Why is tiebreaker only needed in a 2 zone setup?
Pricing for 2 zones:
Pricing for 3 zones:
Because if the nodes lose contact with each other they could form their own cluster - aka split brain.
But 3 nodes means as long as 2 are connected you have a majority.
In recent versions this is not the case. The reason a tiebreaker is needed is that you need to have 3 master eligible nodes in order to be able to allow the cluster to operate even if one node goes down. If you only have 2 nodes both need to be available for the cluster to operate (majority of 3 is 2, majority of 2 is also 2).
The reason 3 zones are cheaper seems to be that the tiebreaker is more expensive than one of your data nodes even though it is smaller. I wonder if this is because deploying across 3 availability zones is better and they are encouraging this best practice through pricing as it leads to less support issues.
Yep, good point.