Subscriber-only nodes and groups v5

Subscriber-only nodes provide a means to scale up the handling of read-only requests in PGD.

Subscriber-only nodes

As the name suggests, a subscriber-only node subscribes only to replication changes from other nodes in the cluster. However, no other nodes receive replication changes from subscriber-only nodes. This is somewhat similar to logical standby nodes. But in contrast to logical standby, the subscriber-only nodes are fully joined to the cluster. They can receive replication changes from all other nodes in the cluster and hence aren't affected by unavailability or parting of any one node in the cluster.

A subscriber-only node is a fully joined PGD node and hence it receives all replicated DDLs and acts on those. It also uses Raft to consistently report its status to all nodes in the cluster. The subscriber-only node doesn't have Raft voting rights and hence can't become a Raft leader or participate in the leader election. Also, while it receives replicated DDLs, it doesn't participate in DDL or DML lock acquisition. In other words, a currently down subscriber-only node doesn't stop a DML lock from being acquired.

The subscriber-only node forms the building block for PGD tree topology. In this topology, a small number of fully active nodes are replicating changes in all directions. A large number of subscriber-only nodes receive only changes but never send any changes to any other node in the cluster. This topology avoids connection explosion due to a large number of nodes yet provides an extremely large number of leaf nodes that you can use to service a large number read-only requests without adding load to the replicating cluster.

Subscriber-only groups

To make use of subscriber-only nodes, first create a PGD group of type subscriber-only. Make it a subgroup of the group from which the member nodes receive the replication changes. Once you create the subgroup, all nodes that intend to become subscriber-only nodes must join the subgroup. You can create more than one subgroup of subscriber-only type, and they can have different parent groups.

Once a node successfully joins the subscriber-only subgroup, it becomes a subscriber-only node and starts receiving replication changes for the parent group. Any changes made directly on the subscriber-only node aren't replicated.

See bdr.create_node_group to know how to create a subgroup of a specific type and belonging to a specific parent group.