[DOCS-11504] Misguided advice to deploy an odd number of members or votes Created: 28/Mar/18  Updated: 30/Oct/23  Resolved: 28/Mar/18

Status: Closed
Project: Documentation
Component/s: manual
Affects Version/s: None
Fix Version/s: Server_Docs_20231030

Type: Bug Priority: Major - P3
Reporter: Andre Spiegel Assignee: Unassigned
Resolution: Works as Designed Votes: 0
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Participants:
Days since reply: 5 years, 46 weeks ago

 Description   

Our docs give the wrong advice that you should have an odd number of voting members in a replica set: https://docs.mongodb.com/manual/core/replica-set-architectures/#deploy-an-odd-number-of-members

There is nothing in the replication protocol or the election algorithm that requires an odd number of voters. Many customers however think that there is such a requirement, and the passage at the above link may reinforce that misunderstanding.

As the page explains one section down, an even number of members does not buy you any additional fault tolerance compared to the next lower odd number, which is why an odd number of members makes sense from an economical standpoint, but there is no technical requirement for this. I suggest removing the first paragraph at the above link entirely, and giving the section a different heading.



 Comments   
Comment by Shannon Bradshaw (Inactive) [ 28/Mar/18 ]

Upon review and some additional internal discussion I don't see any changes warranted. The advice here is the message we should be providing to users. I'm going to go ahead and close this.

Comment by Asya Kamsky [ 28/Mar/18 ]

There is nothing in the replication protocol or the election algorithm that requires an odd number of voters.

There is. If there is a network partition that splits the voting members evenly, there will be no primary. The fact that election protocol requires strict majority is the thing that makes an even number of votes "very bad".

Comment by Asya Kamsky [ 28/Mar/18 ]

I don't think that advice is wrong. You should have an odd number of voting members in your replica set.

The network partition scenario where you can't elect a primary even though every single member of the replica set is up is a good enough reason to give.

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