Question

This one will be interesting…

I have seen many asp.net thumbnail generation tutorials / sample code, but no one has considered the problem of concurrency access when generating thumbnail image dynamically, when one or more user access the same page when the thumbnail needs to be generated.

A simple case, i have a site with property images (houses etc.), images are stored in a folder, the thumbnails are generated (for gallery) when someone first time accesses particular offer, then a handler makes the thumbnails from original larger images, the handler generate each thumbnail only once and then use the generated image in further requests.

What happens if two users access this page in the same time, the handler could run twice on the same file or more, there could be concurrency problem, file opening errors and so on (file needs to be opened for thumbnail generation).

Normally one user gets the thumbnail and other get a blank box without image till they refresh the page (since the first user triggered the thumbnail creation)

So the question is, how to avoid this situations ?

Answer 1

Normally if you are only opening the original image file for reading in order to generate the thumbnail there is no problem accesing it concurrently. Multiple users can open the same file for reading at the same time. Problems arise if you start writing at the same time.