Reduce lock contention in ProducerMetadata#add#22810
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add() is called on every send() via KafkaProducer.waitOnMetadata, so the synchronized method contends with the Sender thread's update()/ retainTopic() calls on every produced record even though the vast majority of calls are just refreshing an already-tracked topic's expiry. Switch the topics map to a ConcurrentHashMap and refresh known topics via a lock-free replace(), which only writes if the topic is still present. For a topic seen for the first time (or one concurrently evicted by retainTopic()), fall back to a synchronized block so the map insert and newTopics bookkeeping happen atomically with retainTopic() - otherwise retainTopic() could expire-and-remove the topic in the gap between the two, leaving it recorded in newTopics without the corresponding map entry, or silently re-inserting an evicted topic without the newTopics bookkeeping needed to trigger an immediate refresh. retainTopic() switches to a conditional remove(topic, expireMs) so a concurrent refresh of an existing topic in add() can't be undone by an eviction decision based on the expiry value read just before the refresh landed. The batch add(Collection<String>, long) overload is unaffected: it stays fully synchronized, remains mutually exclusive with retainTopic(), and only races benignly with the lock-free path (a last-write-wins refresh of the same timestamp semantics). It's used only by KafkaProducer#sendOffsetsToTransaction, not the per-record send() path, so it doesn't need the same treatment. Adds ProducerMetadataTest#testRetainTopic for direct coverage of retainTopic()'s branches, and a JMH benchmark (ProducerMetadataAddBenchmark) comparing add() against a baseline mirroring the prior fully-synchronized implementation. With 8 threads concurrently refreshing a shared pool of 200 already-tracked topics: Benchmark Mode Cnt Score Error Units ProducerMetadataAddBenchmark.addExistingTopicFullySynchronized thrpt 5 6217.287 ± 1279.054 ops/ms ProducerMetadataAddBenchmark.addExistingTopicLockFreeHotPath thrpt 5 27232.344 ± 5404.836 ops/ms ~4.4x higher throughput under this contention pattern.
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Summary
add() is called on every send() via KafkaProducer.waitOnMetadata, so the synchronized method contends with the Sender thread's update()/ retainTopic() calls on every produced record even though the vast majority of calls are just refreshing an already-tracked topic's expiry.
Switch the topics map to a ConcurrentHashMap and refresh known topics via a lock-free replace(), which only writes if the topic is still present. For a topic seen for the first time (or one concurrently evicted by retainTopic()), fall back to a synchronized block so the map insert and newTopics bookkeeping happen atomically with retainTopic() - otherwise retainTopic() could expire-and-remove the topic in the gap between the two, leaving it recorded in newTopics without the corresponding map entry, or silently re-inserting an evicted topic without the newTopics bookkeeping needed to trigger an immediate refresh.
retainTopic() switches to a conditional remove(topic, expireMs) so a concurrent refresh of an existing topic in add() can't be undone by an eviction decision based on the expiry value read just before the refresh landed.
Testing
Adds ProducerMetadataTest#testRetainTopic for direct coverage of retainTopic()'s branches, and a JMH benchmark
(ProducerMetadataAddBenchmark) comparing add() against a baseline mirroring the prior fully-synchronized implementation. With 8 threads concurrently refreshing a shared pool of 200 already-tracked topics:
Benchmark Mode Cnt Score Error Units
ProducerMetadataAddBenchmark.addExistingTopicFullySynchronized thrpt 5 6217.287 ± 1279.054 ops/ms
ProducerMetadataAddBenchmark.addExistingTopicLockFreeHotPath thrpt 5 27232.344 ± 5404.836 ops/ms
~4.4x higher throughput under this contention pattern.