Consumer Lag

Consumer lag is the most important metric for knowing whether a Consumer Group is keeping up with a topic's production rate. Calculated per partition as log end offset - committed offset, it represents how many messages have already been written but not yet processed. Zero lag means the consumer is caught up; lag that grows over time indicates the consumer is processing slower than the producer is writing — a sign you need to investigate processing throughput, increase parallelism (more partitions and consumers, Chapter 6), or optimize the consumer's code.