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:[email protected]] Sent: Monday, July 13, 2015 1:43 PM To: [email protected]; 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:[email protected]] Sent: 13 July 2015 11:48 To: [email protected]<mailto:[email protected]> 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
