We Replaced Redis with MySQL SKIP LOCKED for Inventory Reservation — Oversells Went to Zero

For two years, our Sponsored Placements service booked limited ad inventory through Redis: a counter in Redis, a Redlock around the decrement, and a TTL key per hold.

It oversold. Not catastrophically — consistently. 40–60 double-booked placements a month, each one a manual refund and an apology email to an advertiser.

The root cause was never one bug. It was the architecture: two sources of truth that could not be made atomic with each other. The count lived in Redis; the ownership lived in SQL. No transaction spans both. The Redlock only ever protected the Redis half.

The one mental shift

SKIP LOCKED turns a contended table into a concurrent work queue. Instead of every request fighting over one counter, each request grabs different rows and ignores the ones someone else is holding.