kirktrue commented on code in PR #14640:
URL: https://github.com/apache/kafka/pull/14640#discussion_r1421255259
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clients/src/main/java/org/apache/kafka/clients/consumer/internals/AsyncKafkaConsumer.java:
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@@ -1497,6 +1606,66 @@ private void subscribeInternal(Collection<String>
topics, Optional<ConsumerRebal
}
}
+ /**
+ * This method can be used by cases where the caller has an event that
needs to both block for completion but
+ * also process background events. For some events, in order to fully
process the associated logic, the
+ * {@link ConsumerNetworkThread background thread} needs assistance from
the application thread to complete.
+ * If the application thread simply blocked on the event after submitting
it, the processing would deadlock.
+ * The logic herein is basically a loop that performs two tasks in each
iteration:
+ *
+ * <ol>
+ * <li>Process background events, if any</li>
+ * <li><em>Briefly</em> wait for {@link CompletableApplicationEvent an
event} to complete</li>
+ * </ol>
+ *
+ * <p/>
+ *
+ * Each iteration gives the application thread an opportunity to process
background events, which may be
+ * necessary to complete the overall processing.
+ *
+ * <p/>
+ *
+ * As an example, take {@link #unsubscribe()}. To start unsubscribing, the
application thread enqueues an
+ * {@link UnsubscribeApplicationEvent} on the application event queue.
That event will eventually trigger the
+ * rebalancing logic in the background thread. Critically, as part of this
rebalancing work, the
+ * {@link ConsumerRebalanceListener#onPartitionsRevoked(Collection)}
callback needs to be invoked. However,
+ * this callback must be executed on the application thread. To achieve
this, the background thread enqueues a
+ * {@link ConsumerRebalanceListenerCallbackNeededEvent} on its background
event queue. That event queue is
+ * periodically queried by the application thread to see if there's work
to be done. When the application thread
+ * sees {@link ConsumerRebalanceListenerCallbackNeededEvent}, it is
processed, and then a
+ * {@link ConsumerRebalanceListenerCallbackCompletedEvent} is then
enqueued by the application thread on the
+ * background event queue. Moments later, the background thread will see
that event, process it, and continue
+ * execution of the rebalancing logic. The rebalancing logic cannot
complete until the
+ * {@link ConsumerRebalanceListener} callback is performed.
+ *
+ * @param event Event that contains a {@link CompletableFuture}; it is on
this future that the application thread
+ * will wait for completion
+ * @param timer Overall timer that bounds how long the application thread
will wait for the event to complete
+ * @return {@code true} if the event completed within the timeout, {@code
false} otherwise
+ */
+ private boolean processBackgroundEvents(CompletableApplicationEvent<?>
event, Timer timer) {
+ log.trace("Enqueuing event {} for processing; will wait up to {} ms to
complete", event, timer.remainingMs());
+
+ do {
+ backgroundEventProcessor.process();
+
+ try {
+ Timer pollInterval = time.timer(100L);
Review Comment:
Done. Please see if you agree with the logic.
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