class ActiveRecord::ConnectionAdapters::ConnectionHandler

Active Record Connection Handler ↑

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::Base.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 knowledge about the model. The model needs to pass a connection specification name to the handler, in order to look up the correct connection pool.

Public Class Methods

new () 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 76
def initialize
  # These caches are keyed by pool_config.connection_name (PoolConfig#connection_name).
  @connection_name_to_pool_manager = Concurrent::Map.new(initial_capacity: 2)
end

Public Instance Methods

active_connections? (role = nil)

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_handler.rb, line 157
def active_connections?(role = nil)
  each_connection_pool(role).any?(&:active_connection?)
end
clear_active_connections! (role = nil)

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_handler.rb, line 164
def clear_active_connections!(role = nil)
  each_connection_pool(role).each do |pool|
    pool.release_connection
    pool.disable_query_cache!
  end
end
clear_all_connections! (role = nil) 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 178
def clear_all_connections!(role = nil)
  each_connection_pool(role).each(&:disconnect!)
end
clear_reloadable_connections! (role = nil)

Clears the cache which maps classes.

See ConnectionPool#clear_reloadable_connections! for details.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 174
def clear_reloadable_connections!(role = nil)
  each_connection_pool(role).each(&:clear_reloadable_connections!)
end
connected? (connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)

Returns true if a connection that’s accessible to this class has already been opened.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 200
def connected?(connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)
  pool = retrieve_connection_pool(connection_name, role: role, shard: shard)
  pool && pool.connected?
end
connection_pool_list (role = nil)

Returns the pools for a connection handler and given role. If :all is passed, all pools belonging to the connection handler will be returned.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 95
def connection_pool_list(role = nil)
  if role.nil? || role == :all
    connection_name_to_pool_manager.values.flat_map { |m| m.pool_configs.map(&:pool) }
  else
    connection_name_to_pool_manager.values.flat_map { |m| m.pool_configs(role).map(&:pool) }
  end
end
Also aliased as: connection_pools
establish_connection (config, owner_name: Base, role: Base.current_role, shard: Base.current_shard, clobber: false) 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 115
def establish_connection(config, owner_name: Base, role: Base.current_role, shard: Base.current_shard, clobber: false)
  owner_name = determine_owner_name(owner_name, config)

  pool_config = resolve_pool_config(config, owner_name, role, shard)
  db_config = pool_config.db_config

  pool_manager = set_pool_manager(pool_config.connection_descriptor)

  # If there is an existing pool with the same values as the pool_config
  # don't remove the connection. Connections should only be removed if we are
  # establishing a connection on a class that is already connected to a different
  # configuration.
  existing_pool_config = pool_manager.get_pool_config(role, shard)

  if !clobber && existing_pool_config && existing_pool_config.db_config == db_config
    # Update the pool_config's connection class if it differs. This is used
    # for ensuring that ActiveRecord::Base and the primary_abstract_class use
    # the same pool. Without this granular swapping will not work correctly.
    if owner_name.primary_class? && (existing_pool_config.connection_descriptor != owner_name)
      existing_pool_config.connection_descriptor = owner_name
    end

    existing_pool_config.pool
  else
    disconnect_pool_from_pool_manager(pool_manager, role, shard)
    pool_manager.set_pool_config(role, shard, pool_config)

    payload = {
      connection_name: pool_config.connection_descriptor.name,
      role: role,
      shard: shard,
      config: db_config.configuration_hash
    }

    ActiveSupport::Notifications.instrumenter.instrument("!connection.active_record", payload) do
      pool_config.pool
    end
  end
end
flush_idle_connections! (role = nil)

Disconnects all currently idle connections.

See ConnectionPool#flush! for details.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 185
def flush_idle_connections!(role = nil)
  each_connection_pool(role).each(&:flush!)
end
remove_connection_pool (connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard) 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 205
def remove_connection_pool(connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)
  if pool_manager = get_pool_manager(connection_name)
    disconnect_pool_from_pool_manager(pool_manager, role, shard)
  end
end
retrieve_connection_pool (connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard, strict: false)

Retrieving the connection pool happens a lot, so we cache it in @connection_name_to_pool_manager. 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_handler.rb, line 214
def retrieve_connection_pool(connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard, strict: false)
  pool_manager = get_pool_manager(connection_name)
  pool = pool_manager&.get_pool_config(role, shard)&.pool

  if strict && !pool
    selector = [
      ("'#{shard}' shard" unless shard == ActiveRecord::Base.default_shard),
      ("'#{role}' role" unless role == ActiveRecord::Base.default_role),
    ].compact.join(" and ")

    selector = [
      (connection_name unless connection_name == "ActiveRecord::Base"),
      selector.presence,
    ].compact.join(" with ")

    selector = " for #{selector}" if selector.present?

    message = "No database connection defined#{selector}."

    raise ConnectionNotDefined.new(message, connection_name: connection_name, shard: shard, role: role)
  end

  pool
end