Re: Sessions lock - not using memcached?

On Sep 26, 2010, at 1:10 AM, ROZE Samuel wrote: > Hello, > > I'm working with sessions in my PHP applications. My pages can take long time to be > executed (~500ms) and I've simultaneous requests for the same session (Ajax requests). My > problem is that I close sessions at the end of my PHP script, so my sessions files are locked for > half a second. So, simultaneous requests (for the same session) can't be executed... > simultaneously! > > My question is if I use memcached to store sessions, will my sessions be locked or can I access > to them in the same time? If there aren't it should solve my problem. > > A second question is performances: is memcached sessions faster the PHP default sessions? I > think yes because it doesn't make disk I/O..? > No, the default memcache session handler has no locking. But that presents you with a new problem of a race condition. This is why you shouldn't keep any actual data in sessions: R1:[initial request start, session is read and has X=1] A1:[ajax request 1 reads session, X=1] A2:[user clicks "add 1 to my shopping cart", X = X+1 = 2] A2:[displays shopping cart with X == 2] A2:[faster than A1, session over, write session with X = 2] A1:[session over, write session with X = 1] DOH! There has to be some kind of locking on data between read/write to maintain consistency. One way you can get around this is by *not* using the session for mutable data, and just using it to store a key to the data in your backend, and then only writing when you must. The above becomes R1:[initial request start, session is read and has shopping cart SID=1] A1:[ajax request 1 reads session, SID=1] A2:[user clicks "add 1 to my shopping cart", UPDATE carts SET X = X + 1 WHERE SID = 1] A2:[displays shopping cart with X == 2] A2:[faster than A1, session over, write session with SID=1] A1:[session over, write session with SID=1] Also, the memcache session handler can be dangerous in that it might choose to delete your session before expiration if memcached fills up, so be aware of this. Good luck!