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From Rakesh R <rake...@huawei.com>
Subject RE: ZooKeeper ensemble. Size and Impact ?
Date Mon, 13 Jul 2015 08:12:33 GMT
Hi Srini,

ZooKeeper service will be available if 'quorum' number of servers are running(simple majority
voting factors).

I could see, one of the reason to get a majority vote is to avoid "split-brain" problem. In
a network failure we don't want the two parts of the system to continue as usual. We need
only one part to continue and the other to understand that it is out of the cluster and keep
quiet.

The main reason for suggesting odd number is, with even there won't get much benefit to the
tolerated failures in terms of majority. With 3 and 4 servers, we could see the majority is
2 and 3. But in both the cases, the tolerated number of failure is 1.

Quorum = Leader + Followers,
(2n+1) nodes can tolerate failure of 'n' nodes.

For example, 
n=0, (2*0+1) -> 1 server = standalone. Here there is no quorum majority.
             -> 2 servers = majority is 2. So it needs min 2 servers to form quorum. Tolerated
failure is 0, if >0 failure will drop quorum automatically.

n=1, (2*1+1) -> 3 servers = majority is 2. So it needs min 2 servers to form quorum. Tolerated
failure is 1, if >1 failures will drop quorum automatically.
             -> 4 servers = majority is 3. So it needs min 3 servers to form quorum. Tolerated
failure is 1, if >1 failures will drop quorum automatically.

n=2, (2*2+1) -> 5 servers = majority is 3. So it needs min 3 servers to form quorum. Tolerated
failure is 2, if >2 failures will drop quorum automatically.
             -> 6 servers = majority is 4. So it needs min 4 servers to form quorum. Tolerated
failure is 2, if >2 failures will drop quorum automatically.

n=3, (2*3+1) -> 7 servers = majority is 4. So it needs min 4 servers to form quorum. Tolerated
failure is 3, if >3 failures will drop quorum automatically.
             -> 8 servers = majority is 5. So it needs min 5 servers to form quorum. Tolerated
failure is 3, if >3 failures will drop quorum automatically.


-Rakesh

-----Original Message-----
From: Srinivasan Veerapandian [mailto:srinivasan.veerapandian@ericsson.com] 
Sent: 13 July 2015 11:48
To: user@zookeeper.apache.org
Subject: ZooKeeper ensemble. Size and Impact ?

Hi,

We know ZK demands odd number of servers to provide reliability.
My requirement on having zookeeper in my application is to "know the application status" from
all the clients(Max 100).
And today my application can support deployment 1+1(=2) to N+1(=100) Given this I would like
to go with 2 ZK servers in two different instances because adding one more server for this
purpose would be demand one more instance itself in my 1+1 deployment model.

Questions:


1.       What would happen to ensemble formed ? Would the service goes down automatically
?

2.       What would be the impact if number ZK server instances are even (E.g. 2)

How do I size a Zoo Keeper ensemble (cluster)?
https://cwiki.apache.org/confluence/display/ZOOKEEPER/FAQ

Designing a Zoo Keeper Deployment
http://zookeeper.apache.org/doc/r3.1.2/zookeeperAdmin.html


Thanks,
Srini


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