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Update READ_ONLY_NODES.md

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Philip O'Toole 4 years ago committed by GitHub
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DOC/READ_ONLY_NODES.md
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# Read-only nodes # 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, without adding write latency to the cluster. 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, without adding write latency to the cluster. 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. However it does subscribe to the stream of committed log entries, which are broadcast by the leader. It updates its own copy of the SQLite database, just like any other node in the cluster. 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. Read-only nodes update the local SQLite database, and in every other sense act like any other node in the cluster.
## Querying a read-only node ## Querying a read-only node
Any read request to a read-only node must specify [read-consistency](https://github.com/rqlite/rqlite/blob/master/DOC/CONSISTENCY.md) 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. Any read request to a read-only node must specify [read-consistency](https://github.com/rqlite/rqlite/blob/master/DOC/CONSISTENCY.md) 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.
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Pass `-raft-non-voter=true` to `rqlited` to enable read-only mode. Pass `-raft-non-voter=true` to `rqlited` to enable read-only mode.
## Read-only node management ## Read-only node management
Otherwise read-only nodes join a cluster in the [same manner as a voting node. They can also be removed using the same operations](https://github.com/rqlite/rqlite/blob/master/DOC/CLUSTER_MGMT.md). Read-only nodes join a cluster in the [same manner as a voting node. They can also be removed using the same operations](https://github.com/rqlite/rqlite/blob/master/DOC/CLUSTER_MGMT.md).
### 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.

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