kirktrue commented on code in PR #18795:
URL: https://github.com/apache/kafka/pull/18795#discussion_r1940329129
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clients/src/main/java/org/apache/kafka/clients/consumer/internals/AbstractFetch.java:
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@@ -408,22 +407,52 @@ protected Map<Node, FetchSessionHandler.FetchRequestData>
prepareFetchRequests()
long currentTimeMs = time.milliseconds();
Map<String, Uuid> topicIds = metadata.topicIds();
- for (TopicPartition partition : fetchablePartitions()) {
- SubscriptionState.FetchPosition position =
subscriptions.position(partition);
+ // This is the set of partitions that have buffered data
+ Set<TopicPartition> buffered =
Collections.unmodifiableSet(fetchBuffer.bufferedPartitions());
- if (position == null)
- throw new IllegalStateException("Missing position for
fetchable partition " + partition);
+ // This is the set of partitions that do not have buffered data
+ Set<TopicPartition> unbuffered = fetchablePartitions(buffered);
- Optional<Node> leaderOpt = position.currentLeader.leader;
+ if (unbuffered.isEmpty()) {
+ // If there are no partitions that don't already have data locally
buffered, there's no need to issue
+ // any fetch requests at the present time.
+ return Collections.emptyMap();
+ }
- if (leaderOpt.isEmpty()) {
- log.debug("Requesting metadata update for partition {} since
the position {} is missing the current leader node", partition, position);
- metadata.requestUpdate(false);
+ Set<Integer> bufferedNodes = new HashSet<>();
+
+ for (TopicPartition partition : buffered) {
+ // It's possible that at the time of the fetcher creating new
fetch requests, a partition with buffered
+ // data from a *previous* request is no longer assigned. So before
attempting to retrieve the node
+ // information, check that the partition is still assigned and
fetchable; an unassigned/invalid partition
+ // will throw an IllegalStateException in positionForPartition.
+ //
+ // Note: this check is not needed for the unbuffered partitions as
the logic in
+ // SubscriptionState.fetchablePartitions() only includes
partitions currently assigned.
+ if (!subscriptions.hasValidPosition(partition))
+ continue;
+
+ // A paused partition remains in the fetch buffer even though it's
not returned to the user. Unless
+ // this case is handled, a paused partition will inadvertently
"block" a request from being created to
+ // fetch data for other partitions, leading users to poll
indefinitely waiting for more data.
+ //
+ // See FetchCollector.collectFetch().
+ if (subscriptions.isPaused(partition))
+ continue;
Review Comment:
FYI: I updated the comment after adding my reviewers’ notes.
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