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From <Ben.Yo...@sungard.com>
Subject RE: Leader election in C# client
Date Tue, 26 Nov 2013 10:19:38 GMT
Thanks Flavio, makes sense. I'm trying to convince myself that the effect is the same even
though it doesn't strictly order in the actual sequential order, as long as all the clients
see the same consistent order!

I also notice the code only watches for the previous going away and then assumes it's the
leader, which I think is wrong, it should re-run the check on previous nodes again I think


-----Original Message-----
From: Flavio Junqueira [mailto:fpjunqueira@yahoo.com] 
Sent: 26 November 2013 10:10
To: user@zookeeper.apache.org
Subject: Re: Leader election in C# client

Hi Ben,

I'm not familiar with the C# implementation, but I suspect that:

- The session id is indeed to be able to identify the znode in the case the client sees a
connection loss event and reexecutes the create;
- The sequence number creates an order among the bidders so that when a master crashes (ephemeral
znode goes away), not all clients are notified. This solves the herd effect problem: many
nodes being notified upon a single znode going away.


On Nov 26, 2013, at 9:57 AM, <Ben.Young@sungard.com> wrote:

> Hi,
> I was looking at the leader election algorithm in the .NET client at https://github.com/ewhauser/zookeeper/blob/trunk/src/dotnet/ZooKeeperNet.Recipes/LeaderElection.cs
> There is a point I don't understand and I was wondering if someone could clarify for
me. The recipe creates ephemeral nodes of the form "election-<sessionid>-<sequentialid>",
I think to be able to see if it has already created the node (perhaps due to a retry?). However,
doesn't having the session id in the node name then throw out the sorting of the nodes in
the next step, to find the previous node to watch? I'd have thought it would then be sorting
by session id and not sequential id? Perhaps that doesn't matter as long as only one node
gets picked?
> Thanks,
> Ben
> Ben Young . Senior Software Engineer . Adaptiv . Position, Risk & Operations . SunGard
. Enterprise House, Histon, Cambridge, CB24 9ZR .  Tel +44 (0) 1223 266042 . ben.young@sungard.com
. www.sungard.com

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