GetThumbnailImage not always gets call on ECL Provider

Question

I've improved an Amazon S3 ECL provider got from here and it's working fine. My problem is when I open an image (jpg for example) from the Mountpoint.

Sometimes it shows the image in the General tab and sometimes it shows only the icon (both being .jpg images). If I click on both (either the image or the icon) I can correctly see the image.

Debugging the ECL Provider I see that, when debugging the image that is shown, it enters the code only once and GetThumbnailImage is not called. When debugging the one that shows the icon, the code is called twice, once getting the image like the one above, the second one, it enters into GetThumbnailImage method.

When it is expected to enter GetThumbnailImage method and when not?

Answer 1

The successful flow to display a thumbnail is this:

1. The UI request the ECL item (if you have opened an item) or a list of items (if you are browsing a folder in thumbnail view).
2. The Tridion UI builds up a URL for the thumbnail. This URL will include the etag returned from the ECL provider as well as the requested size - ensuring any change to the thumbnail will result in a new URL.
3. If the browser does not have a cached thumbnail it makes a request to the UI backend.
4. The UI backend server check if it has a cached thumbnail with the correct etag and size. If it does, this is returned with "cache and don't ask me again headers". If not:
5. The UI backend calls the EclService (in the CoreServiceHost) requesting the thumbnail.
6. The EclService calls GetThumbnailImage in the provider and returns the data to the UI backend server.
7. The UI backend server stores the thumbnail in the file system cache and return the image, again with "cache and don't ask me again headers"

So it is expected you do not see multiple calls to GetThumbnailImage for the thumbnail that is successful - as it is picked up by the cache.

For the image not being displayed correctly, check HTTP error response, ECL log files, and general Tridion logs to see if there is an error message.