That describes the kind of infrastructure needed when deploying a clustered database product (all the major commercial products have such). You will need a clustered DB product in your spec. Ignition would indeed make a connection to a single database. All of the HA functionality would be invisible to Ignition. In such a setup, consider having the most critical data buffered in the PLCs, and transferred from PLC FIFO to DB with store-and-forward disabled. This was discussed recently: