Our main/backup solution is based on hot standby redundancy. This means both server servers are running but only one server is active – the other one is inactivein standby mode.

Active means that server is has 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 running the BMS functions: alarm handling, scheduling, trending, executing logic execution, ...
• allows accepts connections from visualization clients
• allow configuration changes via the Web Manager

Inactive Standby means that the server is running but it is that it in an inactive state. It does not perform any of the above mentioned tasks.   However, it gets the current receives all state (e.g. Server Item values, alarm stateschanges (data point values, BMS function configurations, ...) from the active server via a synchronization connectionlink. Thus, the main and the backup server have the some same process image of the underlying building automation system.

Scenarios

Normal operation

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 connectionlink.

Main server failure

Now suppose the main server is down due to a hardware crash or due to a Windows reboot. In this case, the synchronization connection link is lost and interrupted. The backup server recognizes this and switches from inactive standby to active state. This means that it takes over the control, connects to the field devices and performing performs the BMS functions. In addition, the visualization clients are switching to the backup server.

Main server recovery

Now lets assume that the main server recovers and gets back online again. Since the backup server is continuously trying to reestablish the synchronization connection link 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 synchronization link is reestablished, the backup server transfers the current state (data point values, BMS function configurations, ...) to main server. The main server gets active and then resumes control while the backup switches back to inactive state againstandby mode. The visualization clients are switching back to the main server.