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From Andrey Stepachev <oct...@gmail.com>
Subject Re: Getting confused with the "recipe for lock"
Date Fri, 11 Jan 2013 14:48:07 GMT

Yes, this scenario is very likely.
But it will work only for long running tasks (more then session timeout),
for short livinig tasks lock will be unlocked before session timeout,

In case of long living locks, Client1 should track disconnection from zk
cluster and assume, that lock was abandoned (and somehow notify lock owner
about that). Client can know value of session timeout and spawn timer, and
action accordingly program logic. As example it can interrupt thread, which
created lock, and rise some flag, so long running task can know - lock is
not valid.

On Fri, Jan 11, 2013 at 5:46 PM, Zhao Boran <hulunbier@gmail.com> wrote:

> While reading the zookeeper's recipe for
> lock<http://zookeeper.apache.org/doc/trunk/recipes.html#sc_recipes_Locks>,
> I get confused:
> Seems that this recipe-for-distributed-lock can not guarantee *"any
> snapshot in time no two clients think they hold the same lock"*.
> But since zookeeper is so widely adopted, if there were such mistakes in
> the reference doc, someone should have pointed it out long time ago.
> So, what did I misunderstand? please help me!
> Recipe-for-distributed-lock (from
> http://zookeeper.apache.org/doc/trunk/recipes.html#sc_recipes_Locks)
> Locks
> Fully distributed locks that are globally synchronous, *meaning at any
> snapshot in time no two clients think they hold the same lock*. These can
> be implemented using ZooKeeeper. As with priority queues, first define a
> lock node.
>    1. Call create( ) with a pathname of "*locknode*/guid-lock-" and the
>    sequence and ephemeral flags set.
>    2. Call getChildren( ) on the lock node without setting the watch flag
>    (this is important to avoid the herd effect).
>    3. If the pathname created in step 1 has the lowest sequence number
>    suffix, the client has the lock and the client exits the protocol.
>    4. The client calls exists( ) with the watch flag set on the path in the
>    lock directory with the next lowest sequence number.
>    5. if exists( ) returns false, go to step 2. Otherwise, wait for a
>    notification for the pathname from the previous step before going to
> step 2.
> Considering the following case:
>    -
>    Client1 successfully acquired the lock(in step3), with zk node
>    "locknode/guid-lock-0";
>    -
>    Client2 created node "locknode/guid-lock-1", failed to acquire the lock,
>    and watching "locknode/guid-lock-0";
>    -
>    Later, for some reasons(network congestion?), client1 failed to send
>    heart beat message to zk cluster on time, but client1 is still perfectly
>    working, and assuming itself still holding the lock.
>    -
>    But, Zookeeper may think client1's session is timeouted, and then
>    1. deletes "locknode/guid-lock-0"
>       2. sends a notification to Client2 (or send the notification first?)
>       3. but can not send "session timeout" notification to client1 in time
>       (due to network congestion?)
>    -
>    Client2 got the notification, goes to step 2, gets the only node
>    ""locknode/guid-lock-1", which is created by itself; thus, client2
> assumes
>    it hold the lock.
>    -
>    But at the same time, client1 assumes it hold the lock.
> Is this a valid scenario?
> Thanks a lot!


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