Hi,
I think the following should work with the following caveats:
- there's a small danger that if the server blows up before it
releases the lock, that no threads would be able to update your cache,
but since it's only a cache of data. This is easily mitigated by
introducing a task which periodically resets the lock.
- I still believe that there's the possibility that separate instances
will get a non-updated object, even if you use the locking, since I'm
assuming it takes x-amount of time for the changes in MemCache to
propagate throughout the instances. I may be wrong in this assumption
however.
Anyway, your code would go something like:
boolean lockAcquired = false;
try {
lockAcquired = acquireLock();
doStuff();
} finally {
if (lockAcquired) {
releaseLock();
}
}
With the locking code being:
public static boolean acquireLock() {
boolean lockAcquired = true;
try {
while (increment() != 1) {
decrement();
Thread.sleep(500);
}
} catch (Throwable t) {
// Log error
lockAcquired = false;
}
return lockAcquired;
}
public static void releaseLock() {
decrement();
}
private static void decrement() {
MemcacheServiceFactory.getMemcacheService().increment("LOCK",
-1, 0L);
}
private static Long increment() {
return
MemcacheServiceFactory.getMemcacheService().increment("LOCK", 1, 0L);
}
On Dec 14, 9:56 pm, Jay Young <[email protected]> wrote:
> Grr. Just noticed another issue. The cache item that you get the
> index is not the same item as the lock, so really you need two
> different cache items, one to make sure you have a unique value for
> the lock, and the lock itself. The code above is correct, you just
> need different keys for the first two cache reads.
>
> If you can generate a guaranteed-unique name (maybe the thread ID +
> time?), you can drop the first cache item and use that as the lock
> value. This would also prevent the issue with the code above if the
> lockIndex item is ever evicted and re-read at the same time (thus
> returning 0 for both), or if it overflows the long.
>
> (I honestly don't know if this is iron-clad, but intuitively it seems
> like it would work with only very minor edge cases.)
>
> On Dec 14, 4:08 pm, Andrei Cosmin Fifiiþã <[email protected]>
> wrote:
>
> > Hmmmm, super...
> > I was thinking of smth like (val mod 2) == 0/1 (true/false), but the lock
> > reading wasn't safe.
> > Thanks!
>
> > On 14 December 2010 20:57, Jay Young <[email protected]> wrote:
>
> > > Except return lockIndex should just be return true.
>
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