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https://issues.apache.org/jira/browse/TWILL-199?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15797778#comment-15797778
 ] 

ASF GitHub Bot commented on TWILL-199:
--------------------------------------

Github user chtyim commented on a diff in the pull request:

    https://github.com/apache/twill/pull/16#discussion_r94551913
  
    --- Diff: 
twill-core/src/test/java/org/apache/twill/kafka/client/KafkaTest.java ---
    @@ -170,11 +174,128 @@ public void testKafkaClient() throws Exception {
         Cancellable cancel = kafkaClient.getConsumer().prepare().add(topic, 0, 
0).consume(new KafkaConsumer
           .MessageCallback() {
           @Override
    -      public void onReceived(Iterator<FetchedMessage> messages) {
    +      public long onReceived(Iterator<FetchedMessage> messages) {
    +        long nextOffset = Long.MIN_VALUE;
    +        while (messages.hasNext()) {
    +          FetchedMessage message = messages.next();
    +          LOG.info(Charsets.UTF_8.decode(message.getPayload()).toString());
    +          latch.countDown();
    +        }
    +        return nextOffset;
    +      }
    +
    +      @Override
    +      public void finished() {
    +        stopLatch.countDown();
    +      }
    +    });
    +
    +    Assert.assertTrue(latch.await(5, TimeUnit.SECONDS));
    +    cancel.cancel();
    +    Assert.assertTrue(stopLatch.await(1, TimeUnit.SECONDS));
    +  }
    +
    +  @Test
    +  public void testKafkaClientReadFromIdx() throws Exception {
    +    String topic = "testClient";
    +
    +    // Publish 30 messages with indecies the same as offsets within the 
range 0 - 29
    +    Thread t1 = createPublishThread(kafkaClient, topic, Compression.GZIP, 
"GZIP Testing message", 10);
    +    t1.start();
    +    t1.join();
    +    Thread t2 = createPublishThread(kafkaClient, topic, Compression.NONE, 
"Testing message", 10, 10);
    +    t2.start();
    +    t2.join();
    +    Thread t3 = createPublishThread(kafkaClient, topic, 
Compression.SNAPPY, "Snappy Testing message", 10, 20);
    +    t3.start();
    +    t3.join();
    +
    +    final int startIdx = 15;
    +    final CountDownLatch latch = new CountDownLatch(30 - startIdx);
    +    final CountDownLatch stopLatch = new CountDownLatch(1);
    +    final BlockingQueue<Long> offsetQueue = new LinkedBlockingQueue<>();
    +    // Creater a consumer
    +    final SimpleKafkaConsumer consumer = (SimpleKafkaConsumer) 
kafkaClient.getConsumer();
    +    Cancellable initCancel = 
kafkaClient.getConsumer().prepare().add(topic, 0, 0).consume(new KafkaConsumer
    +      .MessageCallback() {
    +      long minOffset = -2; // earliest msg
    +      long maxOffset = -1; // latest msg
    +      @Override
    +      // Use binary search to find the offset of the message with the 
index matching startIdx. Returns the next offset
    --- End diff --
    
    Seems like this unit-test is unnecessarily complicated. All you want to 
test is that the offset being returned from the `onReceived` method is being 
honored, right?


> Get next offset and handle offset error in KafkaConsumer.MessageCallback
> ------------------------------------------------------------------------
>
>                 Key: TWILL-199
>                 URL: https://issues.apache.org/jira/browse/TWILL-199
>             Project: Apache Twill
>          Issue Type: Improvement
>            Reporter: Chengfeng Mao
>
> The method {{void onReceived(Iterator<FetchedMessage> messages)}} in 
> {{KafkaConsumer.MessageCallback}} can be more flexible with the change to 
> {{Long onReceived(Iterator<FetchedMessage> messages)}} so that it can provide 
> additional functionalities:
> 1. To return the next offset to be fetched
> 2. To handle offset non-existence or offset mismatch error and take action on 
> the error
> This method will return null for backward compatibility when it doesn't need 
> to provide the next offset.
> In concrete implementation,  a class of a new interface 
> {{KafkaOffsetProvider}} can be added as a member in 
> {{KafkaConsumer.MessageCallback}} to perform the offset error handling and 
> provide the next offset. Besides, {{KafkaOffsetProvider}} also has methods to 
> provide the following functionalities:
> 1. To fetch earliest/latest offset in Kafka
> 2. To find the offset of a message with timestamp equal to the given 
> timestamp in Kafka
> For backward compatibility, if {{KafkaOffsetProvider}} instance is not 
> provided, its default value will be null and none of its methods will be 
> called.



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