[ZOOKEEPER-1692] Add support for single member ensemble

• Type: Improvement
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Affects Version/s: 3.4.0
• Component/s: quorum
• Assignee: Thawan Kooburat
• Reporter: Thawan Kooburat
• Created: 15/Apr/13 22:49
• Updated: 08/Apr/17 09:34

Description

In the past, we ran into problem where quorum could not be formed multiple times. It take a while to investigate the root cause and fix the problem.

Our current solution is to make it possible to run a quorum with a single member in it. Unlike standalone mode, it has to run as LeaderZooKeeper server, so that the observers can connect to it.

This will allow the operator to use this workaround to bring back the ensemble quickly while investigating the problem in background.

The main problem here is to allow the observers to connect with the leader when the quorum size is reduced to one. We don't want to update the (static) configuration on the observer since it require server restart. We are thinking of allowing the observer to connect to any participant which declared that it is the leader without running the leader election algorithm (because it won't have enough votes).