Our main/backup solution is based on hot standby redundancy. This means both server are running but only one server is active – the other one is inactive.
Active means that server is taken over the control i.e. it is doing all the work:
- connecting to the field network, communicate with the devices and send/receive data point changes
- do the BMS functions: alarm handling, scheduling, trending, executing logic, ...
- allows connections from visualization clients
- allow configuration changes via the Web Manager
Inactive means that the server is running but it is that it does not perform the above mentioned tasks. However, it gets the current state (e.g. Server Item values, alarm states, ...) from the active server via a synchronization connection. Thus, the main and the backup server have the some process image of the underlying building automation system.
The following figure shows the normal situation when both servers are working properly. The main server is active, the backup server is inactive – the backup server is getting all information via the synchronization connection.
Now suppose the main server is down due to a hardware crash or due to a Windows reboot. In this case, the synchronization connection is lost and interrupted. The backup server recognizes this and switches from inactive to active state. This means that it takes over the control, connects to the field devices and performing the BMS functions. In addition, the visualization clients are switching to the backup server.
Now lets assume that the main server recovers and gets online again. Since the backup server is trying to reestablish the synchronization connection to the main server again, the backup server recognizes that the main server is up again. After having established the synchronization connection again, it will transfer the current state (data point values, BMS function configurations, ...) to main server. The main server gets active and the backup switches to inactive state again. The visualization clients are switching back to the main server.