That's a good question, and I'm not sure - I'm fairly new to both ZK and Cassandra. I found this wiki page: http://wiki.apache.org/hadoop/ZooKeeper/FailureScenarios and I think the lock recipe still works, even if a stale read happens. Assuming that wiki page is correct.
There is still subtlety to locking with ZK though, see (Locks based on ephemeral nodes) from the zk mailing list in October: http://mail-archives.apache.org/mod_mbox/zookeeper-user/201110.mbox/thread?0 -Bryce On Fri, 6 Jan 2012 13:36:52 -0800 Drew Kutcharian <d...@venarc.com> wrote: > Bryce, > > I'm not sure about ZooKeeper, but I know if you have a partition > between HazelCast nodes, than the nodes can acquire the same lock > independently in each divided partition. How does ZooKeeper handle > this situation? > > -- Drew > > > On Jan 6, 2012, at 12:48 PM, Bryce Allen wrote: > > > On Fri, 6 Jan 2012 10:03:38 -0800 > > Drew Kutcharian <d...@venarc.com> wrote: > >> I know that this can be done using a lock manager such as ZooKeeper > >> or HazelCast, but the issue with using either of them is that if > >> ZooKeeper or HazelCast is down, then you can't be sure about the > >> reliability of the lock. So this potentially, in the very rare > >> instance where the lock manager is down and two users are > >> registering with the same email, can cause major issues. > > > > For most applications, if the lock managers is down, you don't > > acquire the lock, so you don't enter the critical section. Rather > > than allowing inconsistency, you become unavailable (at least to > > writes that require a lock). > > > > -Bryce >
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