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.

Note that all fs2 backends also support any cats-effect effect, additionally supporting request & response streaming.

Using async-http-client

To use, add the following dependency to your project:

"com.softwaremill.sttp.client3" %% "async-http-client-backend-cats" % "3.8.1" // for cats-effect 3.x
// or
"com.softwaremill.sttp.client3" %% "async-http-client-backend-cats-ce2" % "3.8.1" // for cats-effect 2.x

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 Async 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:

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

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

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

import cats.effect.IO
import org.asynchttpclient.AsyncHttpClientConfig
import sttp.client3.asynchttpclient.cats.AsyncHttpClientCatsBackend

val config: AsyncHttpClientConfig = ???
AsyncHttpClientCatsBackend.usingConfig[IO](config).flatMap { backend => ??? }

or, if you’d like to use adjust the configuration sttp creates:

import cats.effect.IO
import org.asynchttpclient.DefaultAsyncHttpClientConfig
import sttp.client3.SttpBackendOptions
import sttp.client3.asynchttpclient.cats.AsyncHttpClientCatsBackend

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:

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

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

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

import cats.effect.IO
import org.asynchttpclient.AsyncHttpClient
import sttp.client3.asynchttpclient.cats.AsyncHttpClientCatsBackend

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

Using Armeria

To use, add the following dependency to your project:

"com.softwaremill.sttp.client3" %% "armeria-backend-cats" % "3.8.1" // for cats-effect 3.x
// or
"com.softwaremill.sttp.client3" %% "armeria-backend-cats-ce2" % "3.8.1" // for cats-effect 2.x

create client:

import cats.effect.IO
import sttp.client3.armeria.cats.ArmeriaCatsBackend

val backend = ArmeriaCatsBackend[IO]()

// You can use the default client which reuses the connection pool of ClientFactory.ofDefault()

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

import cats.effect.IO
import com.linecorp.armeria.client.WebClient
import com.linecorp.armeria.client.circuitbreaker._
import sttp.client3.armeria.cats.ArmeriaCatsBackend

// Fluently build Armeria WebClient with built-in decorators
val client = WebClient.builder("")
             // Open circuit on 5xx server error status

val backend = ArmeriaCatsBackend.usingClient[IO](client)


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.


This backend doesn’t support non-blocking streaming.


The backend doesn’t support websockets.