2 members cannot form a quorum in a 5 node setup. You cannot guarantee a quorum split across two data centers will withstand the loss of either data center. You must have a tiebreaker node in a third data center.
C On Dec 12, 2015 9:46 PM, "Kaushal Shriyan" <[email protected]> wrote: > On Sat, Dec 12, 2015 at 8:07 PM, Flavio Junqueira < > [email protected]> wrote: > > > Hi Kaushal, > > > > See comments below: > > > > > On 12 Dec 2015, at 04:07, Kaushal Shriyan <[email protected]> > > wrote: > > > > > > Hi, > > > > > > I have couple of questions which are as below :- > > > > > > 1) In a 3 node single ensemble, i have 1 leader and 2 followers. when i > > > explicitly shutdown down the leader for testing purpose, leader > election > > > triggers in between the remaining 2 nodes and i see that one of the > node > > > becomes a leader and the other is a follower. I am not sure if i > > completely > > > understood it. Since there are only 2 nodes how does leader election > > happen > > > because we need odd numbers in order to form a quorum. Please correct > me > > if > > > i am understanding it wrong. > > > > > > > You need an odd number of total members in the ensemble. If you have an > > ensemble of 3, then you can tolerate one crashed server, which means that > > your quorums have size two. That's why two servers can elect a leader > among > > them. > > > > In general, if n is the number of servers in the ensemble, then you can > > tolerate f crashed servers and the relationship between n and f is n = > 2f + > > 1. > > > > > 2) Is it possible to configure 3 nodes each in DC1 and DC2? > > > meaning 1 ensemble consists of 6 nodes with 3 nodes in DC-1 and 3 nodes > > in > > > DC-2 > > > DC-1 1 Leader 2 Followers > > > DC-2 1 Follower 2 Observers. > > > > > > if DC-1 goes down, will there be a leader election between 3 nodes in > > DC-2? > > > Please advise. > > > > No, the two observers won't participate in the election, and the > remaining > > participant in DC-2 will keep waiting for DC-1 to come back up. > > > > This question about tolerating a DC going down using a second DC is a > > classic one. If we allowed DC-2 to elect a leader in the total absence of > > processes of DC-1, then we would be prone to split brain scenarios: DC-1 > > suspects DC-2 and vice-versa, but they are both up. If you want > > availability when a DC going down, you need a third DC. Also, keep in > mind > > that availability here refers to both reads and writes. You can set it up > > to go into read-only mode. > > > > -Flavio > > > > > Thanks Flavio for a detailed explanation. Based on the explanation. > > If i lets say if i set the below config > > DC-1 1 Leader and 2 Followers > DC-2 2 Followers and 1 observer > > Is this a good solution meaning 1 ensemble having 6 nodes. In my earlier > setup it was *1 Follower and 2 Observers in DC-2*. Now i changed it in DC-2 > 2 Followers and 1 observer. In this case there are 2 Followers which will > participate in the leader election to form a quorum. Please comment. > > > Regarding "If you want availability when a DC going down, you need a third > DC. Also, keep in mind that availability here refers to both reads and > writes. You can set it up to go into read-only mode" > > you say set up the below configs for high availability. > > DC-1 1 Leader and 2 Followers > DC-2 2 Followers and 1 observer > DC-3 2 Followers and 1 observer. > > Please advise. > > Regards, > > Kaushal >
