Hello World

We’re glad you’ve decided to give Pedestal a try. We think it’s pretty powerful. Before we get into the heavy lifting though, we should start with some basics.

After reading this guide, you will be able to:

This guide is for beginners who are new to Pedestal. It doesn’t assume that you have any prior experience with Pedestal, Clojure, or any other Clojure-based web framework. You should be familiar with the basics of HTTP: URLs, response codes, and content types.

If you’ve already done some of those other things, you might want to skip ahead to Your First API to start building some logic and multiple routes.

This guide also assumes that you are in a Unix-like development environment, with Java installed. We’ve tested it on Mac OS X and Linux (any flavor) with great results. We haven’t yet tried it on the Windows Subsystem for Linux, but would love to hear from you if you’ve succeeded with it there.

We will be using the Clojure CLI tools to manage dependencies and download libraries. Refer to the Clojure Getting Started page for installation instructions and details.

We’ll take this in small steps. If you get stuck at any point in this guide, please submit an issue about this guide or, hop over to the Pedestal Users mailing list and raise your hand there. You can also get help from the #pedestal channel on the Clojurians Slack.

This guide shows fragments of code as we add them. Sometimes it helps to see the whole thing at once, so you can always check out The Whole Shebang at the end of this guide.

In this guide, we’re going to take a series of small steps. We’ll see all the code to build a Pedestal service starting with an empty directory.

The first time through, we will just do everything "by hand". This is a little more work in typing (or copy-and-pasting) but it will let you see how the pieces connect in the simplest possible use case.

Pedestal services are written in Clojure code, which you then run on a Java Virtual Machine. You will need to have a Java Runtime Environment installed on your computer. Let’s make sure you’re up to date. Fire up a terminal and put in this command:

You should see some output something like this:

If you get an error, such as "Command not found" or some variation of that, you probably need to install Java.

The second thing is to make sure that the Java version is at least 1.7.

OK, so now that you are ready to run a Java applications, let’s move on to the Pedestal part.

We’re going to start this project with an empty directory. In practice, most of the time you’ll generate a project from a template. But for now, it’s more important to see what the pieces are and how they fit together.

I’m going to call my project 'hello-world'. Feel free to call yours something different, but it’s up to you to do the mental translation in the rest of this guide.

A quick note on naming style. Clojure itself uses "kebab case" for its names. That’s lowercase words, separated with hyphens. When you have a long name, it looks like the letters have been skewered. Hence, kebab case. Since Clojure’s own libraries use this style, most applications and libraries do too.

We need a place to keep our code. By convention, that’s in a src directory:

Now we’re going to create a file under "src" to hold our code. Call it "src/hello.clj". It’s going to start with a "namespace declaration". This tell Clojure what namespace to put the code into. (If you’re a Java programmer, you can think of a namespace as similar to a package name.)

This is very similar to using import in Java or require in Ruby. It just makes some names from other namespaces available to us in this namespace.

If you haven’t written any Clojure before, this syntax might look a little strange. The first thing that jumps out at people is the parentheses. Why is there an open paren before the "ns"? In Clojure, every expression is enclosed in its very own set of parentheses. There are no semicolons to end the line or curly braces to close an "if" expression. To find the end of any expression, you just find the matching paren.

The first thing in an expression is the function or a macro to call. In this case ns is a macro that is built in to Clojure. It sets up a namespace and makes the stuff we :require available.

Speaking of the stuff we require, what is io.pedestal.http and io.pedestal.http.route? Those are each namespaces from Pedestal libraries. The io.pedestal.http namespace has functions that let you set up HTTP servers, handle requests, and send responses. The io.pedestal.http.route namespace actually comes from a library dedicated to routing (ingeniously named pedestal.route.)

Where the :require says :as something it means we’re making an alias inside our namespace. So io.pedestal.http will be available as just http and io.pedestal.http.route will be available as route. Aliases don’t change anything about the semantics of the namespace we’re bringing in. They just save us some typing.

Routing is split off into its own library because you can use it by itself, or you can use the HTTP library with a different routing layer. We won’t need to do that in this guide, but other tutorials do some pretty amazing things by substituting modules.

Also, don’t worry about the indentation. Your editor might handle that for you, but in any case, Clojure doesn’t care about whitespace.

Whew. That was a lot to unpack from just the first three lines of code! Let’s pause for a moment to talk about the next steps. We’re making a web service that can say hello. That means we need to do some basic things:

We’re going to do all of those things, but we’re going to do them backwards. In Clojure, you always find the most important, highest-level functions at the bottom of the file. Whenever I read a Clojure source file, I start at the bottom and page upward. So the next thing we’re going to do is write a function that can return a response to a "hello world" request.

A Clojure function returns the value of the last expression in the function. In this case, that will be the map that we construct on line 2. This is in "map literal" syntax, which just means that we’re writing the whole map straight in the source code rather than building it up by calling functions.

The map has two keys and their values:

That’s the whole thing. When our function returns that map, Pedestal will translate that into a full HTTP response complete with content type header and everything.

There’s absolutely nothing special about this map. It’s a plain old Clojure map - this is how Clojure operates, we don’t use classes, we use ordinary maps but care about what particular keys are in the map. This is a response map.

Clojure is a functional language, which means that whenever possible, we create simple functions that have no side-effects: their behavior is defined only by the arguments passed in.

This trivial respond-hello handler function is functional — it can be tested entirely by invoking it, passing in a request map, and making assertions about at the response map it returns. There’s no need to start up a server and send a request to it via HTTP, you can just have your tests call this code directly.

That’s one of the beauties of working in Clojure and Pedestal…​ you can try everything interactively in a running system. Let’s do that before we move on.

Pedestal is built on the shoulders of giants, in the form of great open source technology that many people have contributed to. That gives us great power, but with great power comes great dependencies. We could download all the jar files we need and string together a classpath, but it’s a huge pain. I just made a minimal project and found 57 entries on the classpath.

This is why we can have nice things, but it means we need some help managing those dependencies. Fortunately, Clojure provides tooling for dependency management. We’ll be using the clj tool to run our examples. Please take a few minutes to learn more, then come back and we’ll continue.

Now we can make a deps.edn that tells the clj tool what libraries our service needs. This goes in the main directory hello:

We talked about pedestal.service and pedestal.route before, but we have a new one here. Pedestal works with many different HTTP servers, so we don’t want the core library to depend on all of the possible servers out there. Instead, we let you decide which one to use by adding the library for your chosen service. We’re using Jetty for this guide. It is a fast, stable, and mature HTTP server. Best of all, it doesn’t require any installation ahead of time…​ we can start it up from inside our service. That makes our service more self-contained and portable.

Let’s try out our response function.

The clj tool will download dependencies as needed, add them to the classpath and start a repl in the user namespace. Now we’re able to enter Clojure code to evaluate it (turn it into a value). The first thing we need to do is tell Clojure about our hello namespace:

The require function returns nil on success, which the Clojure REPL printed.

If instead, you get a message like this:

It means you missed the single-quote before "hello" in the require.

Now we can actually test the function:

Well, we got the singularly unexciting result that we can call a function and it returns the map that we told it to. Let’s move on to hooking this up to a route.

It’s worth noting that we passed in an empty map ({}) rather than a full request map. This is perfectly acceptable for testing; in fact we could have even passed a nil, as the respond-hello function doesn’t actually use the request parameter.

In Pedestal, routing is the process of matching an incoming request to a handler ^[1].

Let’s tell Pedestal that we want the route GET /greet to map to our handler function:

This routing table is a single route; the route matches:

When routed in a live service, the respond-hello function will be invoked. We’ve named this route :greet. Every route must have a unique name, though in many cases, Pedestal can automatically provide a reasonable name from other information you provide in the rouet.

We’re building up gradually, and have yet to hook this up to an HTTP server, but it’s still possible to test the routing by hand (using code that could eventually be used in a test suite).

The first thing we need to do is tell Clojure to reload the latest code from hello.clj. That’s what the :reload keyword does in the require call. Without that, you’ll get a "cannot resolve symbol" error when you try to reference hello/routes.

The function try-routing-for lets us ask "What would Pedestal do?" with an incoming HTTP request. We gave it our table of one route, then the query string and HTTP verb.

try-routing-for uses the example routing table, from hello/routes to create a temporary router; it then passes the provided method (:get) and path, /greet, to see how it would be routed; if the route matches, a routing entry is returned, which identifies what was matched, and would be used in a live Pedestal application.

When no route in the table matches, try-routing-for will return nil. The REPL sessions shows that a GET to /greet will be a match, but a PUT or POST will not.

On a match, try-routing-for returns an expanded version of the route that was matched (this is the internal format, which is much more cumbersome than the table format we used to define the route); further, some specific details about what was matched was also included.

You might ask, "where is my respond-hello function in all that?" …​ part of what expand-routes does is to convert your handler function into Pedestal’s native unit of work, an interceptor.

We’re ready for that last step: connecting everything to an HTTP server. That’s one more function (at the end of the file) to create the server.

All we need to do now is run it.

Yep, it didn’t return. Jetty is running and listening for connections, though. Flip to a different window and try curl or use a browser to hit http://127.0.0.1:8890/greet.

It’s alive! Treat yourself to a hot beverage and a high five. Whenever you get tired of poking it, just hit Control-C in the terminal that is running the server to kill it.

Later on, we’ll see how to make Jetty run on its own thread so you can get your REPL prompt back.

This might seem complicated because I’ve used so many words to describe all this. The code is pretty short though. For reference, and in case you’ve hit any snags along the way, here are the complete contents of both files.

You can also see all the code in the GitHub repository for this guide.

We’ve covered a lot of ground in this guide. You have learned how to:

Along the way you’ve also learned a bit of Clojure and some debugging tricks.

The next part in this tutorial adds the ability to receive a query parameter, apply logic to it, and return a different response for an error.