I’m using rake-pipeline on a project at the moment and wanted to implement cache-busting by renaming asset files if they have changes.

A quick look at rake-pipeline-web-filters and we see the Cache Buster filter, it looks just the job:

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output "public"

input "app" do
  match "javascripts/**/*.coffee" do
    coffee_script
  end

  match "javascripts/**/*.js" do
    concat "javascripts/app.js"
    cache_buster
    uglify
    gzip
  end
end

Ok, not so fast. That would have been a rather short blog post…

It turns out that the cache buster doesn’t work if it’s after a concat and this is an issue which is impossible to fix with the way rake-pipeline works.

Rake pipeline filters generate the filenames they output when they are initialized, which means you can’t have a filter which has a filename based on the output of a previous filter as you’d want for cache busting. When the cache buster tries to generate the filename the concat hasn’t yet happened so it has no contents to work with.

If we can’t dynamically change the filename in a filter, how do we write a cache buster for rake-pipeline which actually works? We need to check all the files before rake-pipeline runs and use that knowledge to set the output name, something like:

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output "public"

JS_VERSION = ?? # magic happens here

input "app" do
  match "javascripts/**/*.coffee" do
    coffee_script
  end

  match "javascripts/**/*.js" do
    concat "javascripts/app-#{JS_VERSION}.js"
    uglify
    gzip
  end
end

But how do we set JS_VERSION?

We want to generate some kind of key based on the contents of the files in the javascripts directory which will only change if there’s a change to the files. We could iterate through each of the files and get the MD5 hash of each, then take an MD5 of all the hashes to generate one master hash, or maybe we loop through and find the most recent mtime and generate a key from that? All sounds a bit messy and resource intensive for such a simple task.

If only there was a way we could find out the last change of a file in a directory, some kind of system which tracked all the versions of our files that we could ask…

Turns out git is quite good at tracking changes to files, it’s also rather easy to get a log of what’s changed in a directory and we can ask for the hash of the most recent change with git log -n1 --pretty=format:"%H" app/javascripts.

Plugging that into the above we get:

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output "public"

JS_VERSION = `git log -n1 --pretty=format:"%H" app/javascripts`

input "app" do
  match "javascripts/**/*.coffee" do
    coffee_script
  end

  match "javascripts/**/*.js" do
    concat "javascripts/app-#{JS_VERSION}.js"
    uglify
    gzip
  end
end

Now we have a new JS_VERSION any time there are any changes committed to the app/javascripts directory.

Job done!