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, 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 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’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.

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, 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.

First, let’s start the Pedestal connector.

The start function has done four things:

Now we can use start, stop, and restart functions as we make changes. Mostly, we’ll make code changes, reload the hello namespace, and call restart to quickly stop the connector and start a new one.

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 greet-handler function was invoked, and did its job. To update greet-handler 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 may see slightly different results, but the takeaway is that you can look in the REPL session to verify that requests are being processed by your server.

Here’s a reminder of what greet-handler currently looks like:

We aren’t using the _request argument right now, so we don’t really know what information it provides.

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 connector and start a replacement, using the update functions.

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

Yikes! There’s a lot of content in the request map echoed back to us, and it’s in one huge line.

We need to unpack that a bit to make sense out of it. Let’s add some line breaks and indentation to make it easier to read. We’ll also omit several keys. They may be relevant later, but they’re a distraction now.

Here’s what we are left with.

This is a request map. Again, a generic Clojure map with specific keys. You access its values with the get and get-in functions. 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, which means that some common characters, such as spaces and punctuation, are converted into sequences of several characters. There’s also delimeters between name/value pairs in the query string.

Pedestal parses and slices all that up into a map of parameters for you, which is attached to the request map as the :query-params key. Each parameter name is converted into a keyword. Parameter values are always strings, though, so you may still have to parse them a bit.

Pedestal has several different sources of parameters in the request, which we’ll cover in more detail later. The :params key is the merged map of all params from all sources; :query-params is just the parameters from the query string. You get to choose which of these is right for your particular handler.

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.

You might have noticed that the name query parameter in the URL shows up as the :name key in the :query-params map, but ultimately ends up in a local symbol, greet-name.

It is tempting to just use the symbol name here as well, for consistency, rather than the longer and less obvious greet-name.

Long experience has shown that some symbol names, those that conflict with functions and macros automatically imported from the clojure.core namespace, can cause you great frustration. Using name here shadows the function clojure.core/name.

Consider this easy mistake:

This looks correct at a glance, but consider what happens when it is executed:

clojure.core$name@6b202ecb is how the clojure.core/name function is printed as a string. We can verify that in the REPL:

So because we used greet-name in one place and name in the other we got really strange results; this is partly because Clojure, as a dynamically typed language, is very permissive …​ to a point:

So, the moral of the story is to avoid shadowing names! With that, let us continue on to handling bad input in the request.

In those last couple of curl calls, we did not properly pass up a name query parameter:

This is not desirable behavior; we should do better than returning half a sentence.

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 also 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: it should 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 act as 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-for 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: when greeting-for returns nil (instead of a string), that means the request failed.

Because there are now three possible conditions that can happen in greet-handler, I’ve changed from using an if expression to a cond expression. cond uses pairs of "condition / value" clauses. So if (unmentionables greet-name) returns any truthy value, then cond returns nil and stops evaluating. If (empty? greet-name) 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 compact and to the point. Here’s all the final code.

In this guide, we built upon Hello World to add:

We also learned about the 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.

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.