Re: question about locking in java singleton class
Hi Mahadev,
Thanks for you reply. Yes, I did check the example implementation.
However, it is not suitable for my purpose as such because it needs a
separate ZooKeeper instance for each WriteLock. I want to support
multiple threads through one ZooKeeper instance (don't want to pay for
connect overhead every time just to get a lock). Is this approach bad?
I'm not sure, but there seems to be a possibility of starvation in the
example code (locks are ordered based on session ID rather than znode
id). I'll open a ticket for this.
Thanks,
-Jaakko
On Fri, Jan 15, 2010 at 2:36 AM, Mahadev Konar wrote:
> 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" 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()
>> {
>>
>>
>>
>> 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
>
>
Re: question about locking in java singleton class
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" 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()
> {
>
>
>
> 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
question about locking in java singleton class
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()
{
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
