[GitHub] [kafka] adamkotwasinski opened a new pull request, #12364: MINOR: Improve Javadoc for OffsetCommitCallback-consuming commitAsync…

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adamkotwasinski opened a new pull request, #12364:
URL: https://github.com/apache/kafka/pull/12364

   … methods
   
   Having "async" in Kafka consumer API might encourage users to expect that there is some other internal thread that might deliver offset-commit confirmation, but actually it's usercode that needs to poll() to get the response. This PR attempts to add some disclaimer to cover this situation.
   
   In example, the following blocks infinitely:
   ```
           final Consumer<byte[], byte[]> kc = ....
           kc.assign(Collections.singleton(new TopicPartition("mytopic", 0)));
           kc.poll(2000);
           final CountDownLatch latch = new CountDownLatch(1); // The latch should be released when we get a response.
           kc.commitAsync(new OffsetCommitCallback() {
               @Override
               public void onComplete(final Map<TopicPartition, OffsetAndMetadata> offsets, final Exception exception) {
                   latch.countDown();
               }
           });
           latch.await(); // <- Never releases; adding something trivial like `kc.poll(300)` does help.
   ```
   Obviously the "fix" would be 
   
   
   In detail:
   `KafkaConsumer.commitAsync` calls only send the OFFSET_COMMIT requests, but do not wait for response (as they should be, as they are asynchronous after all) (the "in-flight" request is kept in NetworkClient's `inFlightRequests` collection).
   However, compared to Producer's asynchronous send API, there is no underlying worker thread that is going to get the data and invoke the callback - it is going to happen on the next NetworkClient interaction with remote (AFAICT anything that should finally invoke `NetworkClient.poll`).
   
   
   
   ### Committer Checklist (excluded from commit message)
   - [ ] Verify design and implementation 
   - [ ] Verify test coverage and CI build status
   - [ ] Verify documentation (including upgrade notes)
   


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