Alarm Server Side Synchronization

Synchronization occurs when the primary server establishes or re-establishes a connection with the standby server. Synchronization takes place on startup and then periodically during runtime while connection exists between the Primary and stand by servers. During synchronization the Primary server (or main) updates the data on the standby sever as required with alarm action and event data.

Each time the alarm primary server connects to the standby server, the synchronization process will take place. This will allow primary server to update the standby server with the most recent copy of the primary server's data.

Note: Synchronization occurs only if the primary and standby alarm servers are running and connected.

To configure synchronization between the primary and standby alarm server refer to the Alarm Parameters.

Main-Main Scenario

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.

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 Plant 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.

Configuring Isolation Parameters

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

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 Plant 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.

See Also

Add an Alarm Server Process