Cache-Aside

1. The Incident

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

The product page for item 8412 is this month's bestseller. It is read 4,000 times a minute. The product's row has not changed in three weeks. Your database is replicated, sharded, and carefully tuned. It is answering the same question four thousand times a minute, around the clock. Each time it computes the same 15 ms answer, byte for byte identical. Across the whole catalog, reads run at 12,000 per second at peak