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From Alexander Shraer <shra...@gmail.com>
Subject Re: ZooKeeper ensemble. Size and Impact ?
Date Mon, 13 Jul 2015 16:11:39 GMT
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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