Observability

Decide what to measure so a system can be seen, alerted on, and debugged in production. Getting this wrong is failure mode #6 — ignoring failure: a design that works on the whiteboard but goes dark under load, where the first signal of an outage is a user complaint instead of a page.

When to reach for this

Any production design needs an answer to "how would we know this broke, and how fast?" Reach for this when defining what the system measures, what pages a human, what an acceptable level of service is (SLO), or how a request is traced across services. It is the design move that makes every other block's stress section real — you cannot mitigate a thundering herd or a hot shard you can't see.