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From Ted Dunning <ted.dunn...@gmail.com>
Subject Re: What is the time it takes for one of the follower nodes to be elected as leader
Date Tue, 19 Jul 2011 16:44:38 GMT
Yes.  It is related to session expiration.

The key trade-off is how real-time your client heartbeats can be and how
many false positive detections of failure you can stand to have.  If your
client can be very reliable about delivering heartbeats, then you can
reliably detect failure very quickly.  In a normal Linux environment,
however, it is very hard to guarantee detection without false positives with
expiration less than about 2 seconds.  Remember that it isn't just your
application that has to be pretty precise here, but also the secondary ZK
and the ZK leader.  If you have an unfortunate concatenation of short delays
for whatever reason, you can blow out a short deadline pretty easily.

I do not know what limits Zookeeper sets on lower bounds for session
expiration time.

In general, if you need sub-second fail-over, you probably need to use an
active-active architecture.  For instance, if you are recording streamed
data, the master can write the data normally and the non-master can buffer
several seconds of data.  On failure, the non-master can become master and
write however much of the buffered data as necessary to complete the record.

If you need real-time response from a master, you generally have to go with
something like duplicate masters.  In such a system, requests go to multiple
machines and both reply as quickly as possible.  Duplicate replies are
dropped on the floor.  On failure, you run with the survivor while you
recruit a new duplicate master.



On Tue, Jul 19, 2011 at 9:30 AM, zookenthu <gurupr@gmail.com> wrote:

> Is it related to session timeout? if so what is the recommended minimum for
> session timeout?
>
> --
> View this message in context:
> http://zookeeper-user.578899.n2.nabble.com/What-is-the-time-it-takes-for-one-of-the-follower-nodes-to-be-elected-as-leader-tp6599284p6599379.html
> Sent from the zookeeper-user mailing list archive at Nabble.com.
>

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