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 writes broadcast by the Leader, and update the local SQLite database using the data they receive from the Leader.

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 If you do not do this, you risk receiving significantly out-of-date i.e. stale data.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.