Supported backends

sttp supports a number of synchronous and asynchronous backends. It’s the backends that take care of managing connections, sending requests and receiving responses: sttp defines only the API to describe the requests to be send and handle the response data. It’s the backends where all the heavy-lifting is done.

Choosing the right backend depends on a number of factors: if you are using sttp to explore some data, or is it a production system; are you using a synchronous, blocking architecture or an asynchronous one; do you work mostly with Scala’s Future, or maybe you use some form of a Task abstraction; finally, if you want to stream requests/responses, or not.

Each backend has two type parameters:

  • R[_], the type constructor in which responses are wrapped. That is, when you invoke send() on a request description, do you get a Response[_] directly, or is it wrapped in a Future or a Task?
  • S, the type of supported streams. If Nothing, streaming is not supported. Otherwise, the given type can be used to send request bodies or receive response bodies.

Below is a summary of all the backends. See the sections on individual backend implementations for more information.

Class Response wrapper Supported stream type
HttpURLConnectionBackend None (Id) n/a
TryHttpURLConnectionBackend scala.util.Try n/a
AkkaHttpBackend scala.concurrent.Future akka.stream.scaladsl.Source[ByteString, Any]
AsyncHttpClientFutureBackend scala.concurrent.Future n/a
AsyncHttpClientScalazBackend scalaz.concurrent.Task n/a
AsyncHttpClientMonixBackend monix.eval.Task monix.reactive.Observable[ByteBuffer]
AsyncHttpClientCatsBackend F[_]: cats.effect.Async n/a
AsyncHttpClientFs2Backend F[_]: cats.effect.Async fs2.Stream[F, ByteBuffer]
OkHttpSyncBackend None (Id) n/a
OkHttpFutureBackend scala.concurrent.Future n/a
OkHttpMonixBackend monix.eval.Task monix.reactive.Observable[ByteBuffer]

There are also backends which wrap other backends to provide additional functionality. These include:

  • TryBackend, which safely wraps any exceptions thrown by a synchronous backend in scala.util.Try
  • BraveBackend, for Zipkin-compatible distributed tracing. See the dedicated section.
  • PrometheusBackend, for gathering Prometheus-format metrics. See the dedicated section.

In additional there are also backends for JavaScript:

Class Response wrapper Supported stream type
FetchBackend scala.concurrent.Future n/a
FetchMonixBackend monix.eval.Task monix.reactive.Observable[ByteBuffer]

and Scala Native:

Class Response wrapper Supported stream type
CurlBackend None (id) n/a
CurlTryBackend scala.util.Try n/a