Dual writes: 2 real scaling case studies
2 of the 73 engineering write-ups in our database are tagged with the technique "dual writes". They come from 2 companies (Shopify and Stripe) and were published between 2017 and 2026. 1 of the 2 state concrete scale numbers, for example "$5.1 million sales/minute (Black Friday 2025 peak)" (Shopify). Each card below links to our short summary and the original post.
We replaced Redis with MySQL for inventory reservations—and it scaled
Shopify moved checkout inventory reservations from Redis to MySQL using one row per sellable unit, a bounded pool of 1,000 rows per item/location and SELECT ... FOR UPDATE SKIP LOCKED. The real ceiling was connections held by other checkout code: tagging SQL by business process and measuring hold time in ProxySQL found it, and the cleanup removed 50% of reads and 33% of transactions on the primary.
Online migrations at scale
Stripe's four-step dual-write playbook for migrating data models without downtime.