Question

I have a website running PHP+MySQL. It is a multiuser system and most of the MySQL tables are MyISAM-based.

The following situation got me puzzled for the last few hours:
I have two (concurrent) users A,B. Both of them will do this:

1. Perform a Read Operation on Table 1
2. Perform a Write Operation on another Table 2 (only if the previous Read operation will return a distinct result, e.g. STATUS=”OK”)

B is a little delayed towards A.

So it will occur like this:

• User A performs a read on Table 1 and sees STATUS=”OK”.
• (User A Schedules Write on Table 2)
• User B performs a read on Table 1 and still sees STATUS=”OK”.
• User A performs Write on Table 2 (resulting in STATUS=”NOT OK” anymore)
• User B performs Write on Table 2 (assuming STATUS=”OK”)

I think I could prevent this if Reading Table 1 and Writing to Table 2 were defined as a critical section and would be executed atomically. I know this works perfectly fine in Java with threads etc., however in PHP there is no thread communication, as far as I know.

So the solution to my problem must be database-related, right?
Any ideas?

Thanks a lot!

Answer 1

The Right Way: Use InnoDB and transactions.

The Wrong-But-Works Way: Use the GET_LOCK() MySQL function to obtain an exclusive named lock before performing the database operations. When you’re don, release the lock with RELEASE_LOCK(). Since only one client can own a particular lock, this will ensure that there’s never more than one instance of the script in the “critical section” at the same time.

Pseudo-code:

SELECT GET_LOCK('mylock', 10);
If the query returned "1":
    //Read from Table 1
    //Update Table 2
    SELECT RELEASE_LOCK('mylock');
Else:
    //Another instance has been holding the lock for > 10 seconds...