For every design that is saved on Floorplanner, we create a thumbnail in JPEG format. We use these thumbnails for the gallery, and now we have included them on everyone’s dashboard. However, for various reasons, we do not have a thumbnail available for every design. However, with your help, we are improving the thumbnail database while you our browsing the site!

The thumbnail images are stored on Amazon AWS S3. We know the URL that a thumbnail of a design should have, but we do not know if it actually is available. In the latter case, the result is a nasty image not found placeholder on the dashboard. This of course is not acceptable! We cannot know if a thumbnail exists other than doing a request to the URL and see whether we get an image back from Amazon, or a HTTP 404 status. This is a very time-consuming procedure to run server side so we chose to find a client-side solution.

We found that javascript can be used to check if an image exist. An AJAX call cannot be used, because cross-site calls are not supported. However, the javascript Image object can be used for this purpose.

var img = new Image();
img.onload = function(event) {
  // image was found and loaded successfully
  document.getElementById('img-tag').src = img.src;
};
img.onerror = function(event) {
  // An error occured while loading the image
  document.getElementById('img-tag').src = '/images/thumb-unavailable.jpg';
}
 
// Setting the src property will trigger the events.
img.src = 'http://link.to.amazon.s3/design/thumbnail.jpg';

A nice thumbnail not available image will be shown if the thumbnail file cannot be found on S3. This is much nicer, and the check is completely done client-side! However, we found a way this could even be better by changing the onerror event handler. Instead of displaying a thumbnail not available image, we can simply load a small instance of the Floorplanner application to display a small version of the design. Moreover, we can instruct it to generate a thumbnail JPEG and save it to S3!

So, every time you see a small Floorplanner loading on your dashboard, you are creating a missing thumbnail. Next time, the thumbnail will be available on S3 and there will be no need to start the Floorplanner application. A nice example of distributed computing, mixed with a hint of SETI@home. I like it!