Github

• postmortem

Github platform encountered a novel failure mode when processing a schema migration on a large MySQL table. Schema migrations are a common task at GitHub and often take weeks to complete. The final step in a migration is to perform a rename to move the updated table into the correct place. During the final step of this migration a significant portion of our MySQL read replicas entered a semaphore deadlock. Our MySQL clusters consist of a primary node for write traffic, multiple read replicas for production traffic, and several replicas that serve internal read traffic for backup and analytics purposes. The read replicas that hit the deadlock entered a crash-recovery state causing an increased load on healthy read replicas. Due to the cascading nature of this scenario, there were not enough active read replicas to handle production requests which impacted the availability of core GitHub services.