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https://issues.apache.org/jira/browse/KAFKA-19259?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17956545#comment-17956545
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Arpit Goyal commented on KAFKA-19259:
-------------------------------------
Hi [~lianetm]
Update on the above issue:
It seems I figured out the issue. It seems we are not sending immediate fetches
in the new consumer compared to the classic one. Check this logs , I added
debug logs in awaitNotEmpty function. Ideally the consumer should be making
multiple fetch requests during the 5-second poll period, especially since each
fetch request has maxWaitMs=500ms. Instead, it seems to be waiting the full
poll timeout after receiving just one empty response.
As a next step I will be checking part of the code which is preventing fetches.
{code:java}
[2025-06-06 13:07:26,720] DEBUG [Consumer clientId=console-consumer,
groupId=cg2] Entering awaitNotEmpty with timer remaining: 4949ms
(org.apache.kafka.clients.consumer.internals.FetchBuffer)
[2025-06-06 13:07:26,720] DEBUG [Consumer clientId=console-consumer,
groupId=cg2] Buffer is empty, will wait. Timer remaining: 4949ms, wokenup:
false (org.apache.kafka.clients.consumer.internals.FetchBuffer)
[2025-06-06 13:07:26,720] DEBUG [Consumer clientId=console-consumer,
groupId=cg2] Starting wait at 1749195446720 for 4949ms
(org.apache.kafka.clients.consumer.internals.FetchBuffer)
[2025-06-06 13:07:27,230] DEBUG [Consumer clientId=console-consumer,
groupId=cg2] Received FETCH response from node 1 for request with header
RequestHeader(apiKey=FETCH, apiVersion=17, clientId=console-consumer,
correlationId=44, headerVersion=2): FetchResponseData(throttleTimeMs=0,
errorCode=0, sessionId=963412284, responses=[], nodeEndpoints=[])
(org.apache.kafka.clients.NetworkClient)
[2025-06-06 13:07:27,325] DEBUG [Consumer clientId=console-consumer,
groupId=cg2] Node 1 sent an incremental fetch response with throttleTimeMs = 0
for session 963412284 with 0 response partition(s), 6 implied partition(s)
(org.apache.kafka.clients.FetchSessionHandler)
[2025-06-06 13:07:27,325] DEBUG [Consumer clientId=console-consumer,
groupId=cg2] Removing pending request for fetch session: 963412284 for node:
localhost:9092 (id: 1 rack: null isFenced: false)
(org.apache.kafka.clients.consumer.internals.AbstractFetch)
[2025-06-06 13:07:31,678] DEBUG [Consumer clientId=console-consumer,
groupId=cg2] Wait timed out after 4955ms
(org.apache.kafka.clients.consumer.internals.FetchBuffer)
{code}
> Async consumer fetch intermittent delays on console consumer
> ------------------------------------------------------------
>
> Key: KAFKA-19259
> URL: https://issues.apache.org/jira/browse/KAFKA-19259
> Project: Kafka
> Issue Type: Bug
> Components: clients, consumer
> Affects Versions: 4.0.0
> Reporter: Lianet Magrans
> Assignee: Arpit Goyal
> Priority: Major
> Fix For: 4.1.0
>
> Attachments: Screenshot 2025-05-31 at 10.44.29 PM.png,
> console-consumer-classic-vs-consumer.mov, consumer_KAFKA-19259.log, debug5.log
>
>
> We noticed that fetching with the kafka-console-consumer.sh tool using the
> new consumer shows some intermittent delays, that are not seen when running
> the same with the classic consumer. Note that I disabled auto-commit to
> isolate the delay, and from a first look seems to come from the
> fetchBuffer.awaitNonEmpty logic, that alternatively takes almost the full
> poll timeout (runs "fast", then "slow", and continues to alternate)
> [https://github.com/apache/kafka/blob/0b81d6c7802c1be55dc823ce51729f2c6a6071a7/clients/src/main/java/org/apache/kafka/clients/consumer/internals/AsyncKafkaConsumer.java#L1808]
>
> The difference in behaviour between the 2 consumers can be seen with this
> setup:
> * topic with 6 partitions (I tried with 1 partition first and didn't see the
> delay, then with 3 and 6 I could see it)
> * data populated in topic with producer sending generated uuids to the topic
> in while loop
> * run console consumer (asycn) no commit:
> bin/kafka-console-consumer.sh --topic t1 --bootstrap-server localhost:9092
> --consumer-property group.protocol=consumer --group cg1 --consumer-property
> enable.auto.commit=false
> Here we can notice the pattern that looks like batches, and custom logs on
> the awaitNonEmpty show it take the full poll timeout on alternate poll
> iterations.
> * run same but for classic consumer (consumer-property
> group.protocol=classic) -> not such pattern of intermittent delays
> Produce continuously (I used this)
> while sleep 1; do echo $(uuidgen); done | bin/kafka-console-producer.sh
> --bootstrap-server localhost:9092 --topic t1
> This needs more investigation to fully understand if it's indeed something in
> the fetch path or something else)
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