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From Hulunbier <hulunb...@gmail.com>
Subject Re: Getting confused with the "recipe for lock"
Date Tue, 15 Jan 2013 01:52:25 GMT
Hi Vitalii,

Thanks a lot, got your idea.

Suppose we are measuring the time of events outsides the system(zk & clients) .

And we have no client side time tracking routine.

And t_i < t_k if  i < k

t_0 :

client1 has created lock/node1, client2 has created lock/node2;
client1 thinks itself holding the lock; client2 does not, and watching
lock/node1.

t_1 :

ZK thinks client1's session is timeout(let's say, client1 is actually
failed to send heart-beat message on time, due to a long pause of jvm
gc).

ZK deletes lock/node1,
sends timeout message to client1,
sends "node_not_exist" message to client2 (or send this message before
the deletion, but it does not matter in our case)

but for some reason, link between zk and client1 becomes very unstable,
high packet loss, large amount of packet retransmission,
which leads to a significant packet transmission delay(between client1
and zk only), but the tcp connection is NOT broken.

t_2:

client2 got the "node_not_exist" event, and issues the getChildren Cmd

t_3:

client2 found the only node lock/node2, and thinks itself holding the
lock, and begins acting like a lock owner.

(at the same time, client1 is also thinking itself holding the lock)

t_4:

session_timeout message not reach client1 yet,

client1's jvm gc completed, doing something as the lock-owner.

t_5:

network becomes stable, finally, the session_timeout message sent from
zk reached client1;

client1 thinks itself no longer holding the lock, but it is too late,
it has done something really bad between t_4 and t_5.

--------------------------

Sorry for the grammar, I am not a native English speaker.


On Mon, Jan 14, 2013 at 11:38 PM, Vitalii Tymchyshyn <tivv00@gmail.com> wrote:
> There are two events: disconnected and session expired. The ephemeral nodes
> are removed after the second one. The client  receives both. So to
> implement "at most one lock holder" scheme, client owning lock must think
> it've lost lock ownership since it've received disconnected event. So,
> there is period of time between disconnect and session expired when noone
> should have the lock. It's "safety" time to accomodate for time shifts,
> network latencies, lock ownership recheck interval (in case when client
> can't stop using resource immediatelly and simply checks regulary if it
> still holds the lock).
>
>
>
> 2013/1/14 Hulunbier <hulunbier@gmail.com>
>
>> Hi Vitalii,
>>
>> > I don't see why clock must be in sync.
>>
>> I don't see any reason to precisely sync the clocks either (but if we
>> could ... that would be wonderful.).
>>
>> By *some constrains of clock drift*, I mean :
>>
>> "Every node has a clock, and all clocks increase at the same rate"
>> or
>> "the server’s clock advance no faster than a known constant factor
>> faster than the client’s.".
>>
>>
>> >Also note the difference between disconnected and session
>> > expired events. This time difference is when client knows "something's
>> > wrong", but another client did not get a lock yet.
>>
>> sorry, but I failed to get your idea well; would you please give me
>> some further explanation?
>>
>>
>> On Mon, Jan 14, 2013 at 6:37 PM, Vitalii Tymchyshyn <tivv00@gmail.com>
>> wrote:
>> > I don't see why clock must be in sync. They are counting time periods
>> > (timeouts). Also note the difference between disconnected and session
>> > expired events. This time difference is when client knows "something's
>> > wrong", but another client did not get a lock yet. You will have problems
>> > if client can't react (and release resources) between this two events.
>> >
>> > Best regards, Vitalii Tymchyshyn
>> >
>> >
>> > 2013/1/13 Hulunbier <hulunbier@gmail.com>
>> >
>> >> Thanks Jordan,
>> >>
>> >> > Assuming the clocks are in sync between all participants…
>> >>
>> >> imho, perfect clock synchronization in a distributed system is very
>> >> hard (if it can be).
>> >>
>> >> > Someone with better understanding of ZK internals can correct me, but
>> >> this is my understanding.
>> >>
>> >> I think I might have missed some very important and subtile(or
>> >> obvious?) points of the recipe / ZK protocol.
>> >>
>> >> I just can not believe that, there could be such type of a flaw in the
>> >> lock-recipe,  for so long time,  without anybody has pointed it out.
>> >>
>> >> On Sun, Jan 13, 2013 at 9:31 AM, Jordan Zimmerman
>> >> <jordan@jordanzimmerman.com> wrote:
>> >> > On Jan 12, 2013, at 2:30 AM, Hulunbier <hulunbier@gmail.com>
wrote:
>> >> >
>> >> >> Suppose the network link betweens client1 and server is at very
low
>> >> >> quality (high packet loss rate?) but still fully functional.
>> >> >>
>> >> >> Client1 may be happily sending heart-beat-messages to server without
>> >> >> notice anything; but ZK server could be unable to receive
>> >> >> heart-beat-messages from client1 for a long period of time , which
>> >> >> leads ZK server to timeout client1's session, and delete the
>> ephemeral
>> >> >> node
>> >> >
>> >> > I believe the heartbeats go both ways. Thus, if the client doesn't
>> hear
>> >> from the server it will post a Disconnected event.
>> >> >
>> >> >> But I still feels that, no matter how well a ZK application behaves,
>> >> >> if we use ephemeral node in the lock-recipe; we can not guarantee
"at
>> >> >> any snapshot in time no two clients think they hold the same lock",
>> >> >> which is the fundamental requirement/constraint for a lock.
>> >> >
>> >> > Assuming the clocks are in sync between all participants… The server
>> and
>> >> the client that holds the lock should determine that there is a
>> >> disconnection at nearly the same time. I imagine that there is a certain
>> >> amount of time (a few milliseconds) overlap here. But, the next client
>> >> wouldn't get the notification immediately anyway. Further, when the next
>> >> client gets the notification, it still needs to execute a getChildren()
>> >> command, process the results, etc. before it can determine that it has
>> the
>> >> lock. That two clients would think they have the lock at the same time
>> is a
>> >> vanishingly small possibility. Even if it did happen it would only be
>> for a
>> >> few milliseconds at most.
>> >> >
>> >> > Someone with better understanding of ZK internals can correct me, but
>> >> this is my understanding.
>> >> >
>> >> > -Jordan
>> >>
>> >
>> >
>> >
>> > --
>> > Best regards,
>> >  Vitalii Tymchyshyn
>>
>
>
>
> --
> Best regards,
>  Vitalii Tymchyshyn

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