[Wikipedia-l] Edit locks

[Jimmy Wales]

Here's my understanding of the edit lock problem...

Whenever someone is actively in the process of writing their changes to disk,
the software makes a temporary file (directory) on the disk to let other instances
of the cgi process know that this one is doing some writing to disk.  You don't
want two processes to write to the same file at the same time, because you will
lose one person's changes and possibly the entire file.

Notice that the edit lock is only there when you are *actively writing your changes
to disk*.  This isn't while you are typing them, but only for that instant after
you hit 'submit' and before you get the next screen back.

The problem is that -- for mysterious reasons -- the script sometimes dies and leaves
the edit lock hanging.  All other processes after that refuse to write changes to disk
because they think that someone has the edit lock.

There are several possible solutions.  My own personal solution is wild.  I just
wouldn't use a lock at all.  You run the risk of collisions, but these are so rare
*in my experience* as to be worth ignoring.

Another solution is to have 'per-file' edit locks.  This is more elegant, because 
there isn't any particular reason to lock the ENTIRE system down just because one
person is writing to one file.  If Larry is writing to file A and I am writing to file
B, there is no reason to lock anything.  The main downside to this solution is that
it is still possible for an edit lock to get hung.  The good thing is that only one
page would be affected, not the entire site.

Another solution is to find the reason why the script is leaving the lock, and fix
that bug.  In my experience, this will be impossible.  CGI scripts do in fact die
for no apparent reason sometimes, and it's very hard to trap/debug/etc.  I mean, it
is theoretically possible, but as a practical matter I find that something is always
left hanging.

One of my own personal design philosophies is that everything should fail gracefully.
That is, I accept that my own software sucks and will in fact break.  This is a matter
of humility before the gods of software.  The best that I can hope for is that when my
software does choke, it will do so gracefully.  There will always be bugs and things
that don't work right, but as long as they don't destroy the universe, there's hope.

:-)

--Jimbo




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