If the standby alarm server loses its connection with the primary alarm server, a 'main-main' situation may occur, where both servers communicate and process data from the I/O server. On reconnection the data on both servers will need to be synchronized. With duty mode enabled by default, the servers determine what data to merge. In most instances the data with the latest timestamp is kept while the other is discarded.

UNINTENDED EQUIPMENT OPERATION In a 'main-main' situation do not make any configuration changes to your system until the primary and standby server is reconnected. Configuration changes made to the system during a 'main-main' situation may cause the system to become inoperative. Failure to follow these instructions can result in death, serious injury, or equipment damage.

Deciding which server will be ‘Main’ after a 'Main-Main' scenario

When the connection between the servers is restored, the servers arbitrate to determine which server continues as main and which server becomes the standby.

In Citect SCADA, when configuring the alarm server you select which server is primary and which is standby. By default, the server configured as primary is given the highest priority, thus when the servers arbitrate to determine which becomes main, the primary server will revert to being the main server.

It is likely that when a redundant pair of alarm servers cannot communicate with each other, it is due to one of the servers becoming isolated from the network. The alarm server that has become isolated is also likely to be isolated from the physical devices from which it obtains data, so it will not be obtaining alarm events from the field during the period of isolation.

The parameters [Alarm]IsolationDetectIP1 and [Alarm]IsolationDetectIP2 can be configured so that the system knows which alarm server became isolated. The isolated alarm server can then become the standby when the connection between the alarm servers is restored. This will help avoid the loss of alarm event data during the period of an outage, as the alarm server that was not isolated becomes the main server and synchronizes the events that occurred to the standby.

Duty mode enables a standby server to transfer data to a primary server following a 'main-main' situation.

When the connection between the servers is restored, the servers arbitrate to determine which server switches to standby.

In Citect SCADA duty mode is enabled by default, and helps prevent the data on the server that is deemed to be the standby (after the arbitration process) from not being accessible. In Duty mode the timestamps of the data on the servers are compared. The data with the recent timestamp is kept and is merged into the data on the primary server and the remainder of the data is discarded.

When the primary server receives the more recent data, from the standby server, it merges the data with its own data and then sends an update to the standby server. This allows for primary and standby server to operate with synchronized data sets.