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.

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

Using HttpClient

Firstly, add the following dependency to your project:

"com.softwaremill.sttp.client4" %% "cats" % "4.0.0-M14"

Obtain a cats-effect Resource which creates the backend, and closes the thread pool after the resource is no longer used:

import cats.effect.IO
import sttp.client4.httpclient.cats.HttpClientCatsBackend

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

or, by providing a custom Dispatcher:

import cats.effect.IO
import cats.effect.std.Dispatcher
import sttp.client4.httpclient.cats.HttpClientCatsBackend

val dispatcher: Dispatcher[IO] = ???

HttpClientCatsBackend[IO](dispatcher).flatMap { backend => ??? }

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

import cats.effect.IO
import cats.effect.std.Dispatcher
import java.net.http.HttpClient
import sttp.client4.httpclient.cats.HttpClientCatsBackend

val httpClient: HttpClient = ???
val dispatcher: Dispatcher[IO] = ???

val backend = HttpClientCatsBackend.usingClient[IO](httpClient, dispatcher)

or, obtain a cats-effect Resource with a custom instance of the HttpClient:

import cats.effect.IO
import java.net.http.HttpClient
import sttp.client4.httpclient.cats.HttpClientCatsBackend

val httpClient: HttpClient = ???

HttpClientCatsBackend.resourceUsingClient[IO](httpClient).use { backend => ??? }

This backend is based on the built-in java.net.http.HttpClient available from Java 11 onwards.

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 Armeria

Creation of the backend 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.

Firstly, add the following dependency to your project:

"com.softwaremill.sttp.client4" %% "armeria-backend-cats" % "4.0.0-M14" // for cats-effect 3.x
// or
"com.softwaremill.sttp.client4" %% "armeria-backend-cats-ce2" % "4.0.0-M14" // for cats-effect 2.x

create client:

import cats.effect.IO
import sttp.client4.armeria.cats.ArmeriaCatsBackend

val backend = ArmeriaCatsBackend[IO]()

// You can use the default client which reuses the connection pool of ClientFactory.ofDefault()
ArmeriaCatsBackend.usingDefaultClient[IO]()

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

import cats.effect.IO
import sttp.client4.armeria.cats.ArmeriaCatsBackend

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

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.client4.armeria.cats.ArmeriaCatsBackend

// 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()

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

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.

Streaming

This backend doesn’t support non-blocking streaming.

Websockets

The backend doesn’t support websockets.