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Zookeeper, mail # user - Getting confused with the "recipe for lock"


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Re: Getting confused with the "recipe for lock"
Hulunbier 2013-01-13, 15:05
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
<[EMAIL PROTECTED]> wrote:
> On Jan 12, 2013, at 2:30 AM, Hulunbier <[EMAIL PROTECTED]> 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