Scaling Checkout for Black Friday

Shopify's checkout service faced recurring degradation during BFCM peaks. The previous year saw 4 incidents totalling 38 minutes of degraded service, with an estimated $18M in lost merchant sales. Root cause was Redis memory exhaustion and database deadlocks under concurrent inventory reservation.

Re-architect checkout session management and inventory reservation to handle 500k concurrent sessions without Redis memory exhaustion, database deadlocks, or data inconsistencies.

Introduced a two-tier caching layer (Redis Cluster for hot sessions, PostgreSQL for durability). Replaced synchronous inventory locks with optimistic concurrency and a Sidekiq-based reservation queue. Ran load tests at 150%, 200%, and 300% of projected peak. Coordinated dark-launch with 5% of merchants for 3 weeks pre-BFCM.

BFCM processed 6.3M checkouts in 24 hours — a 40% YoY increase — with zero downtime incidents. Redis memory usage peaked below 65% at maximum load. Inventory accuracy remained 100%.