On Jun 6, 2008, at 17:42 , Benjamin Reed wrote:
> No. Internally we use an atomic broadcast protocol (which is what  
> Paxos would provide us) to keep the replicas in sync, but the  
> protocol is much simpler than Paxos.

After reading QuorumPeer, I think it is safe to say that Zookeeper  
implements Paxos. The mapping between your code and Lamport's  

Zookeeper -> Lamport (Paxos)
Notification (FastLeaderElection.java) -> Prepare request
NEWLEADER (Leader.java) -> Accept message
(zxid, leader id) -> "proposal number"

It seems more or less equivalent to me, and there are no obvious  
problems with it that I can find.

Unrelated observations from browsing the source:

1. syncLimit has slightly different comments in the class header, and  
inline with the variable.

2. ResponderThread: It reads "state" from the enclosing class. This  
variable is not volatile. My weak understanding of the Java memory  
model is that without doing a "synchronizing operation," the JVM would  
be free to optimize away the access of this "state" variable. That is,  
the ResponderThread could get "stuck" in an old state. I could be  
wrong because this stuff is ridiculously hard. See the FAQ:


The accesses to the currentVote volatile variable probably gives you  
the memory barrier guarantees you need, except when LEADING. In that  
case, the ResponderThread never touches another volatile, so in theory  
the JVM could read state once and by happy with it forever after. Will  
it ever actually do that? I would be surprised. However, I think you  
would be safer making state volatile, so ResponderThread would  
*definitely* see a change immediately.

FastLeaderElection.java line 224: The part of the condition after &&  
is not needed: This is the else branch of an if statement, where the  
condition is exactly the first part. Hence, the part after && *must*  
be true.

FastLeaderElection.java: I think it also has accesses of a set of  
variables between threads with no synchronization or volatiles, such  
as logicalclock.


Evan Jones

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