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 sent and handle the response data. Backends do all the heavy-lifting. Typically, a single backend instance is created for the lifetime of the application.

Choosing the right backend depends on a number of factors: whether 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.

Which one to choose?

  • for simple exploratory requests, use the synchronous HttpURLConnectionBackend, or HttpClientSyncBackend if you are on Java11+.
  • if you have Akka in your stack, use the Akka backend
  • if you are using Future without Akka, use the HttpClientFutureBackend if you are on Java11+, or AsyncHttpClientFutureBackend Future otherwise
  • finally, if you are using a functional effect wrapper, use one of the “functional” backends, for ZIO, Monix, Scalaz, cats-effect or fs2.

Each backend has two type parameters:

  • F[_], the effects wrapper for responses. That is, when you invoke send(backend) on a request description, do you get a Response[_] directly, or is it wrapped in a Future or a Task?
  • P, the capabilities supported by the backend, in addition to Effect[F]. If Any, no additional capabilities are provided. Might include Streams (the ability to send and receive streaming bodies) and WebSockets (the ability to handle websocket requests).

Below is a summary of all the JVM backends; see the sections on individual backend implementations for more information:

Class Effect type Supported stream type Supports websockets Fully non-blocking
HttpURLConnectionBackend None (Identity) n/a no no
TryHttpURLConnectionBackend scala.util.Try n/a no no
AkkaHttpBackend scala.concurrent.Future[ByteString, Any] yes (regular & streaming) yes
AsyncHttpClientFutureBackend scala.concurrent.Future n/a yes (regular) no
AsyncHttpClientScalazBackend scalaz.concurrent.Task n/a yes (regular) no
AsyncHttpClientZioBackend zio.Task[Throwable, Byte] yes (regular & streaming) no
AsyncHttpClientMonixBackend monix.eval.Task monix.reactive.Observable[ByteBuffer] yes (regular & streaming) no
AsyncHttpClientCatsBackend F[_]: cats.effect.Concurrent n/a no no
AsyncHttpClientFs2Backend F[_]: cats.effect.Concurrent fs2.Stream[F, Byte] yes (regular & streaming) no
ArmeriaFutureBackend scala.concurrent.Future n/a no yes
ArmeriaScalazBackend scalaz.concurrent.Task n/a no yes
ArmeriaZioBackend zio.Task[Throwable, Byte] no yes
ArmeriaMonixBackend monix.eval.Task monix.reactive.Observable[HttpData] no yes
ArmeriaCatsBackend F[_]: cats.effect.Concurrent n/a no yes
ArmeriaFs2Backend F[_]: cats.effect.Concurrent fs2.Stream[F, Byte] no yes
OkHttpSyncBackend None (Identity) n/a yes (regular) no
OkHttpFutureBackend scala.concurrent.Future n/a yes (regular) no
OkHttpMonixBackend monix.eval.Task monix.reactive.Observable[ByteBuffer] yes (regular & streaming) no
Http4sBackend F[_]: cats.effect.Effect fs2.Stream[F, Byte] no no
HttpClientSyncBackend None (Identity) n/a no no
HttpClientFutureBackend scala.concurrent.Future n/a yes (regular) no
HttpClientMonixBackend monix.eval.Task monix.reactive.Observable[ByteBuffer] yes (regular & streaming) yes
HttpClientFs2Backend F[_]: cats.effect.Concurrent fs2.Stream[F, Byte] yes (regular & streaming) yes
HttpClientZioBackend zio.Task[Throwable, Byte] yes (regular & streaming) yes
FinagleBackend com.twitter.util.Future n/a no no

The backends work with Scala 2.11, 2.12, 2.13 and 3 (with some exceptions for 2.11 and 3).

Backends supporting cats-effect are available in versions for cats-effect 2.x (dependency artifacts have the -ce2 suffix) and 3.x.

All backends that support asynchronous/non-blocking streams, also support server-sent events.

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
  • OpenTracingBackend, for OpenTracing-compatible distributed tracing. See the dedicated section.
  • PrometheusBackend, for gathering Prometheus-format metrics. See the dedicated section.
  • extendable logging backends (with an slf4j implementation) backends. See the dedicated section.
  • ResolveRelativeUrisBackend to resolve relative URIs given a base URI, or an arbitrary effectful function
  • ListenerBackend to listen for backend lifecycle events. See the dedicated section.
  • FollowRedirectsBackend, which handles redirects. All implementation backends are created wrapped with this one.

In addition, there are also backends for Scala.JS:

Class Effect type Supported stream type Supports websockets
FetchBackend scala.concurrent.Future n/a no
FetchMonixBackend monix.eval.Task monix.reactive.Observable[ByteBuffer] no
FetchCatsBackend F[_]: cats.effect.Concurrent n/a no

And a backend for scala-native:

Class Effect type Supported stream type Supports websockets
CurlBackend None (Identity) n/a no

Finally, there are third-party backends: