Witness nodes v5
A witness node is a lightweight node which functions as a data node but which does not store or replicate data. It is used to allow a PGD cluster which uses Raft consensus to have an odd number of voting nodes and therefore be able to achieve a majority when making decisions.
Witness Nodes within PGD groups or regions
One typical use of witness nodes is when a PGD group has two data nodes but resources are not available for the recommended three data nodes. In this case, a witness node can be added to the PGD group to provide a third voting node to local Raft decision making. These decisions are primarily about who will be electing a write leader for the proxies to use. With only two nodes, there could be no consensus over which data node could be write leader. With two data nodes and a witness, there would be two candidates (the data nodes) and three voters (the data nodes and the witness) normally, and when a data node was down, there would still be two voters able to select a write leader.
Witness Node outside regions
At a higher level, witness nodes can be used when there are multiple PGD groups mapped to different regions. For example with three data nodes per-region in two regions, while running normally allows all six data nodes to participate in Raft decisions and obtaining DDL and DML global locks. Even when a data node is down, there are sufficient data nodes to obtain a consensus. But if a network partition occurs and connectivity with the other region is lost, then there are now only three nodes out of six available, which is not enough for a consensus. To avoid this scenario, a witness node in a third region can be deployed as part of the PGD cluster. This witness node will allow a consensus to be achieved for most operational requirements of the PGD cluster while a region is unavailable.