ConnectionHandler is a collection of ConnectionPool objects. It is used for keeping separate connection pools that connect to different databases.
For example, suppose that you have 5 models, with the following hierarchy:
class Author < ActiveRecord::Base
end
class BankAccount < ActiveRecord::Base
end
class Book < ActiveRecord::Base
establish_connection "library_db"
end
class ScaryBook < Book
end
class GoodBook < Book
end
And a database.yml that looked like this:
development:
database: my_application
host: localhost
library_db:
database: library
host: some.library.org
Your primary database in the development environment is “my_application” but the Book model connects to a separate database called “library_db” (this can even be a database on a different machine).
Book, ScaryBook and GoodBook will all use the same connection pool to “library_db” while Author, BankAccount, and any other models you create will use the default connection pool to “my_application”.
The various connection pools are managed by a single instance of ConnectionHandler accessible via ActiveRecord::Core.connection_handler. All Active Record models use this handler to determine the connection pool that they should use.
The ConnectionHandler class is not coupled with the Active models, as it has no knowlodge about the model. The model needs to pass a specification name to the handler, in order to lookup the correct connection pool.
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# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 866 def initialize # These caches are keyed by spec.name (ConnectionSpecification#name). @owner_to_pool = Concurrent::Map.new(initial_capacity: 2) do |h, k| h[k] = Concurrent::Map.new(initial_capacity: 2) end end
Returns true if there are any active connections among the connection pools that the ConnectionHandler is managing.
Returns any connections in use by the current thread back to the pool, and also returns connections to the pool cached by threads that are no longer alive.
Clears the cache which maps classes.
See ActiveRecord::ConnectionAdapters::ConnectionPool#clear_reloadable_connections! for details.
Returns true if a connection that's accessible to this class has already been opened.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 878 def establish_connection(config) resolver = ConnectionSpecification::Resolver.new(Base.configurations) spec = resolver.spec(config) remove_connection(spec.name) message_bus = ActiveSupport::Notifications.instrumenter payload = { connection_id: object_id } if spec payload[:spec_name] = spec.name payload[:config] = spec.config end message_bus.instrument("!connection.active_record", payload) do owner_to_pool[spec.name] = ConnectionAdapters::ConnectionPool.new(spec) end owner_to_pool[spec.name] end
Remove the connection for this class. This will close the active connection and the defined connection (if they exist). The result can be used as an argument for establish_connection, for easily re-establishing the connection.
Retrieving the connection pool happens a lot, so we cache it in @owner_to_pool. This makes retrieving the connection pool O(1) once the process is warm. When a connection is established or removed, we invalidate the cache.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 958 def retrieve_connection_pool(spec_name) owner_to_pool.fetch(spec_name) do # Check if a connection was previously established in an ancestor process, # which may have been forked. if ancestor_pool = pool_from_any_process_for(spec_name) # A connection was established in an ancestor process that must have # subsequently forked. We can't reuse the connection, but we can copy # the specification and establish a new connection with it. establish_connection(ancestor_pool.spec.to_hash).tap do |pool| pool.schema_cache = ancestor_pool.schema_cache if ancestor_pool.schema_cache end else owner_to_pool[spec_name] = nil end end end