intermediate

Consumer

Design consumers for idempotency, acknowledgement timing, concurrency, backpressure, poison-message handling, and observable lag.

Consumers pull or receive messages, deserialize, execute handlers, and acknowledge or commit offsets only after safe processing. Design for idempotency, controlled concurrency, backpressure, poison-message quarantine, and observable consumer lag.

Trade-off: higher concurrency speeds processing but may break per-key ordering unless partitioned correctly.

On interviews: when to ack before versus after processing; handle a message that always throws.

Common pitfalls: unbounded parallel handlers on ordered stream; commit offset before DB transaction commits; no lag alerts.

Checklist:

Idempotent handlers with inbox or natural keys.
Match concurrency to ordering guarantees.
Commit offsets after durable side effects.
Alert on growing lag and DLQ volume.