Remote clients will upload images (and perhaps some instructional files in specially formatted text) to a “drop folder.” Once the upload is complete we need to begin processing these images. It would be an easy, but flawed, solution to just have a script automatically begin processing any files in the folder every few seconds (the files can be move out of the folder once processed); but problems would arise when attempting to process large images which are only partially transfered.
What are some tricks I can use to ensure the files are fully uploaded before processing them?
A few of my own thoughts:
The script can check the validity of the file; ie, a partial jpeg would result in an error and you could respond to that error in the script, this would be fairly CPU intensive though. Some files have special markers on the end, but I can’t count on this, I’m not sure what formats I’ll be dealing with.
I’ve heard of “file handles” but haven’t really figured out the basics of what they are and how I can tell if there is a “file handle” on a particular file. Basically the FTP daemon (actually, I’m on Windows, so “service”) would keep a “handle” on the file while it’s being uploaded and you would know not to process that file. These are just a few of my thoughts but I’m not really sure if they will work or if there are better or more accepted ways of solving this problem.
If you have an server-side script upload system (PHP, ASP, JSP, whatever), you could instruct the script to call another script to process the files, or to create a flag-file indicating the upload is done, something like this.
If your server is Linux-based, you can use lsof to check if the file is open. As your ftp/script/cgi will close the file after upload completes, lsof will not show the file in the list.
If your server is Windows-based, you can use Process Explorer to list the open files.