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T/Mon NRI consists of a Primary unit and a Secondary unit. Both units have the same database and when both are online, the Primary is active and the Secondary is passive. When a unit is active, it means that it is polling and responding as if it were operating on its own. When a unit is passive, it is only polling the Primary unit for any updates. The primary keeps track of COSes, new alarms and ACKs that need to be sent over to the secondary unit. This is done to keep all alarm data in sync at all times.
If the Secondary unit were to go down, the Primary unit will continue to queue up everything that it needs to send over to the Secondary. Once the Secondary comes back online, it will start polling the Primary again and receive everything in the Primary's send queue. The Primary has a maximum of 100,000 entries that it can queue up before it stops adding to the queue.
When the primary unit goes offline and the Secondary unit is still online, it will go active and take over for the Primary. The Secondary will start polling and responding. It also starts building its own queue to send to the Primary when it comes back up. once the Primary comes back up, it will start off passive and poll the Secondary for any changes that occurred while it was offline. After it receives everything and the Secondary's queue has been depleted, it will send a command to the Secondary telling it to go passive. The primary will go active after the Secondary sends a response to the passive request.
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