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" % "3.6.1"  // for ZIO 2.x
"com.softwaremill.sttp.client3" %% "zio1" % "3.6.1" // 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:

-Djdk.httpclient.allowRestrictedHeaders=host

Using async-http-client

To use, add the following dependency to your project:

"com.softwaremill.sttp.client3" %% "async-http-client-backend-zio" % "3.6.1"  // for ZIO 2.x
"com.softwaremill.sttp.client3" %% "async-http-client-backend-zio1" % "3.6.1" // for ZIO 1.x

This backend depends on async-http-client, uses Netty behind the scenes. This backend works with all Scala versions. A Scala 3 build is available as well.

Next you’ll need to define a backend instance. A non-comprehensive summary of how this can be done is as follows:

import sttp.client3._
import sttp.client3.asynchttpclient.zio.AsyncHttpClientZioBackend

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

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

// or, if you'd like to use custom configuration:
import org.asynchttpclient.AsyncHttpClientConfig
val config: AsyncHttpClientConfig = ???
AsyncHttpClientZioBackend.usingConfig(config).flatMap { backend => ??? }

// or, if you'd like to use adjust the configuration sttp creates:
import org.asynchttpclient.DefaultAsyncHttpClientConfig
val sttpOptions: SttpBackendOptions = SttpBackendOptions.Default  
val adjustFunction: DefaultAsyncHttpClientConfig.Builder => DefaultAsyncHttpClientConfig.Builder = ???

AsyncHttpClientZioBackend.usingConfigBuilder(adjustFunction, sttpOptions).flatMap { backend => ??? }

// or, if you'd like to instantiate the AsyncHttpClient yourself:
import org.asynchttpclient.AsyncHttpClient
import zio.Runtime
val asyncHttpClient: AsyncHttpClient = ???
val runtime: Runtime[Any] = ???
val backend = AsyncHttpClientZioBackend.usingClient(runtime, asyncHttpClient)

Using Armeria

To use, add the following dependency to your project:

"com.softwaremill.sttp.client3" %% "armeria-backend-zio" % "3.6.1"  // for ZIO 2.x
"com.softwaremill.sttp.client3" %% "armeria-backend-zio1" % "3.6.1" // 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 => ??? }

Note

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
             .decorator(CircuitBreakerClient.newDecorator(CircuitBreaker.ofDefaultName(),
               CircuitBreakerRule.onServerErrorStatus()))
             .build()

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

Note

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 environment

As an alternative to effectfully or resourcefully creating backend instances, ZIO environment can be used. In this case, a type alias is provided for the service definition:

package sttp.client3.httpclient.zio
type SttpClient = SttpBackend[Task, ZioStreams with WebSockets]

// or, when using async-http-client

package sttp.client3.asynchttpclient.zio
type SttpClient = SttpBackend[Task, ZioStreams with WebSockets]

// or, when using Armeria

package sttp.client3.armeria.zio
type SttpClient = SttpBackend[Task, ZioStreams]

The lifecycle of the SttpClient service is described by ZLayers, which can be created using the .layer/.layerUsingConfig/… methods on AsyncHttpClientZioBackend / HttpClientZioBackend / ArmeriaZioBackend.

The SttpClient companion object contains effect descriptions which use the SttpClient service from the environment to send requests or open websockets. This is different from sttp usage with other effect libraries (which use an implicit backend when .send(backend) is invoked on the request), but is more in line with how other ZIO services work. For example:

import sttp.client3._
import sttp.client3.httpclient.zio._
import zio._
val request = basicRequest.get(uri"https://httpbin.org/get")

val sent: ZIO[SttpClient, Throwable, Response[Either[String, String]]] = 
  send(request)

Streaming

The ZIO based backends support streaming using zio-streams. The following example is using the AsyncHttpClientZioBackend backend, but works similarly with 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 sttp.client3.httpclient.zio.send
import zio.stream._

val s: Stream[Throwable, Byte] =  ???

val request = basicRequest
  .streamBody(ZioStreams)(s)
  .post(uri"...")

send(request)

And receive response bodies as a stream:

import sttp.capabilities.zio.ZioStreams
import sttp.client3._
import sttp.client3.httpclient.zio.{SttpClient, send}

import zio._
import zio.stream._

import scala.concurrent.duration.Duration

val request =
  basicRequest
    .post(uri"...")
    .response(asStreamUnsafe(ZioStreams))
    .readTimeout(Duration.Inf)

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

Websockets

The ZIO backend supports both regular and streaming websockets.

Testing

The ZIO backends also support a ZIO-familiar way of configuring stubs as well. In addition to the usual way of creating a stand-alone stub, you can also define your stubs as effects instead:

import sttp.client3._
import sttp.model._
import sttp.client3.httpclient._
import sttp.client3.httpclient.zio._
import sttp.client3.httpclient.zio.stubbing._

val stubEffect = for {
  _ <- whenRequestMatches(_.uri.toString.endsWith("c")).thenRespond("c")
  _ <- whenRequestMatchesPartial { case r if r.method == Method.POST => Response.ok("b") }
  _ <- whenAnyRequest.thenRespond("a")
} yield ()

val responseEffect = stubEffect *> send(basicRequest.get(uri"http://example.org/a")).map(_.body)

responseEffect.provideLayer(HttpClientZioBackend.stubLayer) // Task[Either[String, String]]

The whenRequestMatches, whenRequestMatchesPartial, whenAnyRequest are effects which require the SttpClientStubbing dependency. They enrich the stub with the given behavior.

Then, the stubLayer provides both an implementation of the SttpClientStubbing dependency, as well as a SttpClient which is backed by the stub.

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 => 
  processEvents(stream.viaFunction(ZioServerSentEvents.parse))))