Majority in voting for a new primary

Just to clarify.

To perform a vote/election, you must have a majority of the replica set nodes up an running.

So if the replica set had 4 nodes, and 2 were down, then because a majority of 4 is 3, then 2 isn’t enough. Even though the 2 nodes could decide between them who is most capable to carry on the be the primary, based upon who is most up to date (as mentioned in the video)

If the set had 6 nodes and 3 were down, the majority of 4 is not available, so a new primary couldn’t be picked.

Why is the majority so important for elections, or is the concept of majority for something completely different.

If I remember correctly somewhere in the videos it is mentioned a replica set must have an odd number of nodes.

Anyway I found the following reference:
https://docs.mongodb.com/manual/core/replica-set-architectures/

The video recommends an odd number.

It’s just this concept of the “Majority” sub group of the replica set, which must be met before a vote can take place. I suppose it is one of things I will have to accept.

However this table from your link, provides some fault tolerance information.

Number of Members Majority Required to Elect a New Primary Fault Tolerance
3 2 1
4 3 1
5 3 2
6 4 2