How a policy holding every mechanism from the semester still fails, and why the last question of the course is a product one.
24 May 2027
Every mechanism this semester has a limit that is a fact about the mechanism rather than a flaw in it. An idempotency key needs storage and a key the client remembers. Backoff adds latency to requests that were about to succeed. Jitter removes any single client’s ability to state its own worst-case wait. A shared budget refuses a layer the attempt it would have recovered on. A breaker fails calls that would have worked. A deadline discards work in flight. Each of those limits can be measured, and none of them tells you whether a given service should retry. That is where the semester ends: engineering can state both numbers precisely — 400 ms added to the 99th percentile, one order in a thousand that stops failing — and cannot choose between them, because the choice depends on what the product is for.
Outline#
- the policy, read cold, with the outcome withheld
- checking each number in it against the others
- what the case against retrying is actually asking for
Teaching team
- Idris Fenn — tutor