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
