...

Observer Based Promotion Observer based self promotion also has merits because we do pull based model already. It could just monitor its lagging from the leader, and when the gap is consistently below a certain threshold for a couple of rounds of fetch, we may call this observer ready to be joining the group. This special logic could be embedded inside the observer when handling BeginQuorumEpoch request sent from the leader. It will put the request in a delayed queue and only reply the leader for a success once the observer assumed itself as in-sync. However at the moment, we want to centralize the information about the quorum instead of letting it scatter around and increase the interaction complexity. In that sense, this approach may not be preferable.

References

• Ongaro, Diego, and John Ousterhout. "In search of an understandable consensus algorithm." 2014 {USENIX} Annual Technical Conference ({USENIX}{ATC} 14). 2014.

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  Ongaro, Diego. Consensus: Bridging theory and practice. Diss. Stanford University, 2014.
• Howard, Heidi, et al. "Raft refloated: Do we have consensus?." ACM SIGOPS Operating Systems Review 49.1 (2015): 12-21.