gitlab-org--gitlab-foss/doc/development/maintenance_mode.md

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Internal workings of GitLab Maintenance Mode (PREMIUM SELF)

Where is Maintenance Mode enforced?

GitLab Maintenance Mode only blocks writes from HTTP and SSH requests at the application level in a few key places within the rails application. Search the codebase for maintenance_mode?.

• the read-only database method, which toggles special behavior when we are not allowed to write to the database. We use this method for possible places where writes could occur in GET requests. Search the codebase for Gitlab::Database.read_only?.
• the read-only middleware, where HTTP requests that cause database writes are blocked, unless explicitly allowed (for example, GET requests).
• Git push access via SSH is denied by returning 401 when gitlab-shell POSTs to /internal/allowed to check if access is allowed.
• Container registry authentication service, where updates to the container registry are blocked.

The database itself is not in read-only mode (except in a Geo secondary site) and can be written by sources other than the ones blocked.