GitHub migrated their entire database fleet with zero downtime. The secret: lots of time testing and careful work.
Between testing, performance tuning, and resolving identified issues, the overall upgrade process took over a year and involved engineers from multiple teams at GitHub. We upgraded our entire fleet to MySQL 8.0 – including staging clusters, production clusters in support of GitHub.com, and instances in support of internal tools. This upgrade highlighted the importance of our observability platform, testing plan, and rollback capabilities. The testing and gradual rollout strategy allowed us to identify problems early and reduce the likelihood for encountering new failure modes for the primary upgrade.