Interfaces Connect to Data Source at Approximately the Same Time

Interfaces Connect to Data Source at Approximately the Same Time

• 
  If the ActiveID equals the failover ID for this copy of the interface, the interface transitions to the primary role and starts sending data to PI. In this situation, it will take two failover update intervals for data to show up in PI. This is because the interface actually transitions to the AssumingControl state for two intervals and queues its data. No data is lost during this two failover update interval start-up delay. The delay is used to prevent contention between the two interfaces trying to assume the role of primary at the same time.
  
• 
  If the ActiveID equals the failover ID for the other copy of the interface, the interface stays in the backup role and starts monitoring the heartbeat of the other copy. For example, if both copies of the interface are down and then one copy is started and the active ID point is equal to the other copy’s failover ID, it will take four update intervals for its data to show up in PI. This is because the interface assumes the backup role at startup and waits to verify the other copy’s heartbeat. If the other copy’s heartbeat point is not being updated, this copy will go through the process of assuming the role of primary. All data collected by the interface during this four interval start-up delay will be sent to PI.
  
• 
  If the ActiveID equals something other than this copy’s or the other copy’s failover ID, the interface transitions to the AssumingControl state. Again, the interface will stay in this state for up to two failover update intervals to avoid contention. For example, if interface IF1 reads the ActiveID, then interface IF2 reads the ActiveID before interface IF1’s update takes place. Interface IF1 will claim primary by setting the ActiveID to its failover ID. Interface IF2 will claim primary by setting the ActiveID to its failover ID. Then both copies wait one failover update interval and read the value of the active ID point. If the ActiveID has changed to the other copy’s failover ID, the interface will transition to the backup role. If the ActiveID is still equal to this copy’s failover ID, the interface will wait one more interval. If, after two intervals, the ActiveID still equals this copy’s failover ID, the interface will transition into the primary role and send the data it has queued during this process.
  

Time Action T+0 Neither interface is running T+1 Event A: IF1 starts and notices the active ID point is 0 so it transitions to active and writes a 1 to the active ID point. T+1.25 Event B: IF2 starts and notices the active ID point is 0 so it transitions to active and writes a 2 to the active ID. T+1.5 Event C: the update from IF1 reaches the data source T+2 Event D: IF1 reads a 1 for the active ID point. Immediately after this, the update from IF2 reaches the data source. T+2.5 IF2 reads a 2 for the active ID point and remains in the active mode. T+3 IF1 reads a 2 for the active ID and immediately transitions to the backup role. The last interface to update the active ID point is the interface that will remain in the primary role.