Initializers are executed when the Rails process is started. That means that initializers are also executed during every deploy.

By default, Rails loads Zeitwerk after the initializers in config/initializers are loaded. Autoloading before Zeitwerk is loaded is now deprecated but because we use a lot of autoloaded constants in our initializers, we had to move the loading of Zeitwerk earlier than these initializers.

A side-effect of this is that in the initializers, config.autoload_paths is already frozen.

To run an initializer before Zeitwerk is loaded, you need put them in config/initializers_before_autoloader. Ruby files in this folder are loaded in alphabetical order just like the default Rails initializers.

Some examples where you would need to do this are:

1. Modifying Rails’ config.autoload_paths
2. Changing configuration that Zeitwerk uses, for example, inflections

Database connections in initializers

Ideally, database connections are not opened from Rails initializers. Opening a database connection (for example, checking the database exists, or making a database query) from an initializer means that tasks like db:drop, and db:test:prepare will fail because an active session prevents the database from being dropped.

To enforce this:

1. We run the clear_active_connections_again initializer to ensure no database connections remain active after initialization.
2. We do not allow database queries in routes. Initializers should be static and fast; issuing queries slows down boot time and can cause subtle failures.

If a database query is made while loading routes, a warning is printed to STDOUT with the query and a backtrace, for example:

InitializerConnections Query: SELECT "projects".* FROM "projects" WHERE "projects"."id" = 1 LIMIT 1
InitializerConnections Backtrace: config/routes.rb:15:in `block (2 levels) in <main>'
InitializerConnections Backtrace: config/routes.rb:9:in `block in <main>'
InitializerConnections Backtrace: lib/initializer_connections.rb:18:in `warn_if_database_connection'
InitializerConnections Backtrace: config/routes.rb:6:in `<main>'
See https://docs.gitlab.com/ee/development/rails_initializers.html#database-connections-in-initializers