IIRC Gitlab uses Redis as well, it get used by CI for the streaming logs while a job is still running before it gets tossed up to object storage. Their statement that they only used a single cluster up to this point is a bit dated as well, I believe they've had a second cluster for nearly a year now for their new docker registry implementation. This new cluster for CI is more like a third.
You're correct. We also have Redis for a few things: Rails caching, Sidekiq jobs, session data, CI, and a few other things. You can learn more about GitLab.com's infrastructure architecture here [1] and more about what Redis is used for here [2].
IIRC Gitlab uses Redis as well, it get used by CI for the streaming logs while a job is still running before it gets tossed up to object storage. Their statement that they only used a single cluster up to this point is a bit dated as well, I believe they've had a second cluster for nearly a year now for their new docker registry implementation. This new cluster for CI is more like a third.