Hi Jaakko,
The lock recipe has already been implemented in zookeeper under
src/recipes/lock (version 3.* I think). It has code to deal with
connectionloss as well. I would suggest that you use the recipe. You can
file jira's in case you see some shortcomings/bugs in the code.
Thanks
mahadev
On 1/14/10 1:32 AM, "Jaakko" <rosvopaalli...@gmail.com> wrote:
> Hi,
>
> I'm trying to provide mutex services through a singleton class
> (methods lock and unlock). It basically follows lock recipe, but I'm
> having a problem how to handle connection loss properly if it happens
> during mutex wait:
>
> pseudocode/snippet:
>
>
> public class SingletonMutex implements Watcher
> {
> private Integer mutex;
> private ZooKeeper zk;
>
> public void process(WatchedEvent event)
> {
> synchronized (mutex)
> {
> mutex.notifyAll();
> }
> }
>
> private String lock()
> {
> <create ephemeral znode>
> <find children and do related checks>
>
> if (there_is_somebody_with_smaller_number)
> {
> mutex.wait();
>
> if (isConnected() == false)
> throw new Exception("foobar");
> }
> }
> }
>
> Question is: If there is a server disconnect during mutex.wait, the
> thread will wake up without having any means to continue (or delete
> the znode), so it throws an exception. However, if this is only due to
> connection loss (and not session expire), the lock znode it has
> created previously will not be deleted, thus resulting in a deadlock.
> One way would be to use sessionId in lock name and check if we already
> have the lock when entering lock method. However, since this is
> singleton class and used by multiple threads, that approach won't
> work. Using thread ID for this purpose or some form of internal
> bookkeeping is also not very robust. Currently I just do zk.close and
> get another instance on connection loss, which seems to solve the
> problem.
>
> Is there any other way to do this?
>
> Thanks for any comments/suggestions,
>
> -Jaakko
