establish_connection

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1042
      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