Witness nodes v5.6
A witness node is a lightweight node that functions as a data node but that doesn't store or replicate data. Use a witness node to allow a PGD cluster that 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 aren't available for the recommended three data nodes. In this case, you can add a witness node 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, it's possible to have no consensus over which data node is write leader. With two data nodes and a witness, there are two candidates (the data nodes) and three voters (the data nodes and the witness). When a data node is down, then, there are still two voters that can select a write leader.
Witness node outside regions
At a higher level, you can use witness nodes when multiple PGD groups are mapped to different regions. For example, with three data nodes per region in two regions, while running normally, all six data nodes can participate in Raft decisions and obtain 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 now only three nodes out of six are available, which isn't enough for a consensus. To avoid this scenario, you can deploy a witness node in a third region 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.