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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.

Methods
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Class Public methods

new()

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 77
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)

  # Backup finalizer: if the forked child skipped Kernel#fork the early discard has not occurred
  ObjectSpace.define_finalizer self, FINALIZER
end

Instance Public methods

active_connections?(role = nil)

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

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 173
def active_connections?(role = nil)
  if role.nil?
    deprecation_for_pool_handling(__method__)
    role = ActiveRecord::Base.current_role
  end

  each_connection_pool(role).any?(&:active_connection?)
end

all_connection_pools()

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 97
      def all_connection_pools
        ActiveRecord.deprecator.warn(<<-MSG.squish)
          The `all_connection_pools` method is deprecated in favor of `connection_pool_list`.
          Call `connection_pool_list(:all)` to get the same behavior as `all_connection_pools`.
        MSG
        connection_name_to_pool_manager.values.flat_map { |m| m.pool_configs.map(&:pool) }
      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.

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 185
def clear_active_connections!(role = nil)
  if role.nil?
    deprecation_for_pool_handling(__method__)
    role = ActiveRecord::Base.current_role
  end

  each_connection_pool(role).each(&:release_connection)
end

clear_all_connections!(role = nil)

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 206
def clear_all_connections!(role = nil)
  if role.nil?
    deprecation_for_pool_handling(__method__)
    role = ActiveRecord::Base.current_role
  end

  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.

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 197
def clear_reloadable_connections!(role = nil)
  if role.nil?
    deprecation_for_pool_handling(__method__)
    role = ActiveRecord::Base.current_role
  end

  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.

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 251
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.

Also aliased as: connection_pools

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 107
def connection_pool_list(role = nil)
  if role.nil?
    deprecation_for_pool_handling(__method__)
    role = ActiveRecord::Base.current_role
    connection_name_to_pool_manager.values.flat_map { |m| m.pool_configs(role).map(&:pool) }
  elsif 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

connection_pools(role = nil)

Alias for: connection_pool_list

establish_connection(config, owner_name: Base, role: Base.current_role, shard: Base.current_shard, clobber: false)

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 131
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_name)

  # 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_class != owner_name)
      existing_pool_config.connection_class = 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_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.

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 218
def flush_idle_connections!(role = nil)
  if role.nil?
    deprecation_for_pool_handling(__method__)
    role = ActiveRecord::Base.current_role
  end

  each_connection_pool(role).each(&:flush!)
end

remove_connection_pool(connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 256
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)

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.

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 265
def retrieve_connection_pool(connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)
  pool_config = get_pool_manager(connection_name)&.get_pool_config(role, shard)
  pool_config&.pool
end