Cache-Aside

1. The Incident

You spent the previous module scaling the database with replicas, shards, and projections, and those changes worked. Now consider what all that infrastructure actually does throughout the day.

Item 8412 is this month's bestseller, so customers read its product page 4,000 times each minute. Although its database row has not changed in three weeks, the replicated, sharded, and carefully tuned database still computes the same 15 ms answer for every request