Hello World, With Parameters

Now that you’ve gone through the basics in the Hello World guide it’s time to go one level deeper. We’re going to add two features. That will be our vehicle to talk about some important concepts in Pedestal.

After reading this guide, you will be able to:

Like hello-world.adoc, this guide is for beginners who are new to Pedestal and may be new to Clojure. It doesn’t assume any prior experience with a 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.adoc 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.

partial$getting-help.adoc

In this guide, we will build on the hello.clj example from the previous guide. We will enhance it to take a single parameter in the URL and return a friendly greeting to that person.

There are some people we don’t want to meet, though. So we’ll also include a list of names that should not be said. Then we can make our handler function a bit smarter. If any forbidden name is uttered, our handler will return a 404 response instead of a cordial greeting.

If you worked through hello-world.adoc, then you already have all the files you need. If not, take a moment to grab the sources from the repository. That will be our starting point for enhancements this time.

HTTP offers a few different ways for the client to send data up to our service. The most basic is probably the URL query parameter. We usually see these on GET requests, but nothing stops a client from using them on other request types.

Because query parameters are so common, Pedestal handles them automatically. You can try that URL with your service right now.

As a reminder, we’re running the server by using the clj tool to start an interactive session (called a REPL, rhymes with ripple) then starting the service in that session. Before we do that, though, let’s discuss interactive development.

It’s really not optimal to be restarting your REPL in order to restart the service every time you need to make a change. In real Clojure development, we rarely restart the REPL. Instead, we make our system friendly to interactive development. The main problem so far is that starting the HTTP service doesn’t return. We can fix that by adding a key to the map that says "return instead of waiting for the server to stop."

The key ::http/join? does the trick. We can now run start-dev in our REPL sessions. Instead of waiting forever, the REPL thread now returns, prints the value of the server (an ugly mess!) and lets us continue interacting. This is how we are going to start things in dev from now on. Let’s try it out.

First, let’s start the service.

Whatever happened to "no news is good news?" Believe it or not, that is all successful output. The big glob at the end is the value of the server. It’s a big map with all the state in it. The most important thing to do with it is hang on to it so we can stop the server later. We do that by putting it in an atom.

Now we can use start-dev, stop-dev, and restart as we make changes.

From another window, let’s send a request with a query parameter.

What we’ve demonstrated is that Pedestal can route the incoming request, GET /greet?name=Michael and isn’t confused by the presence of the query parameter. Ultimately, the respond-hello function was invoked, and did its job. To update respond-hello to match our goal, we first have to discuss the request argument passed to it.

Also, if you look in the console of your REPL, you’ll see that Pedestal logged the request:

You’ll see slightly different results; the qtp part is the thread name, for example. The point is, you can look in the REPL session to verify that requests are being processed by your server.

Here’s a reminder of what respond-hello currently looks like:

We aren’t using the request argument right now, so we don’t really know what it is. (Well, it’s probably not too hard to guess that it has to do with the HTTP request that Pedestal received. From the title of this section, you probably also deduced that it’s a map!)

Let’s take a look at that request by echoing it back to the client. This is a common debugging trick that you can use.

Make sure you’ve update the hello.clj file, then restart your service with that new definition:

Again, this is in two steps: (require :reload …​) to update all the Clojure code, then (hello/restart) to stop the service and start a replacement, using the update functions.

With the update code live, you can use curl again to see the change in the service’s behavior:

Yikes!

We need to unpack that a bit to make sense out of it. It’s a Clojure map, just printed all in one long line. I’ll reformat it with some line breaks. Syntax highlighting will help us read it too. Feel free to paste this map into your favorite text editor with Clojure syntax highlighting to pick it apart yourself.

I’m also going to omit several keys. They will be relevant later, but they’re not interesting now. And anyway, they don’t print very readably because they’re objects rather than data.

Here’s what we are left with.

This is a reference:request-map.adoc. It really is just a Clojure map. You access its values with get and get-in. For now, we’ll look at three interesting keys.

The whole query string appears under the :query-string key. That’s the portion of the URL after, but not including, the '?' character. The query string can be a pain to deal with. It is URL-encoded and may have multiple parameters with delimiters. Pedestal slices that up into a map of parameters for you, which is attached to the :query-params key. Each parameter name gets turned into a keyword. Parameter values are always strings, though, so you may still have to parse them a bit.

You see that :params duplicates the same map as :query-params. Pedestal merges all the different kinds of parameters into one map for convenience. Most of the time, I know when I want a query parameter so I just use :query-params, though.

Inside our handler function, the :name parameter is available inside a nested map. Let’s dig it out and add it to our response.

Here we can just grab the value right out of the map and use it in the response.

Do the restart/reload dance again, and then use curl to try a few different requests with parameters.

Dang. We were doing really well right up until the end there. Obviously we need to do something smarter when the input is missing. Bad user input is a way of life on the web, so let’s not make an error out of this. Instead, we just won’t personalize the greeting.

Again, reload and restart, then try new requests with curl:

Much better! But our handler function is getting a bit unwieldy. I don’t mean that it is a whole six lines long, but rather that it mixes too many concerns. This function parses inputs, applies "domain" logic, and deals with creating a response body. It would be better to refactor this to separate those concerns.

This version separates the concerns nicely. One big benefit is that the greeting-for function is now easier to test. It’s just a pure function - no side effects - that doesn’t require any HTTP or Pedestal machinery to exercise. Likewise, that ok function is side-effect free and quite easy to test. Both functions are likely to get more complex, but each one will only deal with its own complexity.

In this example, we’re treating the case of an empty name the same as if the name parameter just isn’t included. See if you can make the handler more strict so it will return a "400 Bad Request" response if the name parameter is present but zero length.

There are some names that should not be spoken. Let’s enhance our service to avoid them. This will be a relatively simple change. We can hardcode the names as a set, then use that set as a function. (Clojure sets are functions that test for the presence of their argument in the set itself.)

This part needs to go before greeting-for because we’ll use it to make a decision in that function.

Now we need to think about our greeting function. Right now it returns a string. We want to make sure the user gets a 404 if the query names one of the forbidden ones. We could have greeting-for directly return the response map, but that starts to entangle the concerns we just factored out. We’ll do the simplest thing that could possibly work: is greeting-for returns nil, that means the request failed.

Because there are now three possible conditions that can happen in respond-hello, I’ve changed from an clj:if[] to clj:cond[] expression. cond uses pairs of "condition / value" clauses. So if (unmentionables nm) returns any truthy value, then cond returns nil and stops evaluating. If (empty? nm) returns true, then cond returns the literal string "Hello, world!\n" and nothing else. If neither of those happened, then cond sees the keyword :else as a condition^[1]and the code continues to build the string the same way as before.

The second thing you’ll notice is that the handler now makes a decision whether to call our ok helper or the new function not-found. I do hope you weren’t expecting much in not-found! It’s as simple as it gets.

Restart your service and give it a try:

Looks like it mostly works, though it has some trouble with different capitalization. Take a look at the docs for clojure.string and see if you can figure out how to make the comparison case-insensitive.

Once again, we built this thing in small steps, so it may seem like there was a lot to deal with. The final product is still pretty small though. Here’s all the final code.

In this guide, we built upon hello-world.adoc to add:

We also learned about the [reference:request-map], which is central to any non-trivial handler, as well as one approach to dealing with invalid user input in the request.

So far, almost all our responses have been plain text. (When we returned the request map in the body, the content type was actually application/edn but we can ignore that for now.)

Plain text is ugly, so we want to be able to return HTML. Rich clients don’t like HTML so much, so we also want to return JSON sometimes. The next stop on this trail looks at how Pedestal handles content types and response bodies. This will also be our first taste of interceptors, Pedestal’s core unit of work.