Utilizing a Replica Set - MongoDB High Availability - page 57

Understanding the arbiter node's role
An arbiter node doesn't have any dataset, hence it doesn't accept read/write operations.
The arbiter member votes in the replica set election process for a secondary node to be-
come primary . An arbiter node comes useful when you have an even number of rep-
lica set members or not enough servers for a new member.
Note
Add an arbiter node to a replica set only when there is an even number of members. If
you add an arbiter node to a set with an odd number of members, the set might suffer
from tied elections. For more information, you can visit http://docs.mongodb.org/manual/
core/replica-set-arbiter/ .
The following diagram shows you a replica set with an even number of members:
The preceding diagram enables a replica set to have an odd number of votes and prevent
the replica set from the tied election problem.