Your personal blog has cruised for years: a steady stream of readers fetching rendered posts, a trickle of edits from you. Then one evening a post hits the front page of a major aggregator, and for a single hour the read traffic is twenty times the quiet rate — the classic hug of death. The aggregator pays you nothing for downtime, and a reader who stares at a spinner for more than a couple of seconds is gone.
There are only two operations. Reading a post is as cacheable as traffic gets — the same rendered documents, fetched by post ID, tolerable up to a minute stale — so the question is not whether to cache but where the cache attaches. Its position decides which tiers stand behind it when the front-page hour arrives, and which of them face the flood the moment a freshly restarted cache remembers nothing. Editing a post is the opposite temperament: rare, small, and sacred.
The contract follows suit. An edit must be durable before it is acknowledged — an author who saw the save confirmed must find those words intact after any crash, a promise no cache's volatile memory can make. Readers, in exchange, may be served a post up to sixty seconds stale; that minute of leniency is the entire budget this design gets to spend.