Kafka consumer lag: the first alert to set up, and how to read it
Lag tells you whether consumers keep up. Alerting on its trend, not its size, catches stuck consumers before customers do.
Exactly-once delivery doesn't exist. Idempotency is how you fake it safely.
Retries, replays and redeliveries are normal in event pipelines. Here is the dedup-key pattern that keeps them from corrupting your numbers.