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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.

Methods
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Class Public methods
new()
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 834
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
Instance Public methods
active_connections?()

Returns true if there are any active connections among the connection pools that the ConnectionHandler is managing.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 870
def active_connections?
  connection_pool_list.any?(&:active_connection?)
end
clear_active_connections!()

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.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 877
def clear_active_connections!
  connection_pool_list.each(&:release_connection)
end
clear_all_connections!()
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 888
def clear_all_connections!
  connection_pool_list.each(&:disconnect!)
end
clear_reloadable_connections!()

Clears the cache which maps classes.

See ActiveRecord::ConnectionAdapters::ConnectionPool#clear_reloadable_connections! for details.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 884
def clear_reloadable_connections!
  connection_pool_list.each(&:clear_reloadable_connections!)
end
connected?(spec_name)

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 906
def connected?(spec_name)
  conn = retrieve_connection_pool(spec_name)
  conn && conn.connected?
end
connection_pool_list()
Also aliased as: connection_pools
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 841
def connection_pool_list
  owner_to_pool.values.compact
end
connection_pools()
establish_connection(config)
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 846
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_connection(spec_name)

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.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 915
def remove_connection(spec_name)
  if pool = owner_to_pool.delete(spec_name)
    pool.automatic_reconnect = false
    pool.disconnect!
    pool.spec.config
  end
end
retrieve_connection_pool(spec_name)

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 926
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