how to lock a file in ftp site

[Kinsley Turner]

> am trying to write some information into the file, which is located in
> ftp, and this file can be updated by number of people, but if at all i
> download a file from the ftp to my local machine, update it and then
> upload it back to ftp, and at the same time if some one else downloads
> the same file for the modification, then the data will be overwritten.

As far as I'm aware, there's no way to 'lock' a file via FTP.
Some FTP servers might give this sort of functionality, but then
your solution becomes system dependant.

A simple sort of solution might a 'lockfile'.  When you want to
edit this remote file, your machine checks for '<filename>.lock'
which contains (*shrug*) the IP address/username/timestamp of the
person editing the file.  If it doesn't exist, it should try to
create the file, then check again that it indeed managed to get
it's name into the lock.  You still have a contention issue with
multiple-users uploaded the lock-file at the same time.  So on
the re-check, if the file is corrupt, it can be deleted, otherwise
if the lock-owner is you, the edit may continue.  Once the edit
is complete, the lockfile is removed.

You might have to think about the lockfile mechanism a bit - If
you find the lockfile is corrupt (i.e.: more than 1 client tried
to write it at the same time), would you erase it then wait some
random period of seconds?  or just try to achieve the lock again?

What about stale lockfiles?  If the timestamp of the file is more 
than N hours/minutes/seconds old, can anyone remove it?

Anyway, a 100% complete solution to this problem might be difficult,
but a 99% solution is fairly easy.

hope this helps,
-kt

























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