cadonna commented on code in PR #16241:
URL: https://github.com/apache/kafka/pull/16241#discussion_r1635119225
##########
clients/src/main/java/org/apache/kafka/clients/consumer/internals/CommitRequestManager.java:
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@@ -1279,4 +1288,119 @@ static class MemberInfo {
this.memberEpoch = Optional.empty();
}
}
+
+ /**
+ * As the name suggests, this caches the result of an {@link
OffsetFetchRequestState offset fetch request}
+ * <em>in specific cases</em>. The logic resembles that of {@link
ConsumerCoordinator}'s
+ * <code>PendingCommittedOffsetRequest</code> mechanism.
+ *
+ * <p/>
+ *
+ * This handles the case where a user calls {@link
Consumer#poll(Duration)} with a low timeout value, such as 0.
+ * As the timeout value approaches zero, the likelihood that the client
will be able to fetch the offsets from the
+ * broker within the user's timeout also decreases. But we can take
advantage of the fact that {@code poll()}
+ * is typically invoked in a tight loop, so the user's application will
likely call {@code poll()} again to make
+ * a second attempt with the same set of {@link TopicPartition}s as the
first attempt.
+ *
+ * <p/>
+ *
+ * The idea here is to cache the results of the last successful--but
expired--response. An operation may exceed
+ * the time the user allotted, but that doesn't mean that the network
request is aborted. In this scenario, it is
+ * often the case that the client receives a successful response, though
it has technically exceeded the amount
+ * of time the user specified. However, as mentioned above, {@code poll()}
is likely to be invoked again with the
+ * same set of partitions. By caching the successful response from attempt
#1--though it timed out from the user's
+ * perspective--the client is able to satisfy attempt #2 from the cache.
Review Comment:
@lianetm in the legacy consumer, does the fact that the offset fetch request
and the offset commit request use the same network connection ensure that the
requests have an order and the commit request cannot overtake the fetch request
if commit was sent after fetch? Sorry if this question seems dumb, but I am not
familiar with the network part.
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