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 HttpClientSyncBackend.

• if you have Akka in your stack, use the Akka backend

• if you have Pekko in your stack, use the Pekko backend

• if you are using Future without Akka, use the HttpClientFutureBackend

• 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:

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

• OpenTelemetryTracingBackend, for OpenTelemetry-compatible distributed tracing. See the dedicated section.

• OpenTelemetryMetricsBackend, for OpenTelemetry-compatible metrics. 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	yes (regular)
FetchMonixBackend	monix.eval.Task	monix.reactive.Observable[ByteBuffer]	yes (regular & streaming)
FetchZioBackend	zio.Task	zio.stream.Stream[Throwable, Byte]	yes (regular & streaming)
FetchCatsBackend	F[_]: cats.effect.Concurrent	n/a	yes (regular)

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:

• sttp-play-ws for “standard” play-ws (not standalone).

• akkaMonixSttpBackend, an Akka-based backend, but using Monix’s Task & Observable.

• be-kind-rewind, a VCR testing library for Scala