What is consumer lag?

Consumer lag shows how far a consumer group is behind the end of a partition: the partition's log end offset (LOG-END-OFFSET in the kafka-consumer-groups.sh output) minus the group's current committed offset (CURRENT-OFFSET), measured in records, not time. A lag of 0 means the group has consumed everything available. A steadily growing lag means the consumer is reading more slowly than records arrive, which can point to a processing bottleneck. The end offset a consumer sees is the high watermark (or the last stable offset for read_committed), so records that are not yet replicated are not counted. An application can also read its own lag with currentLag() on the consumer.

Expected key concepts: lag, LOG-END-OFFSET, CURRENT-OFFSET, committed offset, records, consumed everything available, growing, bottleneck, high watermark, currentLag