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gitlab-org--gitlab-foss/doc/development/verifying_database_capabilities.md
Toon Claes d13669716a Create idea of read-only database 
In GitLab EE, a GitLab instance can be read-only (e.g. when it's a Geo
secondary node). But in GitLab CE it also might be useful to have the
"read-only" idea around. So port it back to GitLab CE.

Also having the principle of read-only in GitLab CE would hopefully
lead to less errors introduced, doing write operations when there
aren't allowed for read-only calls.

Closes gitlab-org/gitlab-ce#37534.
2017-10-06 22:37:40 +02:00

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Verifying Database Capabilities

Sometimes certain bits of code may only work on a certain database and/or version. While we try to avoid such code as much as possible sometimes it is necessary to add database (version) specific behaviour.

To facilitate this we have the following methods that you can use:

  • Gitlab::Database.postgresql?: returns true if PostgreSQL is being used
  • Gitlab::Database.mysql?: returns true if MySQL is being used
  • Gitlab::Database.version: returns the PostgreSQL version number as a string in the format X.Y.Z. This method does not work for MySQL

This allows you to write code such as:

if Gitlab::Database.postgresql?
  if Gitlab::Database.version.to_f >= 9.6
    run_really_fast_query
  else
    run_fast_query
  end
else
  run_query
end

Read-only database

The database can be used in read-only mode. In this case we have to make sure all GET requests don't attempt any write operations to the database. If one of those requests wants to write to the database, it needs to be wrapped in a Gitlab::Database.read_only? or Gitlab::Database.read_write? guard, to make sure it doesn't for read-only databases.

We have a Rails Middleware that filters any potentially writing operations (the CUD operations of CRUD) and prevent the user from trying to update the database and getting a 500 error (see Gitlab::Middleware::ReadOnly).