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From Stephen Skeirik <skeir...@illinois.edu>
Subject ZooKeeper Client Session Management
Date Fri, 03 Aug 2012 19:10:08 GMT
Dear All,

I am working on a model of the ZooKeeper software for academic purposes. 
Since it is only a model, I don't want to replicate every detail of the 
system. However, I also want to write the model in such a way that it 
can be extended to show lower-level detail without a major rewrite (that 
is, I want to have the details in mind, even if I don't make use of them 
now).

In particular, right now I am working on modelling client session 
management, and I was hoping someone could fill in the gaps in my 
understanding. The standalone case seems straightforward: the 
ZooKeeperServer creates a SessionTrackerImpl which invokes the 
ZooKeeperServer's expire method to end sessions when the session has 
timed out. However, I can't seem to put all the pieces together for the 
distributed case. From my reading, if a client is connected directly to 
the leader, his session management is handled as in the standalone case. 
However, if the client is connected to a Follower, it is different. The 
Followers report all of their session data to the leader, who actually 
manages session expiration. Since the Leader has a SessionTrackerImpl, 
eventually, the Leader's expire() method will be called to initiate 
ending the session.

But the two things I can't seem to figure out are:

1) How does the Follower get notified of that session expiration so that 
it can remove the client from its SessionTracker?

2) Similarly, how does the Follower know to terminate its ServerCnxn 
with the client?

And a related question, which I just thought to ask:

3) Can a client's session persist indefinitely if leader election keeps 
flip-flopping (its unlikely to happen I know, but I'm just curious)?

Any insight would be awesome!
Thanks,

Stephen Skeirik

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