Node types v5.6
Data nodes
A data node in PGD is a node that runs a Postgres instance. It replicates data to all other data nodes. It also participates in the cluster-wide Raft decision-making around locking and leadership. It can be a member of one or more groups and is, by default, a member of the "top level" group that spans all data nodes in the cluster.
Witness nodes
A witness node behaves like a data node in that it participates in the cluster-wide Raft decision-making around locking and leadership. It doesn't replicate or store data, though. The purpose of a witness node is to be available to ensure a majority can be achieved when the cluster seeks a consensus. Witness nodes has more details.
Logical standby nodes
Logical standby nodes are nodes that receive the logical data changes from another node and replicate them locally. PGD can use a logical standby node to replace the node it's replicating if that node becomes unavailable, with some caveats. See Logical standby nodes for more details.
Subscriber-only nodes
A subscriber-only node is a data node that, as the name suggests, only subscribes to changes in the cluster but doesn't replicate changes to other nodes. Subscriber-only nodes can be used as read-only nodes for applications. You create subscriber-only nodes by taking a data node and adding it to a subscriber-only group. See Subscriber-only nodes and groups for more details.