On Wed, Jul 07, 2010 at 10:01:05PM -0400, Bastien Koert wrote: > On Wed, Jul 7, 2010 at 8:47 PM, Paul M Foster <paulf@xxxxxxxxxxxxxxxxx> > wrote: > > On Wed, Jul 07, 2010 at 12:59:30PM -0400, tedd wrote: > > > >> Hi gang: > >> > >> I have *my way* of handling this problem, but I would like to hear > >> how you guys do it. > >> > >> Here's the problem -- let's say you have a database containing names > >> and addresses and you want "approved" users to be able to access the > >> data. As such, a user must login before accessing an editing script > >> that would allow them to review and edit the data -- nothing > >> complicated about that. > >> > >> However, let's say you have more than one user accessing the editing > >> script at the same time and you want to make sure that any changes > >> made to the database are done in the most efficient manner possible. > >> > >> For example, if two users access the database at the same time and > >> are editing different records, then there's no real problem. When > >> each user finishes editing they simply click submit and their changes > >> are recorded in the database. However, if two (or more) users want to > >> access the same record, then how do you handle that? > > > > Use a DBMS? I'm sorry if that seems flippant, but a DBMS handles this by > > queuing the requests, which is one of the advantages of a client-server > > DBMS. > > > > So maybe I don't understand your question. > > > > Paul > > > > -- > > Paul M. Foster > > > > -- > > PHP General Mailing List (http://www.php.net/) > > To unsubscribe, visit: http://www.php.net/unsub.php > > > > > > @Paul, > > The OPs question is about concurrency on the record itself. How to > avoid two users accessing the same record and potentially damaging > each others changes > > My approach is the same as Rob's. Flag it locked and let the second > user gets a read only copy I can't think of a way to do this using MySQL or PostgreSQL. And one of the biggest issues with the solution you suggest is the user who opens a record for writing and then goes out for coffee. Everyone's locked out of the record (for writes) until they come back and finish. Okay, to solve that, we start a timer. But when the locker's time is up, how do we let the locker know they're not allowed to store whatever edits they've made? And how do we fix it so that those locked out are now unlocked? Plus, they're probably in a queue, so we really only let one of them know that they can now make edits. Since this is a PHP list, I assume we're talking about a web interface. So how do we do all this back end jockeying? Javascript is about the only way. But every time you fire off one of these javascript dealies, it has to be on its own timer so that it can let the user know that the original locker is gone and now the golden ticket is yours. It essentially has to sleep and ping, sleep and ping. Actually, it's more like a spinlock. But a spinlock would eat CPU for every user, if it was running on the server. So it would have to be running on the client, and "ping" the server every once in a while. Then you'd have to figure out some kind of messaging infrastrucure for the DBMS, so that it would quickly answer "pings" without tying up a lot of CPU cycles. It would have to be something outside the normal query infrastructure. When you actually get into this, it's an incredibly complex solution. I vote instead for allowing edits to be queued, log changes to the database. If there is a true contention problem, you can look at the journal and see who made what edits in what order and resolve the situation. The best analogy I can think of is when using a DVCS like git, and trying to merge changes where two people have edited the same area of a file. Ultimately, git throws up its hands and asks a human to resolve the situation. Bottom line: I've heard about concurrency problems since I started using databases, and I've never heard of a foolproof solution for them that wasn't incredibly complex. And I don't think I've ever seen a solution in actual practice. If I'm wrong, someone show me where it's been viably solved and how. Paul -- Paul M. Foster -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
