Reloading at the REPL

As Clojure developers, we prefer to live at the REPL, loading and reloading code into the REPL for a lightning fast feedback loop.

It’s common to reload a namespace, and use the REPL (possibly via IDE support) to re-evaluate expressions in our code and see the immediate change. This is true for Pedestal apps as well … we should be able to make changes and see the effects of those changes immediately. But as we’ll see, in practice, accomplishing that takes a little extra structure.

We’re going to show examples of the live reloading failures, then show simple solutions.

Starting Point

Let’s start with yet another basic, "Hello World" application.

(ns org.example.hello
  (:require [io.pedestal.http :as http]))

(defn hello-handler [_request] (1)
  {:status 200
   :body "Hello, World!"})

(def routes
  #{["/hello" :get hello-handler :route-name ::hello]}) (2)

(defn start []
  (-> {::http/port 9999
       ::http/routes routes
       ::http/type :jetty
       ::http/join? false} (3)
      http/create-server
      http/start))

(defonce server (atom nil)) (4)

1	The request map isn’t used here, so we follow the convention of adding a leading underscore; this may also prevent warnings in your IDE.
2	Every route must have a unique :route-name and since we are providing a function, `hello-handler`, it must be explicitly specified.
3	Normally, the call to `io.pedestal.http/start` doesn’t return (until something else shuts down the server); adding `::http/join? false` is necessary for REPL-oriented development.
4	To survive reloading of this namespace, the running server is stored in an Atom.

With that in place, evaluate (reset! server (start)); this will create the server and start it, storing the running system map back into the server atom.

You can now use your favorite HTTP client (such as HTTPIe) to access the /hello route:

> http :9999/hello
HTTP/1.1 200 OK
Content-Security-Policy: object-src 'none'; script-src 'unsafe-inline' 'unsafe-eval' 'strict-dynamic' https: http:;
Content-Type: text/plain
Date: Fri, 18 Nov 2022 23:41:49 GMT
Strict-Transport-Security: max-age=31536000; includeSubdomains
Transfer-Encoding: chunked
X-Content-Type-Options: nosniff
X-Download-Options: noopen
X-Frame-Options: DENY
X-Permitted-Cross-Domain-Policies: none
X-XSS-Protection: 1; mode=block

Hello, World!

So far, so good. You can also see that Pedestal’s default set of interceptors has added a number of security-related headers to the response.

Now we’re ready to make a change. Let’s change the response body to be Hello, Clojure World!. We can change the hello-handler function, reload the namespace, and:

> http -pb :9999/hello (1)
Hello, World!

1	The `-pb` option prints just the response body, omitting the response headers.

That’s strange. It’s as if the code change wasn’t picked up. We can verify that the code change is, in fact, live by evaluating (hello-handler nil) at the REPL:

(hello-handler nil)
=> {:status 200, :body "Hello, Clojure World!"}

What’s happened here is that the value of the org.example.hello/hello-handler Var has changed; it now holds a new Clojure function, one that says "Hello, Clojure World!". However, the routes and start functions captured the value of hello-handler at the time they executed.

We could say ugh, fine and just adapt to a cycle where you reload your REPL code and then restart the server object, but that’s slow and error-prone. We want a fast feedback cycle where that is not necessary, and that goal is entirely achievable.

The first step is to stop the service by evaluating (swap! server http/stop).

Next, change the routes function:

(def routes
  #{["/hello" :get `hello-handler]})

Notice that backtick character before hello-handler. That’s Clojure’s syntax quote; like a single quote, it keeps hello-handler from being evaluated, but unlike single quote, the symbol is namespace qualified. It’s the equivalent of 'org.example.hello/hello-handler.

To understand what’s going on here, we must consider a key stage in Pedestal: route expansion. Route expansion is a step during service startup where the provided routes are converted from one of a number of specifications (such as the table format used here) into an executable format. Part of expansion is a rule that says that a handler that’s a symbol is replaced with the evaluation of that symbol. That’s when org.example.hello/hello-handler is converted from a symbol to function.

Notice that we also no longer have to specify a :route-name; with a handler symbol, a default route name is generated that’s a keyword version of the symbol.

Now, change the function to return "Hello, Programming World!", reload the namespace, and get the /hello URL again:

> http -pb :9999/hello
Hello, Clojure World!

What gives? Where’s our reloading?

Because of the back-ticked symbol, the evaluation of hello-handler occurred later than in the original code, not inside the routes definition, but somewhere inside the call to create-server … but that evaluation and expansion still only happens once, and so there’s still a capture.

We need the route to always reflect the live version of the hello-handler function, and that means we have to avoid capture, not just of the hello-handler function, but of the entire route specification.

Deferred Route Function

Fortunately, Pedestal supports this use case - a route is normally a data structure, but it can also be a zero-arguments function that returns an expanded route.

Stop the service and make these changes:

(ns org.example.hello
  (:require [io.pedestal.http :as http]
            [io.pedestal.http.route :as route])) (1)

(defn hello-handler
  [_request]
  {:status 200
   :body "Hello, Clojure World!"}) (2)

(def routes
  #{["/hello" :get `hello-handler]})

(defn start
  []
  (-> {::http/port 9999
       ::http/routes #(route/expand-routes routes) (3)
       ::http/type :jetty
       ::http/join? false}
      http/create-server
      http/start))

1	We need this additional namespace, `route`.
2	Change the message back.
3	This new route function returns the expanded routes.

Reload and restart the server, and check that we are getting the original behavior:

> http -pb :9999/hello
Hello, Clojure World!

Now, change the message to Hello, JVM World!, reload the namespace, then reload the URL:

> http -pb :9999/hello
Hello, JVM World!

Success! The route function is being evaluated on every incoming request.

Not For Production

Using a function, as we do here, is absolutely not for production. Even a trivial route takes a chunk of time to expand, so doing it on every single request will absolutely trash your production server’s throughput.

Still, we’re not quite done. Let’s say we want to be less formal, and respond on the path /hi instead of /hello. We can change the routes:

(def routes
  #{["/hi" :get `hello-handler]})

But this doesn’t yet work:

> http :9999/hi
HTTP/1.1 404 Not Found
Content-Type: text/plain
Date: Sat, 19 Nov 2022 00:28:29 GMT
Transfer-Encoding: chunked

Not Found

There’s still one more capture: the anonymous function is capturing the value of the routes set. The anonymous function has a lifecycle that starts when the start function is executed, and the routes set’s value is captured at that point in time. In order for route changes to load, we have to ensure that the anonymous route function always returns the current value of routes.

Delaying Route Evaluation

Change the route back to /hello and modify the start function:

(defn start
  []
  (-> {::http/port 9999
       ::http/routes #(route/expand-routes (deref (var routes)))
       ::http/type :jetty
       ::http/join? false}
      http/create-server
      http/start))

Stop and restart the server, and verify that /hello still works.

Now, update the route back to /hi, reload the namespace, and check:

> http -pb :9999/hi
Hello, JVM World!

Done! You can now add, remove, or otherwise change your routes, update interceptors, add constraints … wherever your development takes you, and those changes will be re-evaluated and re-loaded on each request.

How does this work?

The (var routes) captures the Var named routes (rather than the value stored inside the Var), and the deref pulls the current route specification out of the Var, This happens on every invocation of the function - on every request. If you like, you can abbreviate (deref (var routes)) to @#'routes.

Wrap Up

Running code with a long-lived and stateful service creates its own challenges when coding live at the REPL; in this guide we’ve explained how capturing interferes with live reloading, and provided the necessary steps to keep your REPL flowing as you develop your application.

Just remember … you want to make sure your production application doesn’t use these reloading techniques if you want to meet your SLAs (service level agreements)!