In short: it seems leader can treat observers as quorum members. Steps to repro:
1. I have a following ensemble configuration: # servers list server.1=localhost:2881:3881 server.2=localhost:2882:3882 server.3=localhost:2883:3883:observer server.4=localhost:2884:3884 server.5=localhost:2885:3885:observer 2. I'm bringing up servers 1,2,3 and it's enough for quorum (1 and 2). 3. I'm shutting down the one from the quorum who is the follower. As I understand, expected result is that leader will start a new election round so that to regain quorum. But the real situation is that it just says goodbye to that follower, and is still operable. (When I'm shutting down 3rd one -- observer -- leader starts trying to regain a quorum). Is this a bug, or a feature? -- Regards, Sergey