How is the fault tolerance of etcd ensured and why is an odd number of replicas needed?

etcd ensures fault tolerance by using a distributed consensus based on the Raft algorithm. Data is replicated across multiple nodes (replicas), and a majority consensus is required to confirm a record — this guarantees data integrity and consistency even in the event of node failures.

Why an odd number of replicas:

• To achieve a majority (quorum), more than half of the nodes are needed.
• With an odd number of replicas, a clear majority can always be determined.
• For example, with 3 replicas, the majority is 2; with 4, it is also 3, but 4 is even, which can lead to ambiguity during split votes.

Thus, an odd number of replicas minimizes the likelihood of split votes and increases fault tolerance.