File sharing, locking, etc... between multiple users...

[David Vaughan]

On 23/02/2006, at 5:00, "Jonathan Lynch" <jonathandlynch at gmail.com>  
wrote:
>
> any other tips?

Rob's comment about supporting 15-30 users is valid although it  
depends also on the transaction rates they can achieve.

Jim's discussion of pre-emptive rights for certain actions or user  
types will add complexity to the locking and checking process. My own  
view is that I would adopt a real client-server database if I were  
going to that level. Given we are talking about a small system and  
user base, pre-emption should not be critical. We are relying on a  
process to give up a lock anyway, which is something else you should  
consider (removing stale locks where a process has died or improperly  
quit). Jim's general strategy of reserving line 1 for metadata is  
very nice. I would stick to flags for locking, though, simply for the  
small speed gain.

To create a flag, generate a random number and create a file of that  
name .flag. To see if you can go ahead, first count the number of  
flag files and, if one, check that it is yours. Remember to remove  
the flag under all exit conditions and consider the tradeoffs of a  
cleanup daemon which arbitrarily removes flags older than X, where X  
is entirely your problem in your context :-). At the least you should  
be able to post a warning message.

Finally, you may need to time your flag setting/checking loop under  
various conditions so you can set a safe minimum wait time to avoid  
races while the maximum does not delay processing too much. We should  
be talking a range of milliseconds.

HTH

regards
David