Read-only nodes

rqlite supports adding read-only nodes. You can use this feature to add read scalability to the cluster if you need a high volume of reads, or want to distribute copies of the data nearer to clients – but don’t want those nodes counted towards the quorum. These types of nodes are also known as non-voting nodes.

What this means is that a read-only node doesn’t participate in the Raft consensus system i.e. it doesn’t contribute towards quorum, nor does it cast a vote during the leader election process. Just like voting nodes, however, read-only nodes still subscribe to the stream of committed log entries broadcast by the Leader, and update the SQLite database using the log entries they receive from the Leader.

Querying a read-only node

Any read request to a read-only node must specify read-consistency level none. If any other consistency level is specified (or no level is explicitly specified) the read-only node will redirect the request to the leader.

To ensure a read-only node hasn’t become completely disconnected from the cluster, you will probably want to set the freshness query parameter.

Enabling read-only mode

Pass -raft-non-voter=true to rqlited to enable read-only mode.

Read-only node management

Read-only nodes join a cluster in the same manner as a voting node. They can also be removed using the same operations.

Handling failure

If a read-only node becomes unreachable, the leader will continually attempt to reconnect until the node becomes reachable again, or the node is removed from the cluster. This is exactly the same behaviour as when a voting node fails. However, since read-only nodes do not vote, a failed read-only node will not prevent the cluster commiting changes via the Raft consensus mechanism.

Automatic clustering

Read-only nodes are fully compatible with DNS-based, Consul-based, and etcd-based Automatic Clustering methods. These nodes utilize the configured autoclustering to locate the cluster leader and join as standard read-only nodes. However, it’s important to note that read-only nodes are not capable of bootstrapping a cluster. Therefore, setting the -bootstrap-expect flag to a non-zero value on a read-only node during launch will result in rqlited terminating.