CURRENT-OFFSET and LAG of kafka consumer group that has no active members

Question

How are these two set? Behaviour that I observe with kafka-consumer-groups.sh is that when new message is appended to a certain partition, it increments at first its LOG-END-OFFSET and LAG columns, and after some time, CURRENT-OFFSET column gets incremented and LAG column gets decremented, although no offset was actually commited by any consumer, as there are no active consumers. Am I right, and is this always happening with consumer groups that have no active members, or is there a possibility to turn off the second stage, that simulates commiting offsets by non-existing consumers? This is actually confusing, you have to take into account the information that there are no active members in a consumer group, in order to have the right perspective of what the CURRENT-OFFSET and LAG columns actually mean (not much in that case).

OK, it seems that the consumer actually does continuously connect and poll the messages and commits the offsets, but in a volatile fashion (disconnecting each time) so that kafka-consumer-groups.sh always reports as if there are no active members in a group. This is a flink job that acts this way. Is that possible?

Answer 1

If the retention policy kicks in, and deletes old messages, the lag could decrease (if published logs are less than deleted ones), since the CURRENT-OFFSET positions itself at the earliest avaliable log.

I'd check what's the retention policy for your topic, since this may be due to deleted messages: The lag doesn't care about purgued messages, only active ones.

Answer 2

This has nothing to do with connecting to and disconnecting from the kafka cluster, that would be way to slow and ineffective. It has to do with the way that flink kafka consumer is implemented, which is described here: Flink Kafka Connector

The committed offsets are only a means to expose the consumer's progress for monitoring purposes.

What it basically does, it does not subscribe to topics as standard consumers that use consumer groups and their standard coordinators and leader mechanisms, but it directly assigns partitions, and only commits offsets to a consumer group for monitoring purposes, although it has methods of using these offsets for continuation too, see here , but anyway, that is why these groups appear to kafka as non having active members, and still getting offsets commited.