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+---
+stage: Data Stores
+group: Database
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Verifying Database Capabilities
+
+Sometimes certain bits of code may only work on a certain database
+version. While we try to avoid such code as much as possible sometimes it is
+necessary to add database (version) specific behavior.
+
+To facilitate this we have the following methods that you can use:
+
+- `ApplicationRecord.database.version`: returns the PostgreSQL version number as a string
+  in the format `X.Y.Z`.
+
+This allows you to write code such as:
+
+```ruby
+if ApplicationRecord.database.version.to_f >= 11.7
+  run_really_fast_query
+else
+  run_fast_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`).