m1a2st commented on PR #17263:
URL: https://github.com/apache/kafka/pull/17263#issuecomment-2379334067

   The main reason is that when a consumer needs to close, the 
`ConsumerCoordinator` must also close. Therefore, when the 
`ConsumerCoordinator` attempts to close, it calls the `commitOffsetsSync` [1] 
method. Inside this method, it first invokes `ensureCoordinatorReady` [2], 
which sends a `FindCoordinatorRequest` [3] to request server information. The 
topic requested here is `__consumer_offsets`. When the server receives this 
request, it uses `metadataCache.getTopicMetadata `[4] to look up the topic. 
However, since the` __consumer_offsets` topic is not present, the server 
returns an error code `COORDINATOR_NOT_AVAILABLE`, causing the process to 
continue until it times out.
   [1]
   
https://github.com/apache/kafka/blob/7c429f3514dd50e98f42c74595c35b5f0b32b7d7/clients/src/main/java/org/apache/kafka/clients/consumer/internals/ConsumerCoordinator.java#L1144
   [2] 
   
https://github.com/apache/kafka/blob/7c429f3514dd50e98f42c74595c35b5f0b32b7d7/clients/src/main/java/org/apache/kafka/clients/consumer/internals/AbstractCoordinator.java#L278
   [3] 
   
https://github.com/apache/kafka/blob/7c429f3514dd50e98f42c74595c35b5f0b32b7d7/clients/src/main/java/org/apache/kafka/clients/consumer/internals/AbstractCoordinator.java#L904
   [4] 
https://github.com/apache/kafka/blob/7c429f3514dd50e98f42c74595c35b5f0b32b7d7/core/src/main/scala/kafka/server/KafkaApis.scala#L1661


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