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 <rake...@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.veerapand...@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:rake...@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.veerapand...@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 > > >
