runners/flink/src/main/java/org/apache/beam/runners/flink/translation/functions/ImpulseSourceFunction.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.beam.runners.flink.translation.functions;

import org.apache.beam.sdk.util.WindowedValue;
import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.Iterables;
import org.apache.flink.api.common.state.ListState;
import org.apache.flink.api.common.state.ListStateDescriptor;
import org.apache.flink.api.common.typeutils.base.BooleanSerializer;
import org.apache.flink.runtime.state.FunctionInitializationContext;
import org.apache.flink.runtime.state.FunctionSnapshotContext;
import org.apache.flink.streaming.api.checkpoint.CheckpointedFunction;
import org.apache.flink.streaming.api.functions.source.SourceFunction;

/**
 * Source function which sends a single global impulse to a downstream operator. It may keep the
 * source alive although its work is already done. It will only shutdown when the streaming job is
 * cancelled.
 */
public class ImpulseSourceFunction
    implements SourceFunction<WindowedValue<byte[]>>, CheckpointedFunction {

  /** Keep source running even after it has done all the work. */
  private final boolean keepSourceAlive;

  /** Indicates the streaming job is running and the source can produce elements. */
  private volatile boolean running;

  /** Checkpointed state which indicates whether the impulse has finished. */
  private transient ListState<Boolean> impulseEmitted;

  public ImpulseSourceFunction(boolean keepSourceAlive) {
    this.keepSourceAlive = keepSourceAlive;
    this.running = true;
  }

  @Override
  public void run(SourceContext<WindowedValue<byte[]>> sourceContext) throws Exception {
    if (Iterables.isEmpty(impulseEmitted.get())) {
      synchronized (sourceContext.getCheckpointLock()) {
        // emit single impulse element
        sourceContext.collect(WindowedValue.valueInGlobalWindow(new byte[0]));
        impulseEmitted.add(true);
      }
    }
    // Do nothing, but still look busy ...
    // we can't return here since Flink requires that all operators stay up,
    // otherwise checkpointing would not work correctly anymore
    //
    // See https://issues.apache.org/jira/browse/FLINK-2491 for progress on this issue
    if (keepSourceAlive) {
      // wait until this is canceled
      final Object waitLock = new Object();
      while (running) {
        try {
          // Flink will interrupt us at some point
          //noinspection SynchronizationOnLocalVariableOrMethodParameter
          synchronized (waitLock) {
            // don't wait indefinitely, in case something goes horribly wrong
            waitLock.wait(1000);
          }
        } catch (InterruptedException e) {
          if (!running) {
            // restore the interrupted state, and fall through the loop
            Thread.currentThread().interrupt();
          }
        }
      }
    }
  }

  @Override
  public void cancel() {
    this.running = false;
  }

  @Override
  public void snapshotState(FunctionSnapshotContext context) {}

  @Override
  public void initializeState(FunctionInitializationContext context) throws Exception {
    impulseEmitted =
        context
            .getOperatorStateStore()
            .getListState(new ListStateDescriptor<>("impulse-emitted", BooleanSerializer.INSTANCE));
  }
}