Future-based backends

There are several backend implementations which are scala.concurrent.Future-based. These backends are asynchronous, sending a request is a non-blocking operation and results in a response wrapped in a Future.

Apart from the ones described below, also the Akka backend is Future-based.


Supported stream type

Websocket support



yes (regular)


akka.stream.scaladsl.Source[ByteString, Any]

yes (regular & streaming)






yes (regular)




Using HttpClient

To use, you don’t need any extra dependencies, core is enough:

"com.softwaremill.sttp.client3" %% "core" % "3.6.2"

You’ll need the following imports:

import sttp.client3.HttpClientFutureBackend
import scala.concurrent.ExecutionContext.Implicits.global

Create the backend using:

val backend = HttpClientFutureBackend()

or, if you’d like to instantiate the HttpClient yourself:

import java.net.http.HttpClient

val client: HttpClient = ???  
val backend = HttpClientFutureBackend.usingClient(client)

This backend is based on the built-in java.net.http.HttpClient available from Java 11 onwards.

Host header override is supported in environments running Java 12 onwards, but it has to be enabled by system property:


Using async-http-client

To use, add the following dependency to your project:

"com.softwaremill.sttp.client3" %% "async-http-client-backend-future" % "3.6.2"

And some imports:

import sttp.client3._
import sttp.client3.asynchttpclient.future.AsyncHttpClientFutureBackend

This backend depends on async-http-client and uses Netty behind the scenes.

Next you’ll need to create the backend instance:

val backend = AsyncHttpClientFutureBackend()

or, if you’d like to use custom configuration:

import org.asynchttpclient.AsyncHttpClientConfig

val config: AsyncHttpClientConfig = ???
val backend = AsyncHttpClientFutureBackend.usingConfig(config)

or, if you’d like to use adjust the configuration sttp creates:

import org.asynchttpclient.DefaultAsyncHttpClientConfig

val sttpOptions: SttpBackendOptions = SttpBackendOptions.Default  
val adjustFunction: DefaultAsyncHttpClientConfig.Builder => DefaultAsyncHttpClientConfig.Builder = ???
val backend = AsyncHttpClientFutureBackend.usingConfigBuilder(adjustFunction, sttpOptions)

or, if you’d like to instantiate the AsyncHttpClient yourself:

import org.asynchttpclient.AsyncHttpClient

val asyncHttpClient: AsyncHttpClient = ???  
val backend = AsyncHttpClientFutureBackend.usingClient(asyncHttpClient)

Using OkHttp

To use, add the following dependency to your project:

"com.softwaremill.sttp.client3" %% "okhttp-backend" % "3.6.2"

and some imports:

import sttp.client3.okhttp.OkHttpFutureBackend
import scala.concurrent.ExecutionContext.Implicits.global

Create the backend using:

val backend = OkHttpFutureBackend()

or, if you’d like to instantiate the OkHttpClient yourself:

import okhttp3.OkHttpClient

val asyncHttpClient: OkHttpClient = ???  
val backend = OkHttpFutureBackend.usingClient(asyncHttpClient)

This backend depends on OkHttp and fully supports HTTP/2.

Using Armeria

To use, add the following dependency to your project:

"com.softwaremill.sttp.client3" %% "armeria-backend" % "3.6.2"

add imports:

import sttp.client3.armeria.future.ArmeriaFutureBackend

create client:

val backend = ArmeriaFutureBackend()

// You can use the default client which reuses the connection pool of ClientFactory.ofDefault()

or, if you’d like to instantiate the WebClient yourself::

import com.linecorp.armeria.client.circuitbreaker._
import com.linecorp.armeria.client.WebClient

// Fluently build Armeria WebClient with built-in decorators
val client = WebClient.builder("https://my-service.com")
             // Open circuit on 5xx server error status
val backend = ArmeriaFutureBackend.usingClient(client)


A WebClient could fail to follow redirects if the WebClient is created with a base URI and a redirect location is a different URI.

This backend is build on top of Armeria and doesn’t support host header override. Armeria’s ClientFactory manages connections and protocol-specific properties. Please visit the official documentation to learn how to configure it.


The Akka backend supports streaming using akka-streams.

Other backends don’t support non-blocking streaming.


Some of the backends (see above) support regular and streaming websockets.