What is consumer lag?

Consumer lag is the difference between the partition's Log End Offset (LEO) and the consumer's committed offset. Lag=0 means the consumer is fully caught up. Growing lag indicates the consumer is processing slower than messages are being produced. Consumer lag is the most critical health metric for consumer applications. Alert on lag that continuously grows — it indicates a consumer bottleneck that will eventually cause memory/processing backlog.

Expected key concepts: lag, LEO, committed offset, caught up, growing, bottleneck, health metric, alert