Connection-Level Client-Side API

Connection-Level Client-Side API

The connection-level API is the lowest-level client-side API Akka HTTP provides. It gives you full control over when HTTP connections are opened and closed and how requests are to be send across which connection. As such it offers the highest flexibility at the cost of providing the least convenience.

Opening HTTP Connections

With the connection-level API you open a new HTTP connection to a target endpoint by materializing a Flow returned by the Http().outgoingConnection(...) method. Here is an example:

import akka.http.scaladsl.Http
import akka.http.scaladsl.model._

import scala.concurrent.Future

implicit val system = ActorSystem()
implicit val materializer = ActorMaterializer()

val connectionFlow: Flow[HttpRequest, HttpResponse, Future[Http.OutgoingConnection]] =
val responseFuture: Future[HttpResponse] =
  Source.single(HttpRequest(uri = "/"))

Apart from the host name and port the Http().outgoingConnection(...) method also allows you to specify socket options and a number of configuration settings for the connection.

Note that no connection is attempted until the returned flow is actually materialized! If the flow is materialized several times then several independent connections will be opened (one per materialization). If the connection attempt fails, for whatever reason, the materialized flow will be immediately terminated with a respective exception.

Request-Response Cycle

Once the connection flow has been materialized it is ready to consume HttpRequest instances from the source it is attached to. Each request is sent across the connection and incoming responses dispatched to the downstream pipeline. Of course and as always, back-pressure is adequately maintained across all parts of the connection. This means that, if the downstream pipeline consuming the HTTP responses is slow, the request source will eventually be slowed down in sending requests.

Any errors occurring on the underlying connection are surfaced as exceptions terminating the response stream (and canceling the request source).

Note that, if the source produces subsequent requests before the prior responses have arrived, these requests will be pipelined across the connection, which is something that is not supported by all HTTP servers. Also, if the server closes the connection before responses to all requests have been received this will result in the response stream being terminated with a truncation error.

Closing Connections

Akka HTTP actively closes an established connection upon reception of a response containing Connection: close header. The connection can also be closed by the server.

An application can actively trigger the closing of the connection by completing the request stream. In this case the underlying TCP connection will be closed when the last pending response has been received.

The connection will also be closed if the response entity is cancelled (e.g. by attaching it to Sink.cancelled) or consumed only partially (e.g. by using take combinator). In order to prevent this behaviour the entity should be explicitly drained by attaching it to Sink.ignore.


Currently Akka HTTP doesn't implement client-side request timeout checking itself as this functionality can be regarded as a more general purpose streaming infrastructure feature.

It should be noted that Akka Streams provide various timeout functionality so any API that uses streams can benefit from the stream stages such as idleTimeout, backpressureTimeout, completionTimeout, initialTimeout and throttle. To learn more about these refer to their documentation in Akka Streams (and Scala Doc).

For more details about timeout support in Akka HTTP in general refer to Akka HTTP Timeouts.

Stand-Alone HTTP Layer Usage

Due to its Reactive-Streams-based nature the Akka HTTP layer is fully detachable from the underlying TCP interface. While in most applications this "feature" will not be crucial it can be useful in certain cases to be able to "run" the HTTP layer (and, potentially, higher-layers) against data that do not come from the network but rather some other source. Potential scenarios where this might be useful include tests, debugging or low-level event-sourcing (e.g by replaying network traffic).

On the client-side the stand-alone HTTP layer forms a BidiStage that is defined like this:

 * The type of the client-side HTTP layer as a stand-alone BidiFlow
 * that can be put atop the TCP layer to form an HTTP client.
 * {{{
 *                +------+
 * HttpRequest  ~>|      |~> SslTlsOutbound
 *                | bidi |
 * HttpResponse <~|      |<~ SslTlsInbound
 *                +------+
 * }}}
type ClientLayer = BidiFlow[HttpRequest, SslTlsOutbound, SslTlsInbound, HttpResponse, NotUsed]

You create an instance of Http.ClientLayer by calling one of the two overloads of the Http().clientLayer method, which also allows for varying degrees of configuration.