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


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Getting confused with the "recipe for lock"
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!
NEW: Monitor These Apps!
elasticsearch, apache solr, apache hbase, hadoop, redis, casssandra, amazon cloudwatch, mysql, memcached, apache kafka, apache zookeeper, apache storm, ubuntu, centOS, red hat, debian, puppet labs, java, senseiDB