ZIO backends

The ZIO backends are asynchronous. Sending a request is a non-blocking, lazily-evaluated operation and results in a response wrapped in a zio.Task. There’s a transitive dependency on the zio or zio1 modules.

The *-zio modules depend on ZIO 2.x. For ZIO 1.x support, use modules with the *-zio1 suffix.

Using HttpClient

To use, add the following dependency to your project:

"com.softwaremill.sttp.client3" %% "zio" % "3.8.3"  // for ZIO 2.x
"com.softwaremill.sttp.client3" %% "zio1" % "3.8.3" // for ZIO 1.x

Create the backend using:

import sttp.client3.httpclient.zio.HttpClientZioBackend

HttpClientZioBackend().flatMap { backend => ??? }

// or, if you'd like the backend to be created in a Scope:
HttpClientZioBackend.scoped().flatMap { backend => ??? }

// or, if you'd like to instantiate the HttpClient yourself:
import java.net.http.HttpClient
val httpClient: HttpClient = ???
val backend = HttpClientZioBackend.usingClient(httpClient)

This backend is based on the built-in java.net.http.HttpClient available from Java 11 onwards. The backend is fully non-blocking, with back-pressured websockets.

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


Using Armeria

To use, add the following dependency to your project:

"com.softwaremill.sttp.client3" %% "armeria-backend-zio" % "3.8.3"  // for ZIO 2.x
"com.softwaremill.sttp.client3" %% "armeria-backend-zio1" % "3.8.3" // for ZIO 1.x

add imports:

import sttp.client3.armeria.zio.ArmeriaZioBackend

create client:

ArmeriaZioBackend().flatMap { backend => ??? }

// or, if you'd like the backend to be wrapped in a Scope:
ArmeriaZioBackend.scoped().flatMap { backend => ??? }

// You can use the default client which reuses the connection pool of ClientFactory.ofDefault()
ArmeriaZioBackend.usingDefaultClient().flatMap { backend => ??? }


The default client factory is reused to create ArmeriaZioBackend if a SttpBackendOptions is unspecified. So you only need to manage a resource when SttpBackendOptions is used.

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

ArmeriaZioBackend.usingClient(client).flatMap { backend => ??? }


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. Armeria’s ClientFactory manages connections and protocol-specific properties. Please visit the official documentation to learn how to configure it.

ZIO layers

As an alternative to effectfully or resourcefully creating backend instances, ZIO layers can be used. In this scenario, the lifecycle of a SttpBackend service is described by ZLayers, which can be created using the .layer/.layerUsingConfig/… methods on HttpClientZioBackend / ArmeriaZioBackend.

The layers can be used to provide an implementation of the SttpBackend dependency when creating services. For example:

import sttp.client3._
import sttp.client3.httpclient.zio._
import zio._

class MyService(sttpBackend: SttpBackend[Task, Any]) {
  def runLogic(): Task[Response[String]] = {
    val request = basicRequest.response(asStringAlways).get(uri"https://httpbin.org/get")

object MyService {
  val live: ZLayer[SttpBackend[Task, Any], Any, MyService] = ZLayer.fromFunction(new MyService(_))

ZLayer.make[MyService](MyService.live, HttpClientZioBackend.layer())


The ZIO based backends support streaming using zio-streams. The following example is using the HttpClientZioBackend.

The type of supported streams is Stream[Throwable, Byte]. The streams capability is represented as sttp.client3.impl.zio.ZioStreams. To leverage ZIO environment, use the SttpClient object to create request send effects.

Requests can be sent with a streaming body:

import sttp.capabilities.zio.ZioStreams
import sttp.client3._
import zio.stream._
import zio.Task

val sttpBackend: SttpBackend[Task, ZioStreams] = ???
val s: Stream[Throwable, Byte] =  ???

val request = basicRequest


And receive response bodies as a stream:

import sttp.capabilities.zio.ZioStreams
import sttp.client3._

import zio._
import zio.stream._

import scala.concurrent.duration.Duration

val sttpBackend: SttpBackend[Task, ZioStreams] = ???

val request =

val response: ZIO[Any, Throwable, Response[Either[String, Stream[Throwable, Byte]]]] = sttpBackend.send(request)


The HttpClient ZIO backend supports both regular and streaming websockets.


A stub backend can be created through the .stub method on the companion object, and configured as described in the testing section.

A layer with the stub SttpBackend can be then created by simply calling ZLayer.succeed(sttpBackendStub).

Server-sent events

Received data streams can be parsed to a stream of server-sent events (SSE):

import zio._
import zio.stream._

import sttp.capabilities.zio.ZioStreams
import sttp.client3.impl.zio.ZioServerSentEvents
import sttp.model.sse.ServerSentEvent
import sttp.client3._

def processEvents(source: Stream[Throwable, ServerSentEvent]): Task[Unit] = ???

basicRequest.response(asStream(ZioStreams)(stream =>