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. Backends do all the heavy-lifting.
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
, orHttpClientSyncBackend
if you are on Java11+. - if you have Akka in your stack, use the Akka backend
- if you are using
Future
without Akka, use theHttpClientFutureBackend
if you are on Java11+, orAsyncHttpClientFutureBackend
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 invokesend(backend)
on a request description, do you get aResponse[_]
directly, or is it wrapped in aFuture
or aTask
?P
, the capabilities supported by the backend, in addition toEffect[F]
. IfAny
, no additional capabilities are provided. Might includeStreams
(the ability to send and receive streaming bodies) andWebSockets
(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 |
akka.stream.scaladsl.Source[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 |
zio.stream.Stream[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 |
zio.stream.Stream[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 |
zio.stream.Stream[Throwable, Byte] |
yes (regular & streaming) | yes |
FinagleBackend |
com.twitter.util.Future |
n/a | no | no |
The backends work with Scala 2.11, 2.12 and 2.13 (with some exceptions for 2.11). Moreover, HttpURLConnectionBackend
, AsyncHttpClientFutureBackend
, AsyncHttpClientZioBackend
, HttpClientSyncBackend
, HttpClientFutureBackend
and HttpClientZioBackend
are additionally built with Scala 3.
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 inscala.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 functionListenerBackend
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:
- sttp-play-ws for “standard” play-ws (not standalone).
- akkaMonixSttpBackend, an Akka-based backend, but using Monix’s
Task
&Observable
.