kafka

There is no status code and no caller waiting, so latency and error rate stop meaning what they meant. The signals that replace them are age, lag and completion, and they answer a question the request-response set never had to ask.

More partitions should mean more throughput, so why did the backlog deepen? Because rebalance behavior depends on the group protocol, and the way consumers divide work decides who pays for it.

One partition has one consumer, by design. If the key distribution sends a third of the traffic to one partition, the parallelism you bought is unavailable to the work that needs it.

Trace a retry through the broker, database and external provider. The guarantee changes at each boundary, and a recorded intent does not prove a payment or email completed.

A published event is an API with no version negotiation and consumers you cannot enumerate. The design is a registry that refuses an incompatible change at publish time, because after publication your options are all bad.