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From Rakesh <rakeshr.apa...@gmail.com>
Subject Re: ZooKeeper ensemble. Size and Impact ?
Date Tue, 14 Jul 2015 04:41:40 GMT
Thank you Alex for the info.

Hi Srini, I think Observer would be fine for your case. Probably you can try it out.

-Rakesh

On 13 Jul 2015 21:41, Alexander Shraer <shralex@gmail.com> wrote:
>
> In 3.4 releases you can't connect an observer to a standalone zookeeper 
> server, but in 3.5.0 
> if you set standaloneEnabled=false your server will run in a "distributed" 
> mode even if its the only one and 
> you'll be able to have observers or reconfigure adding more servers later 
> if needed. 
>
> On Mon, Jul 13, 2015 at 5:34 AM, Rakesh R <rakeshr@huawei.com> wrote: 
>
> > 
> > >>>>> Is it so that only ensemble would be down but other functions
would 
> > be up and running like data-sync ... ? 
> > Say, if a ZooKeeper server lost connection with the quorum. It will 
> > shutdown all the services and try to join the quorum by starting internal 
> > election algo. There is a special type of "read-only" server, on connection 
> > lost, it will automatically transition to r-o mode and serve only the 
> > requests from r-o client. Please visit 
> > http://zookeeper.apache.org/doc/trunk/zookeeperAdmin.html for more 
> > details about r-o feature. 
> > 
> > >>>>> My need to run only 2 ZKS as I'm ok with have +1 copy of the
data. 
> > Is there a way to run a dummy ZKS in any of the instance ? 
> > There is a typical 'Observer' server mode which will act as an observer 
> > and only syncup data with the Leader server, but I'm not really sure 
> > whether it will work along with Standalone server. I haven't tried yet, 
> > probably you can do a try 
> > http://zookeeper.apache.org/doc/r3.5.0-alpha/zookeeperObservers.html 
> > 
> > To begin with, you can run both as Participant and later if you want to 
> > change servers you can use reconfig feature, 
> > http://zookeeper.apache.org/doc/trunk/zookeeperReconfig.html 
> > In 1+1 deployment, tolerated failure is 0 and you should ensure both 
> > servers are up & running for the availability of ZooKeeper service. I could

> > see one advantage of this approach is, you have a backup 'dataDir'. 
> > Administrator can use this if one is lost. 
> > 
> > 
> > -Rakesh 
> > -----Original Message----- 
> > From: Srinivasan Veerapandian [mailto:srinivasan.veerapandian@ericsson.com 
> > ] 
> > Sent: 13 July 2015 15:01 
> > To: user@zookeeper.apache.org 
> > Subject: RE: ZooKeeper ensemble. Size and Impact ? 
> > 
> > Rakesh & Garry, 
> > 
> > 
> > 
> > Thanks for the information and details.  From both of your responses I can 
> > see that, more failures will cause drop of quorum automatically. 
> > 
> > Is it so that only ensemble would be down but other functions would be up 
> > and running like data-sync ... ? Sorry If this is very basic question. 
> > 
> > 
> > 
> > I see a note below note, does this means we can form ensemble with 
> > leaderServes turned ON. 
> > 
> > Turning on leader selection is highly recommended when you have more than 
> > three ZooKeeper servers in an ensemble. 
> > http://zookeeper.apache.org/doc/r3.3.2/zookeeperAdmin.html 
> > 
> > My need to run only 2 ZKS as I'm ok with have +1 copy of the data. Is 
> > there a way to run a dummy ZKS in any of the instance ? 
> > 
> > 
> > 
> > Thanks, 
> > 
> > Srini 
> > 
> > -----Original Message----- 
> > From: Rakesh R [mailto:rakeshr@huawei.com] 
> > Sent: Monday, July 13, 2015 1:43 PM 
> > To: user@zookeeper.apache.org; Srinivasan Veerapandian 
> > Subject: RE: ZooKeeper ensemble. Size and Impact ? 
> > 
> > 
> > 
> > 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<mailto: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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