Two things quietly decide whether a small application survives being used: whether its reads are indexed, and whether its writes can be repeated safely. They are usually taught as separate subjects - one belongs to databases, the other to API design. They are the same subject seen twice. An index is how reads survive scale. Idempotency is how writes survive retries. Every example below is from the application that serves this page, including a failure that happened while it was being writ...