Using Rmagick with eventmachine

Question

I am trying to create a very simple web service with Goliath and Grape. All that my service would do is that given an image path and a target dimension it would return the new geometry of the image. The images are stored in the same server as the host of the web service.

So I have this code within Grape:

# some Grape code omitted
get "/" do
  EM.defer {
    image = Magick::Image.read('path to image').first
    image.change_geometry('3000x3900') do |cols, row, img|
      return {width: cols, height: row}
    end
  }
end

When I visit the endpoint in the browser all I get is this string

"#<ConditionVariable:0x007ffd9de1f6e8>"

Without EM.defer it returns the following json but with very low requests/secs (about 4 reqs/sec):

{width: 'new width', height: 'new heigth'}

How could I make the Rmagick operations non blocking and make it return the results?

See http://eventmachine.rubyforge.org/EventMachine.html#defer-class_method, `op` parameter is the `&block` from your code, it's executed in a separate thread, the `callback` parameter on the other hand is the block of code the reactor triggers once `EM.defer` returns the result. See if you can return response from EM.defer's callback block — bbozo, Dec 06 '13 at 14:14

score 0 · Answer 1 · answered Dec 06 '13 at 11:19

0

It looks like you mixed things up a bit.

Call to RMagick is always blocking, since it requires the image to be processed. The good news is the call to grape itself is non-blocking. It means, that while your single clients must wait for claimed 1/3 secs to complete, another clients are still able to queue requests.

I guess your testing environment simply calls the respective service one-by-one, waiting for requests to complete. Instead of that you are to call the service from separate threads, getting results as they are ready.

Hope it helps.

answered Dec 06 '13 at 11:19

Aleksei Matiushkin

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I am aware that Rmagick would block my application, hence I tried using EM.defer with it. Perhaps there is something wrong with my implementation of EM.defer? – Lester Celestial Dec 06 '13 at 11:24
I suggest you have `Sinatra` behind your `Grape`, right? You don’t need to re-implement `Sinatra` functionality, please refer to `threaded` option [here](http://www.sinatrarb.com/intro.html). Anyway, implementing `EM#defer` inside of the `get` block is wrong, since client waits for the response here and you can’t prevent blocking from within already blocked method. – Aleksei Matiushkin Dec 06 '13 at 11:32
You can, however, not block the reactor thread for all other requests, this is why OP uses EM.defer – bbozo Dec 06 '13 at 14:12

Using Rmagick with eventmachine

1 Answers1