Pluggable Client Transports / HTTPS Proxy Support

The client side infrastructure has (unstable) support to plug different transport mechanisms underneath. A client side transport is represented by an instance of akka.http.scaladsl.ClientTransport:

trait ClientTransport {
  def connectTo(host: String, port: Int, settings: ClientConnectionSettings)(implicit system: ActorSystem): Flow[ByteString, ByteString, Future[OutgoingConnection]]

A transport implementation defines how the client infrastructure should communicate with a given host.


In our model, SSL/TLS runs on top of the client transport, even if you could theoretically see it as part of the transport layer itself.

Configuring Client Transports

A ClientTransport is configured slightly differently for the various layers of the HTTP client. Right now, configuration is only possible with code (and not through config files). There’s currently no predefined way that would allow you to select different transports per target host (but you can easily define any kind of strategy by implementing ClientTransport yourself).

Connection Pool Usage

The ConnectionPoolSettings class allows setting a custom transport for any of the pool methods. Use ConnectionPoolSettings.withTransport to configure a transport and pass those settings to one of the pool methods like Http().singleRequest, Http().superPool, or Http().cachedHostConnectionPool.

Single Connection Usage

You can configure a custom transport for a single HTTP connection by passing it to the Http().outgoingConnectionUsingTransport method.

Predefined Transports


The default transport is ClientTransport.TCP which simply opens a TCP connection to the target host.


A transport that connects to target servers via an HTTPS proxy. An HTTPS proxy uses the HTTP CONNECT method (as specified in RFC 7231 Section 4.3.6) to create tunnels to target servers. The proxy itself should transparently forward data to the target servers so that end-to-end encryption should still work (if TLS breaks, then the proxy might be fussing with your data).

Instantiate the HTTPS proxy transport using ClientTransport.httpsProxy(proxyAddress).

Implementing Custom Transports

Implement ClientTransport.connectTo to implement a custom client transport.

Here are some ideas for custom (or future predefined) transports:

  • SSH tunnel transport: connects to the target host through an SSH tunnel
  • Per-host configurable transport: allows choosing transports per target host

Usage Examples

Use HTTPS Proxy with Http().singleRequest


import akka.http.scaladsl.{ ClientTransport, Http }

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

val proxyHost = "localhost"
val proxyPort = 8888

val httpsProxyTransport = ClientTransport.httpsProxy(InetSocketAddress.createUnresolved(proxyHost, proxyPort))

val settings = ConnectionPoolSettings(system).withTransport(httpsProxyTransport)
Http().singleRequest(HttpRequest(uri = ""), settings = settings)
The source code for this page can be found here.