The big project that I have been working on for quite some time now is a complete change to the architecture of the add-ons manager backend. It’s a big scary prospect since (IMHO) the code is pretty crucial to the success of Firefox and many other Mozilla based applications. Without extensions I don’t think we’d be where we are today, in fact it was because of extensions that I got involved in the project in the first place.

The current incarnation of the extension manager has served us well over the past years but for some time it has been under strain. There are always more features we want to add but certain aspects of its design make it hard to do that. Things like changing from an RDF based persistent storage to anything else are hard because the concepts are so ingrained in the code. It’s now at the point where changing anything (particularly in the startup code) is very difficult since unexpected side effects are almost sure to happen. I tried various approaches to slowly iterating the current code to something better but ultimately changing anything required changing everything so I finally bit the bullet and concluded that a rewrite was the way forward (I know, normally they aren’t the right choice but I’m, pretty confident that it is in this case).

It is that rewrite that I’ve been working on on and off for something like half a year, if not longer. Some of the nice benefits that we should see:

  • Switching from RDF to a SQLite storage model (and making it easier to switch in the future)
  • Startup and other performance improvements (probably not immediately though)
  • Less bothersome dialogs for the user
  • Vastly simpler APIs for application/extension developers including:
    • Proper separation of the backend and UI code
    • Support for pluggable add-on types

I didn’t actually mean for this post to be this long actually (after all it is about simplification), it was just this last point that I felt like demonstrating. I just wrote the basic part of the code that implements the automatic daily background update checks. It uses the new API that will be available and as well as checking for updates it also downloads and installs any it finds silently (currently, I’ll be adding the notification and controls etc. soon). It struck me just how simple it was to write this:

AddonManager.getAddonsByType(null, function(addons) {
  addons.forEach(function(addon) {
    if (addon.permissions & AddonManager.PERM_CANUPGRADE) {
        onUpdateAvailable: function(addon, install) {

That is some 11 lines of code to perform an update check to the add-on’s update server, retrieve information about the newest available update and start downloading and installing it.