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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 is 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.
So 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.
Creating a cluster
To create a cluster you must launch a node that can act as leader. Do this as follows:
rqlited -http localhost:4001 -raft localhost:4002 ~/node.1
With this command a single node is started, listening for API requests on port 4001 and listening on port 4002 for intra-cluster communication. This node stores its state at ~/node.1.
To join a second node to this leader, execute the following command:
rqlited -http localhost:4003 -raft localhost:4004 -join http://localhost:4001 ~/node.2
Once executed you now have a cluster of two nodes. Of course, for fault-tolerance you need a 3-node cluster, so launch a third node like so:
rqlited -http localhost:4005 -raft localhost:4006 -join http://localhost:4001 ~/node.3
This entire example assumes, for the sake of clarity, that all nodes are running on the same host.