Primary Stops Updating Heartbeat Point for less than 5 Intervals

Primary Stops Updating Heartbeat Point for less than 5 Intervals

• 
  Every failover update interval, the backup interface reads the value of the heartbeat point for the primary interface and remembers the value internally.
  
• 
  If on the third update interval the value of the primary’s heartbeat point has not changed, the backup transitions into a temporary PrimaryStale state. This state allows the primary two additional failover update intervals of time to recover before the backup assumes control. If there was no recovery time for the primary, thrashing might result due to latency in the system architecture.
  
• 
  Upon entering the PrimaryStale state, the backup stops discarding queued data.
  
• 
  The backup interface remains in the PrimaryStale state for two failover update intervals waiting for the primary interface to come back to life.
  
• 
  Before two failover update intervals elapse, the primary resumes normal operation and updates its heartbeat point.
  
• 
  At the next failover update interval, the backup will see that the primary has resumed updating its heartbeat point and will transition back to the backup role.
  
• 
  When the backup exits the PrimaryStale state, it will send its queued data to the PI Server.
  

Time Action T+0 Both interfaces are running with IF1 in the primary role. T+2.25 Event A: IF1 stops updating its heartbeat point for any reason. T+2.5 IF2 reads the active ID point and IF1’s heartbeat point. Both are good so IF2 discards data older than time 0.5. T+3.5 Event B: IF2 notices that IF1’s heartbeat point has not updated. IF2 discards data older than Time 1.5. T+4.5 IF2 notices that IF1 still has not updated its heartbeat point. IF2 stops discarding queued data. T+5.5 IF2 notices that IF1 still has not updated its heartbeat point. T+5.75 Event C: IF1 resumes operation. The active ID point is equal to IF1’s ID, so IF1 updates its heartbeat point and continues operating normally. T+6.5 IF2 notices IF1 has updated its heartbeat point. IF2 will send the data it has in its queue and then return to normal backup operation. IF1 was not collecting data from time 2.25 to time 5.75. IF2 will send the data it received from time 1.5 to time 6.5. This will lead to two periods of overlapping data: from time 1.5 to time 2.25 and from time 5.75 to time 6.5.