Section checkpoint

Reliability

8 questions drawn from the 4 chapters in this section. If most land on the first try, you are ready for the next section. If not, the explanation after each answer says which idea to revisit.

Checkpoint

8 questions · first-try answers earn the most XP

1. Why does the server need an atomic 'claim' on the idempotency key before doing the real work?

2. Why is this pattern called 'exactly once' in quotes rather than literally?

3. Which operations are safe to retry automatically after a timeout?

4. What does a bulkhead pattern isolate, and why?

5. Why should traces usually be sampled rather than recorded for every request?

6. What does a correlation id let you do across a wall of logs?

7. During a failover, replication lag forces you to choose between what two options?

8. Why does the chapter insist that an untested failover does not really exist?

What this checkpoint covers

Idempotency & Exactly-Once Illusion
Why retries double-charge you, and the one table that stops it.
Failures, Retries & Rate Limits
Timeouts, backoff, circuit breaker story.
Observability: Logs, Metrics, Traces
Three questions you must answer at 3am, and which tool answers which.
Multi-Region & Disaster Recovery
RPO, RTO, active-active vs active-passive, and what a region outage really costs.

Put it to work in a drill

A slow fraud check joins the consumer
Senior
One bad record keeps retrying a whole batch
Senior
Checkout fails when analytics is down
Beginner
Some orders never reach the warehouse
Intermediate
Orders span payment, restaurant and courier
Senior

Next section: Distributed Systems