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From Matthieu Morel <>
Subject Re: Some Questions regarding features of S4
Date Thu, 12 Jan 2012 09:59:16 GMT
On 1/12/12 10:25 AM, omkar wrote:
> I have the following questions about S4?
>  1.
>     What fault tolerance mechanism(apart from checkpoints, which we know
>     will be part of S40.4.0 ) does S40.3.0 supports?

Currently fault tolerance relies on an automatic fail-over from standby 
nodes, coordinated through Zookeeper: if an S4 node crashes, this is 
detected by Zookeeper, which notifies standby nodes. One of these 
standby nodes takes over the task for the logical partition previously 
assigned to the failed node, and start receiving and processing events 
for that partition. In 0.3, the stand-by node starts from an empty state.

Note that more sophisticated mechanisms - aiming at limiting event and 
state loss - are possible to implement, and we are considering them as 
well, but there is no implementation yet.

>  2.
>     Will the events leave the S4cluster in the same order as they enter
>     the S4cluster? If yes, I would like to know what mechanism it uses
>     which qauranteesthat.

I don't think there is any guarantee about that, since the streams are 
split among various independent nodes. But you could implement some kind 
of piggy-backing to track ordering of messages, then reorder them before 
they leave the cluster. That might be costly though.



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