Write-Behind

1. The Incident

Product asks for a small feature: a view counter on every product page. "12,483 people viewed this today." One line of UI. You ship it the obvious way. Every page view runs UPDATE products SET views = views + 1.

At peak, the bestseller gets 12,000 views per second. That now means 12,000 UPDATEs per second, all on the same row. Databases protect a row being written with a lock. So those updates form a queue, one behind another, each waiting for the last.

Latency on that update climbs past a second. Connections pile up waiting for their turn