establish_connection

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1043
      def establish_connection(config, owner_name: Base, role: ActiveRecord::Base.current_role, shard: Base.current_shard)
        owner_name = config.to_s if config.is_a?(Symbol)

        pool_config = resolve_pool_config(config, owner_name)
        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::Base.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