Choosing a Consistency Model Without Sacrificing All Latency
Two weeks before Black Friday, a developer on the checkout crew added a cached reserve count. The read path used eventual consistency — maybe six seconds stale — because the latency SLA was 99th percentile under 50 ms. Orders started failing mid-transaction. The database was consistent; the cache wasn't. That is the kind of bug that makes people swear off eventual consistency forever. But here is the thing: you can choose a model without sacrificing all latency. You just have to know where each tradeoff actually lands. This article is a site guide for units running two or more database — the polyglot reality that most mid-sized companies live in. We cover the consistency models that survive traffic, the ones that quietly break, and the maintenance overheads nobody budgets for. No ivory tower diagrams. Just manufacturing scars.