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. Search the codebase for Gitlab::Database.main.read_only?.
• the read-only middleware, where HTTP requests that cause database writes are blocked, unless explicitly allowed.
• 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.