cats-effect backend

The Cats Effect backend is asynchronous. It can be created for any type implementing the cats.effect.Concurrent typeclass, such as cats.effect.IO. Sending a request is a non-blocking, lazily-evaluated operation and results in a wrapped response. There’s a transitive dependency on cats-effect.

To use, add the following dependency to your project:

"com.softwaremill.sttp.client3" %% "async-http-client-backend-cats" % "3.0.0-RC12"

You’ll need the following imports and implicits to create the backend:

import sttp.client3._
import sttp.client3.asynchttpclient.cats.AsyncHttpClientCatsBackend
import cats.effect._

// an implicit `cats.effect.ContextShift` in required to create the backend; here, for `cats.effect.IO`:
implicit val cs: ContextShift[IO] = IO.contextShift(

This backend depends on async-http-client, uses Netty behind the scenes.

Alternatively, the http4s backend can also be created for a type implementing the cats-effect’s Effect typeclass, and supports streaming as in fs2.

Next you’ll need to define a backend instance. This can be done in two basic ways:

  • by creating an effect, which describes how the backend is created, or instantiating the backend directly. In this case, you’ll need to close the backend manually
  • by creating a Resource, which will instantiate the backend and close it after it has been used

A non-comprehensive summary of how the backend can be created is as follows:

// the type class instance needs to be provided explicitly (e.g. `cats.effect.IO`). 
// the effect type must implement the Concurrent typeclass
AsyncHttpClientCatsBackend[IO]().flatMap { backend => ??? }

or, if you’d like to use a custom configuration:

import org.asynchttpclient.AsyncHttpClientConfig
val config: AsyncHttpClientConfig = ???
AsyncHttpClientCatsBackend.usingConfig[IO](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 = ???
AsyncHttpClientCatsBackend.usingConfigBuilder[IO](adjustFunction, sttpOptions).flatMap { backend => ??? }

or, if you’d like the backend to be wrapped in cats-effect Resource:

AsyncHttpClientCatsBackend.resource[IO]().use { backend => ??? }

or, if you’d like to instantiate the AsyncHttpClient yourself:

import org.asynchttpclient.AsyncHttpClient

val asyncHttpClient: AsyncHttpClient = ???  
val backend = AsyncHttpClientCatsBackend.usingClient[IO](asyncHttpClient)


This backend doesn’t support non-blocking streaming.


The backend doesn’t support websockets.