If you've ever spent any time learning Rails then you probably read one of the editions of Agile Web Development with Rails, and if you're like me (skeptical & pedantic) then you probably asked yourself: what the hell does Rails have to do with Agile development? At the time, I assumed that Dave and David were merely capitalizing on the buzz around Agile; however, even if that's the case, I think they did manage to highlight one of my favorite aspects to building websites with Rails: The ability to make a change, reload the page and see the results makes you a much more agile programmer - where 'agile' is defined as: Characterized by quickness, lightness, and ease of movement; nimble

It turns out, it's not very hard to get that same productivity advantage in Clojure as well. I would go so far as to say that the ability to change the server while it's running is assumed if you're using emacs+slime; however, what's not often mentioned is that it's also possible (and trivial) to reload your server code (while it's running) even if you're using IntelliJ, scripts, or anything else.

The majority of the servers I'm working on these days have some type of web UI; therefore, I tie my server side code reloading to a page load. Specifically, each time a websocket is opened the server reloads all of the namespaces that I haven't chosen to ignore. The code below can be found in pretty much every Clojure application that I work on.

(defonce ignored-namespaces (atom #{}))

(defn reload-all []
  (doseq [n (remove (comp @ignored-namespaces ns-name) (all-ns))]
    (require (ns-name n) :reload )))

Like I said, when I open a new websocket, I call (reload-all); however, the (reload-all) fn can be called on any event. When discussing this idea internally at DRW, Joe Walnes pointed out that you could also watch the file system and auto-reload on any changes. That's true, and the important take-away is that you can easily become more productive simply by finding the appropriate hook for what you're working on, and using the code above.

The ignored-namespaces are important for not reloading namespaces that don't ever need to be reloaded (user); other times you'll have a namespace that doesn't behave properly if it's reloaded (e.g. I've found a record + protocol issue in the past, so I don't dynamically reload defrecords in general).

The change-reload webpage-test loop is nice for making changes and seeing the results very quickly - and I strongly prefer it to having to stop and start servers to see new functionality.