# class ActiveRecord::ConnectionAdapters::ConnectionHandler

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.

### Public Class Methods

new() click to toggle source

    # File 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

### Public Instance Methods

active_connections?() click to toggle source

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

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 902
    def active_connections?
      connection_pool_list.any?(&:active_connection?)
    end

clear_active_connections!() click to toggle source

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 lib/active_record/connection_adapters/abstract/connection_pool.rb, line 909
    def clear_active_connections!
      connection_pool_list.each(&:release_connection)
    end

clear_all_connections!() click to toggle source

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 920
    def clear_all_connections!
      connection_pool_list.each(&:disconnect!)
    end

clear_reloadable_connections!() click to toggle source

Clears the cache which maps classes.

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

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 916
    def clear_reloadable_connections!
      connection_pool_list.each(&:clear_reloadable_connections!)
    end

connected?(spec_name) click to toggle source

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

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 938
    def connected?(spec_name)
      conn = retrieve_connection_pool(spec_name)
      conn && conn.connected?
    end

connection_pool_list() click to toggle source

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 873
    def connection_pool_list
      owner_to_pool.values.compact
    end

Also aliased as: connection_pools
connection_pools()
Alias for: connection_pool_list
establish_connection(config) click to toggle source

    # File 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_connection(spec_name) click to toggle source

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 lib/active_record/connection_adapters/abstract/connection_pool.rb, line 947
    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) click to toggle source

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 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

### Private Instance Methods

owner_to_pool() click to toggle source

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 977
    def owner_to_pool
      @owner_to_pool[Process.pid]
    end

pool_from_any_process_for(spec_name) click to toggle source

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 981
    def pool_from_any_process_for(spec_name)
      owner_to_pool = @owner_to_pool.values.reverse.find { |v| v[spec_name] }
      owner_to_pool && owner_to_pool[spec_name]
    end