This tutorial will walk you through creating your first http4s service and calling it with http4s’ client.

Create a new directory, with the following build.sbt in the root:

scalaVersion := "2.13.4" // Also supports 2.12.x

val http4sVersion = "0.21.32"

// Only necessary for SNAPSHOT releases
resolvers += Resolver.sonatypeRepo("snapshots")

libraryDependencies ++= Seq(
  "org.http4s" %% "http4s-dsl" % http4sVersion,
  "org.http4s" %% "http4s-blaze-server" % http4sVersion,
  "org.http4s" %% "http4s-blaze-client" % http4sVersion

// Uncomment if you're using Scala 2.12.x
// scalacOptions ++= Seq("-Ypartial-unification")

This tutorial is compiled as part of the build using mdoc. Each page is its own REPL session. If you copy and paste code samples starting from the top, you should be able to follow along in a REPL.

$ sbt console

Your first service

An HttpRoutes[F] is a simple alias for Kleisli[OptionT[F, *], Request, Response]. If that’s meaningful to you, great. If not, don’t panic: Kleisli is just a convenient wrapper around a Request => F[Response], and F is an effectful operation. We’ll teach you what you need to know as we go, or if you prefer you can read these introductions first:

Defining your service

Wherever you are in your studies, let’s create our first HttpRoutes. Start by pasting these imports into your SBT console:

import cats.effect._, org.http4s._,,

You also will need a ContextShift and a Timer. These come for free if you are in an IOApp.

implicit val cs: ContextShift[IO] = IO.contextShift(global)
implicit val timer: Timer[IO] = IO.timer(global)

Using the http4s-dsl, we can construct an HttpRoutes by pattern matching the request. Let’s build a service that matches requests to GET /hello/:name, where :name is a path parameter for the person to greet.

val helloWorldService = HttpRoutes.of[IO] {
  case GET -> Root / "hello" / name =>
    Ok(s"Hello, $name.")
// helloWorldService: HttpRoutes[IO] = Kleisli(
//   org.http4s.HttpRoutes$$$Lambda$8265/1868696275@1c73464b
// )

Returning content in the response

In order to return content of type T in the response an EntityEncoder[T] must be used. We can define the EntityEncoder[T] implictly so that it doesn’t need to be explicitly included when serving the response.

In the example below, we’re defining a tweetEncoder and then explicitly using it to encode the response contents of a Tweet, which can be seen as Ok(getTweet(tweetId))(tweetEncoder).

We’ve defined tweetsEncoder as being implicit so that we don’t need to explicitly reference it when serving the response, which can be seen as getPopularTweets().flatMap(Ok(_)).

case class Tweet(id: Int, message: String)

implicit def tweetEncoder: EntityEncoder[IO, Tweet] = ???
implicit def tweetsEncoder: EntityEncoder[IO, Seq[Tweet]] = ???

def getTweet(tweetId: Int): IO[Tweet] = ???
def getPopularTweets(): IO[Seq[Tweet]] = ???

val tweetService = HttpRoutes.of[IO] {
  case GET -> Root / "tweets" / "popular" =>
  case GET -> Root / "tweets" / IntVar(tweetId) =>
// tweetService: HttpRoutes[IO] = Kleisli(
//   org.http4s.HttpRoutes$$$Lambda$8265/1868696275@2b55ed5d
// )

Running your service

http4s supports multiple server backends. In this example, we’ll use blaze, the native backend supported by http4s.

We start from a BlazeServerBuilder, and then mount the helloWorldService under the base path of / and the remainder of the services under the base path of /api. The services can be mounted in any order as the request will be matched against the longest base paths first. The BlazeServerBuilder is immutable with chained methods, each returning a new builder.

Multiple HttpRoutes can be combined with the combineK method (or its alias <+>) by importing cats.implicits._ and org.http4s.implicits._. Please ensure partial unification is enabled in your build.sbt.

scalacOptions ++= Seq("-Ypartial-unification")

import cats.syntax.all._
import org.http4s.server.blaze._
import org.http4s.implicits._
import org.http4s.server.Router
val services = tweetService <+> helloWorldService
// services:[[IO, β$0$], Request[IO], Response[IO]] = Kleisli(
// )
val httpApp = Router("/" -> helloWorldService, "/api" -> services).orNotFound
// httpApp:[IO, Request[IO], Response[IO]] = Kleisli(
//   org.http4s.syntax.KleisliResponseOps$$Lambda$8365/281612239@6425b471
// )
val serverBuilder = BlazeServerBuilder[IO](global).bindHttp(8080, "localhost").withHttpApp(httpApp)
// serverBuilder: BlazeServerBuilder[IO] = org.http4s.server.blaze.BlazeServerBuilder@62e2f7a0

The bindHttp call isn’t strictly necessary as the server will be set to run using defaults of port 8080 and the loopback address. The withHttpApp call associates the specified routes with this http server instance.

We start a server resource in the background. The server will run until we cancel the fiber:

val fiber = serverBuilder.resource.use(_ => IO.never).start.unsafeRunSync()
// fiber: Fiber[IO, Nothing] = Tuple(
//   Bind(
//     Async(
//       cats.effect.internals.IOAsync$$$Lambda$8258/681007058@66d9c1a4,
//       false,
//       StackTrace(
//         List(
//           cats.effect.internals.IOTracing$.buildFrame(IOTracing.scala:48),
//           cats.effect.internals.IOTracing$.buildCachedFrame(IOTracing.scala:39),
//           cats.effect.internals.IOTracing$.cached(IOTracing.scala:34),
//           cats.effect.internals.IOAsync$.apply(IOAsync.scala:30),
//           cats.effect.IO$.async(IO.scala:1263),
//           cats.effect.internals.IOFromFuture$.apply(IOFromFuture.scala:37),
//           cats.effect.internals.IOStart$.fiber(IOStart.scala:48),
//           cats.effect.internals.IOStart$.$anonfun$apply$1(IOStart.scala:42),
//           cats.effect.internals.IOStart$.$anonfun$apply$1$adapted(IOStart.scala:29),
//           cats.effect.internals.IORunLoop$RestartCallback.start(IORunLoop.scala:447),
//           cats.effect.internals.IORunLoop$.cats$effect$internals$IORunLoop$$loop(IORunLoop.scala:156),
//           cats.effect.internals.IORunLoop$.$anonfun$suspendAsync$1(IORunLoop.scala:321),
//           cats.effect.internals.IORunLoop$.$anonfun$suspendAsync$1$adapted(IORunLoop.scala:320),
//           cats.effect.internals.IORunLoop$RestartCallback.start(IORunLoop.scala:447),
//           cats.effect.internals.IORunLoop$.cats$effect$internals$IORunLoop$$loop(IORunLoop.scala:156),
//           cats.effect.internals.IORunLoop$.start(IORunLoop.scala:38),
//           cats.effect.IO.unsafeRunAsync(IO.scala:274),
//           cats.effect.internals.IOPlatform$.unsafeResync(IOPlatform.scala:39),
//           cats.effect.IO.unsafeRunTimed(IO.scala:342),
//           cats.effect.IO.unsafeRunSync(IO.scala:256),
//           repl.MdocSession$App.<init>(,
//           repl.MdocSession$.app(,
//           mdoc.internal.document.DocumentBuilder$$doc$.$anonfun$build$2(DocumentBuilder.scala:89),
//           scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$,
//           scala.util.DynamicVariable.withValue(DynamicVariable.scala:62),
//           scala.Console$.withErr(Console.scala:196),
//           mdoc.internal.document.DocumentBuilder$$doc$.$anonfun$build$1(DocumentBuilder.scala:89),
//           scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$,
//           scala.util.DynamicVariable.withValue(DynamicVariable.scala:62),
//           scala.Console$.withOut(Console.scala:167),
//           mdoc.internal.document.DocumentBuilder$$doc$.build(

Use curl, or your favorite HTTP client, to see your service in action:

$ curl http://localhost:8080/hello/Pete

Cleaning up

We can shut down the server by canceling its fiber.


Running your service as an App

Every ServerBuilder[F] has a .serve method that returns a Stream[F, ExitCode]. This stream runs forever without emitting any output. When this process is run with .unsafeRunSync on the main thread, it blocks forever, keeping the JVM (and your server) alive until the JVM is killed.

As a convenience, cats-effect provides an cats.effect.IOApp trait with an abstract run method that returns a IO[ExitCode]. An IOApp runs the process and adds a JVM shutdown hook to interrupt the infinite process and gracefully shut down your server when a SIGTERM is received.

import cats.effect._
import org.http4s.HttpRoutes
import org.http4s.implicits._
import org.http4s.server.blaze._
object Main extends IOApp {

  val helloWorldService = HttpRoutes.of[IO] {
    case GET -> Root / "hello" / name =>
      Ok(s"Hello, $name.")

  def run(args: List[String]): IO[ExitCode] =
      .bindHttp(8080, "localhost")

You may also create the server within an IOApp using resource:


object MainWithResource extends IOApp {

  def run(args: List[String]): IO[ExitCode] =
      .bindHttp(8080, "localhost")
      .use(_ => IO.never)