Read-only nodes

rqlite supports adding read-only nodes to a cluster. These nodes let you increase read scalability when handling large volumes of queries or when you want to place copies of your data closer to clients.

An rqlite node can serve thousands of queries per second, assuming you use the default read consistency level. Don’t add read-only nodes unless you are sure you need them.

Read-only nodes are especially useful at the network edge. If the link between the edge and voting nodes is slow or unreliable, a read-only node at the edge can continue serving queries locally, even during outages. The data may be stale, but this can be an acceptable trade-off for many applications.

Also called non-voting nodes, read-only nodes do not participate in Raft consensus: they do not count towards quorum and do not vote in Leader elections. They do, however, receive the Leader’s stream of writes and apply them to their local SQLite database, just like voting nodes.

Querying a read-only node

A read request to a read-only node must use a read-consistency level of none or auto. If any other level is specified — or if no level is set explicitly — the node will transparently forward the request to the Leader, negating the benefits of using a read-only node.

To ensure a read-only node hasn’t become completely disconnected from the cluster, set the freshness query parameter to limit data staleness. If you do not do this, you risk receiving significantly out-of-date i.e. stale data.

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 periodically attempt to reconnect to the node until the node becomes reachable again, unless 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 from processing write requests.

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 setting 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 with an error.