At GitHub we use MySQL as our main datastore. While repository data lies in
git, metadata is stored in MySQL. This includes Issues, Pull Requests, Comments etc. We also auth against MySQL via a custom git proxy (babeld). To be able to serve under the high load GitHub operates at, we use MySQL replication to scale out read load.
We have different clusters to provide with different types of services, but the single-writer-multiple-readers design applies to them all. Depending on growth of traffic, on application demand, on operational tasks or other constraints, we take replicas in or out of our pools. Depending on workloads some replicas may lag more than others.
Displaying up-to-date data is important. We have tooling that helps us ensure we keep replication lag at a minimum, and typically it doesn’t exceed
1 second. However sometimes lags do happen, and when they do, we want to put aside those lagging replicas, let them catch their breath, and avoid sending traffic their way until they are caught up.
Read more at GitHub Engineering