Rack Middleware: Driven Almost to Completion

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Continuing the unexpectedly long implementation of my Rack::ThumbNailer Rack middleware, I next describe that the middleware needs to pull images from the target application:

      it "should pull the image from the target application" do
        Rack::ThumbNailer.should_receive(:rack_image)
        get "/foo.jpg", :thumbnail => 1
      end

I make the rack_image another class method due to my inability to stub the target application. To make that pass:

    def call(env)
      req = Rack::Request.new(env)
      if !req.params['thumbnail'].blank?
        filename = "foo"

        ThumbNailer.rack_image(@app, env)
        ThumbNailer.mk_thumbnail(filename)

        thumbnail = ::File.new(filename).read
        [200, { }, thumbnail]
      else
        @app.call(env)
      end
    end

When creating the thumbnail file, the mk_thumbnail class method should use the result of ThumbNailer.rack_image. To describe this in RSpec, I add a default output for ThumbNailer.rack_image in the before(:each) setup block:

    context "with thumbnail param" do
      before(:each) do
#...
        Rack::ThumbNailer.
          stub!(:rack_image).
          and_return("image data")
      end

I can then specify that mk_thumbnail should use the return value of rack_image ("image data"):

      it "should generate a thumbnail" do
        Rack::ThumbNailer.
          should_receive(:mk_thumbnail).
          with(anything(), "image data")
        get "/foo.jpg", :thumbnail => 1
      end

To make that pass, I add a second argument in the call method:

    def call(env)
      req = Rack::Request.new(env)
      if !req.params['thumbnail'].blank?
        filename = "foo"

        image = ThumbNailer.rack_image(@app, env)
        ThumbNailer.mk_thumbnail(filename, image)

        thumbnail = ::File.new(filename).read
        [200, { }, thumbnail]
      else
        @app.call(env)
      end
    end

Rather than stick with the anthing() matcher in the mk_thumbnail example, I can specify the filename for the cached copy of the image. It should be a concatenation of the cache directory, plus the path_info for the image in the target application.

To support this, I add a default cache directory to the constructor for Rack::ThumbNailer:

  class ThumbNailer
    DEFAULT_CACHE_DIR = '/var/cache/rack/thumbnails'

    def initialize(app, options = { })
      @app = app
      @options = {:cache_dir => DEFAULT_CACHE_DIR}.merge(options)
    end
...

The make-the-thumbnail-file example can then read:

      it "should generate a thumbnail" do
        Rack::ThumbNailer.
          should_receive(:mk_thumbnail).
          with("/var/cache/rack/thumbnails/foo.jpg", "image data")
        get "/foo.jpg", :thumbnail => 1
      end

To make that pass, I update the call method to:

    def call(env)
      req = Rack::Request.new(env)
      if !req.params['thumbnail'].blank?
        filename = @options[:cache_dir] + req.path_info

        image = ThumbNailer.rack_image(@app, env)
        ThumbNailer.mk_thumbnail(filename, image)

        thumbnail = ::File.new(filename).read
        [200, { }, thumbnail]
      else
        @app.call(env)
      end
    end

Nice, that is a full-featured thumbnail generating bit of Rack middleware, save for the two class methods, which I largely borrow from my spike the other day (I deleted the code, but kept the notes):

    private
    def self.rack_image(app, env)
      http_code, headers, body = app.call(env)

      img_data = ''
      body.each do |data|
        img_data << data
      end
      img_data
    end


    def self.mk_thumbnail(filename, image_data)
      path = filename.sub(/\/[^\/]+$/, '')
      FileUtils.mkdir_p(path)

      ImageScience.with_image_from_memory(image_data) do |img|
        img.resize(200, 150) do |small|
          small.save cache_filename
        end
      end
    end

I would prefer to test these as well, but my inability to set expectations / mock these instance method makes such an endeavor more trouble that it is worth. So I keep them small, focused and rely on the axiom that one does not test private methods.

Tomorrow, I will add cache hit/miss functionality and that should finish of my middleware.