Committed Offset and Consumer Lag

A consumer’s position is where it will fetch next. A committed offset is the group’s saved restart position for a partition. After processing offset 49 durably, committing 50 says to resume at 50. Saving an offset does not itself move the current reader or prove that external output succeeded.

If offsets 50 and 52 finish while 51 remains unfinished, the safe completed prefix ends before 51. Committing 53 would skip 51 after restart. Track completion per partition and make replay safe for output that completed before its offset was committed.

Committed-offset lag compares an end boundary with saved progress. Specify the boundary and inspect each partition. Offset distance may include gaps from compaction or transaction records, so it is not always a readable-record count or a time delay. Also monitor oldest unfinished work, errors, and whether retention has already removed required input.

Reference: Kafka fundamentals lab.