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   * 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.sdk.nexmark.sources;
  
  import static org.junit.Assert.assertEquals;
  import static org.junit.Assert.assertFalse;
  import static org.junit.Assert.assertTrue;
  
  import java.io.IOException;
  import java.util.HashSet;
  import java.util.Random;
  import java.util.Set;
  import org.apache.beam.sdk.io.UnboundedSource.CheckpointMark;
  import org.apache.beam.sdk.io.UnboundedSource.UnboundedReader;
  import org.apache.beam.sdk.nexmark.NexmarkConfiguration;
  import org.apache.beam.sdk.nexmark.model.Event;
  import org.apache.beam.sdk.nexmark.sources.generator.Generator;
  import org.apache.beam.sdk.nexmark.sources.generator.GeneratorCheckpoint;
  import org.apache.beam.sdk.nexmark.sources.generator.GeneratorConfig;
  import org.apache.beam.sdk.options.PipelineOptions;
  import org.apache.beam.sdk.testing.TestPipeline;
  import org.junit.Ignore;
  import org.junit.Test;
  import org.junit.runner.RunWith;
  import org.junit.runners.JUnit4;
  
  /** Test UnboundedEventSource. */
  @RunWith(JUnit4.class)
  public class UnboundedEventSourceTest {
    private GeneratorConfig makeConfig(long n) {
      return new GeneratorConfig(NexmarkConfiguration.DEFAULT, System.currentTimeMillis(), 0, n, 0);
    }
  
    /**
     * Helper for tracking which ids we've seen (so we can detect dups) and confirming reading events
     * match the model events.
     */
    private static class EventIdChecker {
      private final Set<Long> seenPersonIds = new HashSet<>();
      private final Set<Long> seenAuctionIds = new HashSet<>();
  
      public void add(Event event) {
        if (event.newAuction != null) {
          assertTrue(seenAuctionIds.add(event.newAuction.id));
        } else if (event.newPerson != null) {
          assertTrue(seenPersonIds.add(event.newPerson.id));
        }
      }
  
      public void add(int n, UnboundedReader<Event> reader, Generator modelGenerator)
          throws IOException {
        for (int i = 0; i < n; i++) {
          assertTrue(modelGenerator.hasNext());
          Event modelEvent = modelGenerator.next().getValue();
          assertTrue(reader.advance());
          Event actualEvent = reader.getCurrent();
          assertEquals(modelEvent.toString(), actualEvent.toString());
          add(actualEvent);
        }
      }
    }
  
    /**
     * Check aggressively checkpointing and resuming a reader gives us exactly the same event stream
     * as reading directly.
     */
    @Ignore("TODO(https://github.com/apache/beam/issues/19077) Test is flaky. Fix before reenabling.")
    @Test
    public void resumeFromCheckpoint() throws IOException {
      Random random = new Random(297);
      int n = 47293;
      GeneratorConfig config = makeConfig(n);
      Generator modelGenerator = new Generator(config);
  
      EventIdChecker checker = new EventIdChecker();
      PipelineOptions options = TestPipeline.testingPipelineOptions();
      UnboundedEventSource source = new UnboundedEventSource(config, 1, 0, false);
      UnboundedReader<Event> reader = source.createReader(options, null);
  
      while (n > 0) {
        int m = Math.min(459 + random.nextInt(455), n);
        System.out.printf("reading %d...%n", m);
        checker.add(m, reader, modelGenerator);
        n -= m;
        System.out.printf("splitting with %d remaining...%n", n);
        CheckpointMark checkpointMark = reader.getCheckpointMark();
        reader = source.createReader(options, (GeneratorCheckpoint) checkpointMark);
      }
  
      assertFalse(reader.advance());
    }
  }
  