how to lock a file in ftp site

Kinsley Turner kingsleyturner at westpac.com.au
Tue Dec 20 00:55:37 CET 2005



> I've come across this problem - instead of uploading the file to it's
> 
> intended directory, upload it to a temp directory, with a unique name
> (say uuuuuyyyymmddhhmmss where uuuuu is username and the rest are
> obvious) and then *rename* it to it's correct name in the real
> destination directory. Do the move in a try... except block and if it
> fails, take evasive action.
> 
> In FTP a rename (i.e. move) is usually an atomic action but an upload is
> not. You have an unavoidable race condition here trying to lock a file
> (even if ftp allowed it) but with the method described above, you can
> avoid it, but get the same effect.

This is a good method for transferring files, it handles broken-transfers
especially well (if something on the other end is waiting for a particular 

filename.)

But I think it just moves the locking problem from the
upload to a rename.  What happends if two clients try to rename their
files at a half-second interval: both succeed, but the first-clients 
changes
are clobbered.  Actually, speaking of atomic functions, maybe you could
use directory creation instead of a rename, because only one directory
creation can succeed.  This might be able to be incorperated into some
sort of locking mechanism.

cheers,
-kt























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