache.flink.cep.functions.PatternProcessFunction;
import org.apache.flink.cep.pattern.Pattern;
import org.apache.flink.cep.pattern.conditions.SimpleCondition;
import org.apache.flink.streaming.api.datastream.DataStream;
import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
import org.apache.flink.util.Collector;

import java.util.List;
import java.util.Map;

public class CEPPatternAB {

	public static void main(String[] args) throws Exception {

		StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();

		DataStream<String> stringInputStream = env
				.fromElements("A1", "C", "A2", "A3", "B", "E")
				.assignTimestampsAndWatermarks(
						WatermarkStrategy // using ingestion time for this example
								.<String>forMonotonousTimestamps()
								.withTimestampAssigner((e, t) -> System.currentTimeMillis()));

		Pattern<String, ?> pattern = Pattern.<String>begin("A")
				.where(new SimpleCondition<String>() {
					@Override
					public boolean filter(String s) throws Exception {
						return s.startsWith("A");
					}
				}).next("B") // next enforces strict contiguity
				.where(new SimpleCondition<String>() {
					@Override
					public boolean filter(String s) throws Exception {
						return s.equals("B");
					}
				});

		PatternStream<String> patternStream = CEP.pattern(stringInputStream, pattern);

		DataStream<String> processed = patternStream.process(new PatternProcessFunction<String, String>() {
			@Override
			public void processMatch(
					Map<String, List<String>> map,
					Context context,
					Collector<String> out) throws Exception {

				out.collect(map.toString());
			}
		});

		processed.printToErr(); // making the results easier to find amidst a sea of logging

		env.execute();
	}
}