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Cluster Guidelines
This document describes, in detail, how to create and manage a rqlite cluster.
Practical cluster size
Clusters of 3, 5, 7, or 9, nodes are most practical. Clusters with a greater number become unweildy.
Clusters with an even-number of nodes
There is no point running clusters with even numbers of nodes. To see why this imagine you have one cluster of 3 nodes, and a second of 4 nodes. In both cases a majority of nodes are required for the cluster to reach consensus. A majority is defined as N/2 + 1 where N is the number of nodes in the cluster. For a 3-node a majority is 2; for a 4-node cluster a majority is 3. Therefore a 3-node cluster can tolerate the failure of a single node, and 4-node cluster can also only tolerate a failure of a single node. A 4-node cluster is no more fault-tolerant than a 3-node cluster, so running a 4-node cluster provides no advantage over a 3-node cluster. Only a 5-node cluster can tolerate the failure of 2 nodes.
An analogous argument applies to 5-node vs. 6-node clusters, and so on.