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Version: 2.x

Tutorial: How to Build a RESTful Web Service

ZIO provides good support for building RESTful web services. Using Service Pattern, we can build web services that are modular and easy to test and maintain. On the other hand, we have several powerful official and community libraries that can help us to work with JSON data types, and databases and also work with HTTP protocol.

In this tutorial, we will learn how to build a RESTful web service using ZIO. The corresponding source code for this tutorial is available on GitHub. If you haven't read the ZIO Quickstart: Building RESTful Web Service yet, we recommend you read it first and download and run the source code, before reading this tutorial.

Installation​

We need to add the following dependencies to our project:

libraryDependencies ++= Seq(
  "dev.zio"  %% "zio-http" % "3.0.0-RC6"
)

For this tutorial, we will be using the ZIO HTTP library, which is a library for building HTTP applications using ZIO.

Introduction to The Route Data Type​

Before we start to build a RESTful web service, we need to understand the Route data type. It is a data type that models a route in an HTTP application.

We can think of the Route[Env, Err] as a description of an HTTP route that accepts a Request that matches the route pattern and returns a Response. It can use the environment of type Env and may fail with an error of type Err.

A simple Route can be defined as follows:

import zio.http._

val helloRoute: Route[Any, Nothing] =
  Method.GET / "hello" -> handler(Response.text("Hello, world!"))

We can say that Route[Any, Nothing] is a function that takes a Request and returns a Response. It doesn't require any services from the environment and won't fail.

Having multiple routes, we can collect them into a single Routes data type:

import zio.http._

val routes: Routes[Any, Nothing] =
  Routes(
    Method.GET / "hello" -> handler(Response.text("Hello, world!")),
    Method.GET / "greet" -> handler(Response.text("Hello, ZIO!"))
  )

Finally, we can serve the routes using the Server.serve method:

import zio._
import zio.http._

object MainApp extends ZIOAppDefault {
  def run = Server.serve(routes).provide(Server.default)
}

Modeling Http Applications​

Let's try to model some HTTP applications using the Route data type. So first, we are going to learn some basic Route constructors and how to combine them to build more complex HTTP applications.

Creation of a Handler​

The Handler.succeed constructor creates a Handler that always returns a successful response:

import zio.http._

val app: Handler[Any, Nothing, Any, Response] =
  Handler.succeed(Response.text("Hello, world!"))

We have the same constructor for failures called Http.fail. It creates a Handler application that always returns a failed response:

import zio._
import zio.http._

val app: Handler[Any, Response, Any, Nothing] = 
  handler(ZIO.fail(Response.internalServerError("Something went wrong")))

We can also create a Handler form a function. The Handler.fromFunction constructor takes a total function of type A => B and then creates a Handler that accepts an A and returns a B:

import zio.http._

val app: Handler[Any, Nothing, Int, Double] = Handler.fromFunction[Int](_ / 2.0)

Handlers can be effectual. We have a couple of constructors that can be used to create a Handler that are effectual:

  • Http.fromZIO
  • Http.fromStream
  • Http.fromFunctionZIO
  • Http.fromFile

There are lots of other constructors, to learn more about them, please refer to the Handler page in the ZIO HTTP documentation.

Combining Handlers​

The Handler data type is composable like ZIO. We can create new complex Handler by combining existing simple ones by using flatMap, zip, andThen, orElse, and ++ methods:

import zio.http._

val a           : Handler[Any, Nothing, Int, Double]    = ???
val b           : Handler[Any, Nothing, Double, String] = ???
def c(i: Double): Handler[Any, Nothing, Long, String]   = ???

val d = a >>= c // a flatMap c (combine two handlers sequentially)
val f = a >>> b // a andThen b (pipe output of the a handler to input of the b handler)
val h = a <> b  // a orElse b  (run a, if it fails, run b)

Built-in Request and Response Data Types​

Until now, we have learned how to create a Handler with some simple request and response types, e.g. String and Int in a Handler[Any, Nothing, String, Int]. But, in real life, when we want to deal with HTTP requests and responses, we need to have a more complex type for the request and response.

ZIO HTTP provides a type Request for HTTP requests and a type Response for HTTP responses. It has a built-in decoder for Request and an encoder for Response. So we don't need to worry about the details of how requests and responses are decoded and encoded.

The Response type has a default apply constructor in its companion object that takes a status, headers, and, HTTP data to create a Response:

object Response {
  def apply[R, E](
    status: Status = Status.Ok,
    headers: Headers = Headers.empty,
    data: HttpData = HttpData.Empty
  ): Response = ???
}

Other than the default constructor, we have several helper methods to create a Response. Here are some of them:

  1. Response.ok: Creates a successful response with 200 status code.
  2. Response.text("Hello World"): Creates a successful response with 200 status code and a body of Hello World.
  3. Response.status(Status.BadRequest): Creates a response with a status code of 400.
  4. Response.html("<h1>Hello World</h1>"): Creates a successful response with 200 status code and an HTML body of <h1>Hello World</h1>.
  5. Response.redirect("/"): Creates a successful response that redirects to the root path.

On the other hand, we do not need to create a Request instead, we need to pattern-match incoming requests to decompose them and determine the appropriate action to take.

Each incoming request can be extracted into two parts using pattern matching:

  • HTTP Method (GET, POST, PUT, etc.)
  • Path (e.g. /, /greeting, /download)

Let's see an example of how to pattern match on incoming requests:

import zio.http._

val httpApp: Route[Any, Nothing] =
  Method.GET / "greet" / string("name") -> 
    handler { (name: String, _: Request) =>
      Response.text(s"Hello $name!")
    }

Using this DSL we only access the method and path of the incoming request. If we need to access the query string, the body, and more, we need to use the following DSL:

import zio._
import zio.http._

val httpApp: Route[Any, Response] =
  Method.GET / "greet" -> 
    handler { (req: Request) =>
      if (req.url.queryParams.nonEmpty)
        ZIO.succeed(Response.text(s"Hello ${req.url.queryParams("name").mkString(" and ")}!"))
      else
        ZIO.fail(Response.badRequest("Missing query parameter 'name'"))
    }

Until now, we have learned how to create Route and Routes applications that handle HTTP requests. In the next section, we will learn how to create HTTP servers that can serve HTTP routes.

Creating HTTP Server​

To start an HTTP server, the ZIO HTTP requires a Routes of type Routes[Env, Response] and returns an effect that requires a Server from the environment and never produces a value and never fails:

object Server {
  def server[R](
    http: Routes[Env, Response]
  ): ZIO[R with Server, Nothing, Nothing] = ???
}

Greeting App​

First, we need to define a request handler that will handle GET requests to the /greet path:

import zio._
import zio.http._

object GreetingRoutes {
  def apply(): Routes[Any, Response] =
    Routes(
      // GET /greet?name=:name
      Method.GET / "greet" -> handler { (req: Request) =>
        if (req.url.queryParams.nonEmpty)
          ZIO.succeed(
            Response.text(
              s"Hello ${req.url.queryParams("name").map(_.mkString(" and "))}!"
            )
          )
        else 
          ZIO.fail(Response.badRequest("The name query parameter is missing!"))
      },

      // GET /greet
      Method.GET / "greet" -> handler(Response.text(s"Hello World!")),

      // GET /greet/:name
      Method.GET / "greet" / string("name") -> handler {
        (name: String, _: Request) =>
          Response.text(s"Hello $name!")
      }
    )

}

In the above example, we have defined three routes:

  • The first case matches a request with a path of /greet and a query parameter name.
  • The second case matches a request with a path of /greet with no query parameters.
  • The third case matches a request with a path of /greet/:name and extracts the name from the path.

Next, we need to create a server for GreetingRoutes:

import zio._
import zio.http._

object MainApp extends ZIOAppDefault {
  def run =
    Server.serve(GreetingRoutes()).provide(Server.default)
}

Now, we have three endpoints in our server. We can test the server according to the steps mentioned in the corresponding quickstart.

Note that if we have written other routes along with GreetingRoutes, such as DownloadRoutes, CounterRoutes, and UserRoutes, we can combine them together and start a server for that routes:

import zio.http._

Server.serve(
  GreetingRoutes() ++ DownloadRoutes() ++ CounterRoutes() ++ UserRoutes()
).provide(Server.default)

Conclusion​

In this tutorial, we have learned the basic building blocks of writing HTTP servers. We learned how to write routes and handlers. And finally, we saw how to create an HTTP server that can handle HTTP applications.

All the source code associated with this article is available on the ZIO Quickstart project.