Http4s v0.20 Introduction

6 minute read

What is http4s?

http4s is a typeful, functional, streaming HTTP for Scala.

The current status of each version

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This introduction is based on the stable version 0.20

How to install?

Add following configuration into your build.sbt

val Http4sVersion = "0.20.17"
libraryDependencies += Seq(
  "org.http4s"  %% "http4s-blaze-server"  % Http4sVersion, // http server implementation
  "org.http4s"  %% "http4s-blaze-client"  % Http4sVersion, // http client implementation
  "org.http4s"  %% "http4s-circe"         % Http4sVersion, // supply some utility methods to convert the Encoder/Decoder of circe to the EntityEncoder/EntityDecoder of http4s
  "org.http4s"  %% "http4s-dsl"           % Http4sVersion  // supply lots of syntax to parse request
 )

Add following package into you import header

import org.http4s.dsl.io._
import org.http4s.implicits._
import org.http4s.circe._

Core Concept

Request

sealed abstract case class Request[F[_]](
  method: Method = Method.GET,
  uri: Uri = Uri(path = "/"),
  httpVersion: HttpVersion = HttpVersion.`HTTP/1.1`,
  headers: Headers = Headers.empty,
  body: EntityBody[F] = EmptyBody,
  attributes: Vault = Vault.empty
) extends Message[F]

The Request model contains all the information needed by a http request. The body of http request is EntityBody which is a fs2.Stream[F, Byte].

Obviously when the server receive a request, it’s not an easy work to convert Stream to a data model. So there is a type class called EntityDecoder to help us to do this work.

trait EntityDecoder[F[_], T] { 
  def decode(msg: Message[F], strict: Boolean): DecodeResult[F, T]
}

its consumer is defined in Message which is the parent of Request

trait Message[F[_]] {
  def attemptAs[T](implicit decoder: EntityDecoder[F, T]): DecodeResult[F, T] =
    decoder.decode(this, strict = false)

  def as[A](implicit F: MonadError[F, Throwable], decoder: EntityDecoder[F, A]): F[A] =
    attemptAs.leftWiden[Throwable].rethrowT
}

Actually, the EntityDecoder didn’t help us too much, because the decode method still get a Message which contains Stream body. In term of the widely usage of JSON and circe in the RestfulAPI, http4s supply jsonOf to let us convert the Decoder of circe to EntityDecoder

import org.http4s.circe._

implicit val personDecoder: Decoder[Person] = new Decoder[Person] {
  override def apply(c: HCursor): Decoder.Result[Person] =
    for {
      name <- c.get[String]("name")
      age <- c.get[Option[Int]]("age")
      phoneNumbers <- c.get[List[String]]("phone")
    } yield Person(name, age, phoneNumbers)
}
implicit val personEntityDecoder: EntityDecoder[IO, Person] = jsonOf[IO, Person]

But when you want to send a http request by a http client, you need to convert a data model to Stream. http4s supply another type class called EntityEncoder to do this work and also supply jsonOfEncoder to let you use JSON and circe Encoder.

final case class Entity[+F[_]](body: EntityBody[F], length: Option[Long] = None)
trait EntityEncoder[F[_], A] {
  def toEntity(a: A): Entity[F]
}

trait Message[F[_]] {
  def withEntity[T](b: T)(implicit w: EntityEncoder[F, T]): Self
}

import org.http4s.circe._

implicit val personEncoder: Encoder[Person] = new Encoder[Person] {
  override def apply(a: Person): Json = Json.obj(
    "name" -> a.name.asJson,
    "age" -> a.age.asJson,
    "phone" -> a.phoneNumbers.asJson
  )
}
implicit val personEntityEncoder: EntityEncoder[IO, Person] = jsonEncoderOf[IO, Person]

Response

final case class Response[F[_]](
  status: Status = Status.Ok,
  httpVersion: HttpVersion = HttpVersion.`HTTP/1.1`,
  headers: Headers = Headers.empty,
  body: EntityBody[F] = EmptyBody,
  attributes: Vault = Vault.empty
) extends Message[F]

Except the status attribute, the usage of EntityEncoder and EntityDecoder, most parts of Response are same as Request.

When a server want to return a http response, the EntityEncoder will be used.

case class Person(name: String, age: Option[Int], phoneNumbers: List[String])
implicit val personEntityEncoder: EntityEncoder[IO, Person] = jsonEncoderOf[IO, Person]

Ok(Person("Job", Some(18), List("111111"))) // send back Ok status with body

When a clent want to parse a http response, the EntityDecoder will be used.

trait Client[F[_]] {
  def expect[A](req: F[Request[F]])(implicit d: EntityDecoder[F, A]): F[A]
  def expect[A](uri: Uri)(implicit d: EntityDecoder[F, A]): F[A]
  def fetchAs[A](req: Request[F])(implicit d: EntityDecoder[F, A]): F[A]
  def fetchAs[A](req: F[Request[F]])(implicit d: EntityDecoder[F, A]): F[A]
}

Routes

  type Http[F[_], G[_]] = Kleisli[F, Request[G], Response[G]]
  type HttpApp[F[_]] = Http[F, F]
  type HttpRoutes[F[_]] = Http[OptionT[F, ?], F]

A HttpRoutes is just a function which take Request as input and produce Response as output.

Usually we will build differnt Routes for differnt business logic, so the HttpRoutes won’t return Response for every Request, that’s why it return Option[Response].

But the http server should be able to handle all the Request, so we need to give a default route, then the HttpRoutes will become HttpApp which is used in Server

Middleware

A middleware is just a function HttpApp[F[_], G[_]] => HttpApp[F[_], G[_]]

Usage

How to start a server?

object Main extends IOApp {
  def helloWorldRoutes: HttpRoutes[IO] = HttpRoutes
      .of[IO]({
        case GET -> Root / "hello" => Ok("Hello!!")
        })
  override def run(args: List[String]): IO[ExitCode] = {
    val resource = for {
      server <- BlazeServerBuilder[IO]
        .bindHttp(8888, "0.0.0.0")
        .withHttpApp(helloWorldRoutes.orNotFound)
        .resource
    } yield server
    resource.use(_ => IO.never)
  }
}

How to create a route?

HttpRoutes is a type alias of Kleisli[OptionT[F, ?], Request[F], Response[F]] which is complicated, http4s supply HttpRoutes.of to help us convert a paritial function Request[F] => Response[F] to HttpRoutes

def helloWorldRoutes: HttpRoutes[IO] = HttpRoutes
    .of[IO]({
      case GET -> Root / "hello" => Ok("Hello!!")
     })

HttpRoutes is the main place of business logic, in which we will parse Request, do computaion then construct the Response.

How to parse the Uri of Request?

To parse the Uri, http4s supply a set of dsl to help us and it already give the dsl implementation of IO.

Here we wil use IO as our effect, so we need to import its dsl when we parse the Uri

import org.http4s.dsl.io._

  • Method

      case (GET|POST) -> Root / "hello" =>
  //       ^
  //     Method

  • Path parameter

      case GET -> Root / "hello" / name / IntVar(id) / LongVar(hash) =>
  //                            ^        ^            ^
  //                         String     Int          Long

  • Query Paramerter

    • Required query parameter

        // url is /hello?user_name=john

  object UserName extends QueryParamDecoderMatcher[String]("user_name")
  case GET -> Root / "hello" :? UserName(name) =>
  //                                      ^
  //                            Required query parameter(String)

    • Optional query parameter

        // url is /hello?user_name=john
  // or /hello

  object UserName extends OptionalQueryParamDecoderMatcher[String]("user_name")
  case GET -> Root / "hello" :? UserName(name) =>
  //                                      ^
  //                           Optional query parameter(Option[String])

    • Multiple query parameter

        // url is /hello?user_name=john&age=18

  object UserName extends QueryParamDecoderMatcher[String]("user_name")
  object Age extends OptionalQueryParamDecoderMatcher[Int]("age")
  case GET -> Root / "hello" :? UserName(name) +& Age(age) =>
  //                                           ^
  //                                 Multiple query parameter

    • Optional query parameter which occurs multiple times

        // url is /hello?user_name=john&user_name=lili
  // or /hello

  object UserNames extends OptionalMultiQueryParamDecoderMatcher[String]("user_name")
  case GET -> Root / "hello" :? UserNames(names) =>
  //                                      ^
  //                        Occurs multiple times query parameter(List[String])

How to get the hearder of Request?

  case request@GET -> Root / "hello" =>
    println(request.headers)
    Ok()

How to get the body of Request?

Assume we have a model Person in our program

case class Person(name: String, age: Option[Int], phoneNumbers: List[String])

And we already defined the EntityDecoder of Person

object Person {
  implicit val personDecoder: Decoder[Person] = new Decoder[Person] {
    override def apply(c: HCursor): Decoder.Result[Person] =
      for {
        name <- c.get[String]("name")
        age <- c.get[Option[Int]]("age")
        phoneNumbers <- c.get[List[String]]("phone")
      } yield Person(name, age, phoneNumbers)
  }

  implicit val personEntityDecoder: EntityDecoder[IO, Person] = jsonOf[IO, Person]
}

Then if the body of Request is a JSON

{
  "name": "Job",
  "age": 18,
  "phone": ["1", "2"]
}

We can get the body of Request like this

  case request@POST -> Root / "hello" =>
    println(request.as[Person])
    Ok()

How to construct a Response?

http4s already defined the Status model for each status code, it’s a simple entry of Response. we need to choose the staus code first, then pass the header or body to it.

Here we will use 200 to give an example.

  • Header

    Ok(Header("Vary", "Origin,Access-Control-Request-Methods"))

  • Body

    Assume we already defined the EntityEncoder of Person

    object Person {
  implicit val personEncoder: Encoder[Person] = new Encoder[Person] {
    override def apply(a: Person): Json = Json.obj(
      "name" -> a.name.asJson,
      "age" -> a.age.asJson,
      "phone" -> a.phoneNumbers.asJson
    )
  }

  implicit val personEntityEncoder: EntityEncoder[IO, Person] = jsonEncoderOf[IO, Person]
}

    Then we can return Person as body like this

    Ok(Person("Job", Some(18), List("1111")))

How to add middleware?

How to support CORS?

def helloWorldRoutes: HttpRoutes[IO] = CORS(HttpRoutes
    .of[IO]({
      case GET -> Root / "hello" => Ok("Hello!!")
      }))

How to create a client?

val resource = for {
 client <- BlazeClientBuilder[IO](scala.concurrent.ExecutionContext.Implicits.global).resource
} yield client

resource.use(client => client.expect[Person](Uri.unsafefromString("http://localhost:8888/hello"))).unsafeRunSync()

How to send a GET request?

val response: IO[Person] = client.expect[Person](Uri.unsafefromString("http://localhost:8888/hello"))

How to send a POST request?

val request: Request[IO] = Request[IO](Method.POST, Uri.unsafefromString("http://localhost:8888/hello")).withEntity(Person("Jon", Some(18), List("1111")))
val response: IO[Response[IO]] = client.expect[String](IO(request))

Summary

Now you should be able to start a simple http server and send http request to remote server. if you want to try more feature, please clone the following repo and have a fun.

git clone git@github.com:sjmyuan/http4s-intro.git

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