An overview of PGD 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.
The data node is also the foundation on which the other three nodes are built.
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 that the cluster can achieve a majority it 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. You can use subscriber-only nodes as read-only nodes for applications. You create subscriber-only nodes by specifying a data node is subscriber-only when you create the node and then adding it to a subscriber-only group. See Subscriber-only nodes and groups for more details.