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

Class Public methods

new()

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 75
      def initialize
        # These caches are keyed by pool_config.connection_specification_name (PoolConfig#connection_specification_name).
        @owner_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
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Instance Public methods

active_connections?(role = ActiveRecord::Base.current_role)

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

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 161
      def active_connections?(role = ActiveRecord::Base.current_role)
        connection_pool_list(role).any?(&:active_connection?)
      end
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all_connection_pools()

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 117
      def all_connection_pools
        owner_to_pool_manager.values.flat_map { |m| m.pool_configs.map(&:pool) }
      end
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clear_active_connections!(role = ActiveRecord::Base.current_role)

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 code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 168
      def clear_active_connections!(role = ActiveRecord::Base.current_role)
        connection_pool_list(role).each(&:release_connection)
      end
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clear_all_connections!(role = ActiveRecord::Base.current_role)

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 179
      def clear_all_connections!(role = ActiveRecord::Base.current_role)
        connection_pool_list(role).each(&:disconnect!)
      end
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clear_reloadable_connections!(role = ActiveRecord::Base.current_role)

Clears the cache which maps classes.

See ConnectionPool#clear_reloadable_connections! for details.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 175
      def clear_reloadable_connections!(role = ActiveRecord::Base.current_role)
        connection_pool_list(role).each(&:clear_reloadable_connections!)
      end
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connected?(spec_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 code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 216
      def connected?(spec_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)
        pool = retrieve_connection_pool(spec_name, role: role, shard: shard)
        pool && pool.connected?
      end
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connection_pool_list(role = ActiveRecord::Base.current_role)

Also aliased as: connection_pools
📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 121
      def connection_pool_list(role = ActiveRecord::Base.current_role)
        owner_to_pool_manager.values.flat_map { |m| m.pool_configs(role).map(&:pool) }
      end
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connection_pools(role = ActiveRecord::Base.current_role)

Alias for: connection_pool_list

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

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 126
      def establish_connection(config, owner_name: Base, role: ActiveRecord::Base.current_role, shard: Base.current_shard)
        owner_name = StringConnectionOwner.new(config.to_s) if config.is_a?(Symbol)

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

        # Protects the connection named `ActiveRecord::Base` from being removed
        # if the user calls `establish_connection :primary`.
        if owner_to_pool_manager.key?(pool_config.connection_specification_name)
          remove_connection_pool(pool_config.connection_specification_name, role: role, shard: shard)
        end

        message_bus = ActiveSupport::Notifications.instrumenter
        payload = {}
        if pool_config
          payload[:spec_name] = pool_config.connection_specification_name
          payload[:shard] = shard
          payload[:config] = db_config.configuration_hash
        end

        if ActiveRecord.legacy_connection_handling
          owner_to_pool_manager[pool_config.connection_specification_name] ||= LegacyPoolManager.new
        else
          owner_to_pool_manager[pool_config.connection_specification_name] ||= PoolManager.new
        end
        pool_manager = get_pool_manager(pool_config.connection_specification_name)
        pool_manager.set_pool_config(role, shard, pool_config)

        message_bus.instrument("!connection.active_record", payload) do
          pool_config.pool
        end
      end
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flush_idle_connections!(role = ActiveRecord::Base.current_role)

Disconnects all currently idle connections.

See ConnectionPool#flush! for details.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 186
      def flush_idle_connections!(role = ActiveRecord::Base.current_role)
        connection_pool_list(role).each(&:flush!)
      end
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remove_connection_pool(owner, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 221
      def remove_connection_pool(owner, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)
        if pool_manager = get_pool_manager(owner)
          pool_config = pool_manager.remove_pool_config(role, shard)

          if pool_config
            pool_config.disconnect!
            pool_config.db_config
          end
        end
      end
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retrieve_connection_pool(owner, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)

Retrieving the connection pool happens a lot, so we cache it in @owner_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 code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 235
      def retrieve_connection_pool(owner, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)
        pool_config = get_pool_manager(owner)&.get_pool_config(role, shard)
        pool_config&.pool
      end
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while_preventing_writes(enabled = true)

Prevent writing to the database regardless of role.

In some cases you may want to prevent writes to the database even if you are on a database that can write. while_preventing_writes will prevent writes to the database for the duration of the block.

This method does not provide the same protection as a readonly user and is meant to be a safeguard against accidental writes.

See READ_QUERY for the queries that are blocked by this method.

📝 Source code 
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 102
      def while_preventing_writes(enabled = true)
        unless ActiveRecord.legacy_connection_handling
          raise NotImplementedError, "`while_preventing_writes` is only available on the connection_handler with legacy_connection_handling"
        end

        original, self.prevent_writes = self.prevent_writes, enabled
        yield
      ensure
        self.prevent_writes = original
      end
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