Cache-Aside

1. The Incident

You spent the last module scaling the database: replicas, shards, projections. It worked. And now look at what all that machinery is actually doing all day.

The product page for item 8412 (this month's bestseller) is read 4,000 times a minute. The product's row hasn't changed in three weeks. Your database, replicated and sharded and carefully tuned, is answering the same question four thousand times a minute, computing the same 15 ms answer, byte-for-byte identical, around the clock