How many instances of ETCD are required for fault tolerance?

To ensure high availability in an ETCD cluster, it is recommended to have an odd number of instances — at least 3. This is because ETCD uses the Raft consensus algorithm, which requires a majority (quorum) to make decisions.

For example, with 3 instances, the cluster can withstand the failure of one node and continue to operate. If there are 2 instances, losing one will result in loss of quorum and service inaccessibility. With 5 instances, it can withstand the failure of two nodes.

In summary:

• Minimum of 3 instances for basic high availability
• Preferably an odd number for optimal quorum operation

This ensures a balance between reliability and infrastructure costs.