Monitor a FTP site for arrival of new/updated files

[python at bdurham.com]

Any suggestions on a best practice way to monitor a remote FTP
site for the arrival of new/updated files? I don't need specific
code, just some coaching on technique based on your real-world
experience including suggestions for a utility vs. code based
solution.
My goal is to maintain a local collection of files synced with a
remote FTP site and when I download a new/updated file locally,
run a script to process it. The arrival and format of the files
that I need to sync with are beyond my control (eliminating a
rsync solution) ... all I have is a generic FTP connection to a
specific FTP address. Note: The remote site I'm monitoring may
have multiple uploads occuring at the same time.
My basic strategy is to poll the remote directory on a regular
basis and compare the new directory listing to a previous
snapshot of the directory listing. If a file timestamp or size
has changed (or a new file has appeared), then track this file as
a changed file. Once a file has been marked as changed, wait <N>
polling cycles for the file timestamp and size to remain stable,
then download it, and trigger a local script to process the file.
In addition to detecting new or changed files, I would compare
remote directory listings to my local sync folder and delete
local files that no longer exist on the remote site.
My concern about using a utility is the utility's ability to
detect when a remote file has finished being updated. I don't
want to download files that are still in the process of being
updated - I only want to download new/updated files after they've
been closed on the remote site.
Any ideas appreciated!
Thanks,
Malcolm
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