How to read messages from kafka consumer group without consuming?
I'm managing a kafka queue using a common consumer group across multiple machines. Now I also need to show the current content of the queue. How do I read only those messages within the group which haven't been read, yet making those messages again readable by other consumers in the group which actually processes those messages. Any help would be appreciated.
Answer
In Kafka, the notion of "reading" messages from a topic and that of "consuming" them are the same thing. At a high level, the only thing that makes a "consumed" message unavailable to a consumer is that consumer setting its read offset to a value beyond that of the message in question. Thus, you can turn off the autocommit feature of your consumers and avoid committing offsets in cases where you'd like only to "read" but not to "consume".
A good proxy for getting "all messages which haven't been read" is to compare the latest committed offset to the highwater mark offset per partition. This provides a notion of "lag" that indicates how far behind a given consumer is in its consumption of a partition. The fetch_consumer_lag CLI function in pykafka is a good example of how to do this.