How we migrated the database behind every Vercel build

This post details the migration of Vercel's build warm pool state from Redis to DynamoDB. The migration was performed live, in phases, using feature flags and shadow writes/reads to ensure data consistency and minimal downtime. The core challenge was adapting to DynamoDB's higher latency compared to Redis, which required redesigning the supply loop to handle slower reads by overlapping operations and accepting a small degree of stale state. The schema was redesigned around container IDs and access patterns, incorporating time-aware indexes for status counts and a separate table for billing mappings. The migration process involved rigorous monitoring and phased rollouts, with two failures encountered and addressed during the process.