Add filesize or hash after URL to get correct caching

Question

I am administrating a web page were we have an HTML dokument linking to PDF-files. The PDF-files gets updated from time to time, but we don't want to change the file names. This means that the users get old cached copies of the files, and have to refresh the files manually in order to get the newest file.

I added the following code to the links:

onClick="this.href=this.href.split('?')[0]+'?'+new Date().getTime()">

This solved the problem were the users got old files, but introduced a problem were the user needs to load PDFs even though they have not been updated. This causes more server load, and longer wait times for the users. Is it possible to get a similar code were the script checks a hash or the file size of the target file and adds that to the URL behind the questionmark? If this is possible I would overcome all my problems.

Answer 1

I dont know where you got access to but i assume you can use php.

So you should append an md5 (generated by md5_file()) as parameter to your string. The parameter will only change, if you upload a new pdf (mtime() will have the same effect)