UniInt End User

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UniInt Interface User Manual
PI BASemAPI

Figure 4: Timing chart for a manual failover to the backup interface.

Manual Failover

Initiating a manual failover requires that the backup interface be in a state equal to or better than the current primary copy of the interface. For example, if the current primary has a connection to the PI Server and the backup does not, manual failover is not authorized. The backup interface will know the state of the primary by the value of the heartbeat point as explained in the next section.
Manual failover will result in overlapping data of between of up to two update intervals. Manual failover can be accomplished by changing the value of the active ID point on the data source from the current primary’s failover ID to the current backup’s failover ID. For example, if interface copy IF1 is currently the primary and interface copy IF2 is currently the backup, manual failover can be accomplished by changing the active ID point from a value of 1 to a value of 2 on the data source.
If the active ID point on the data source is changed, actions taken by the interface during this type of failover are as follows.

The primary will notice that the value of the active ID point on the data source is equal to the failover ID of the other copy and immediately transition to the backup state.
Similarly, the backup will notice that the value of the active ID point on the data source is now its failover ID and will immediately transition to the primary state.

Following is a depiction of the failover timing chart related to the manual failover scenario where the active ID point on the data source is changed.

Time	Action
T+0	Both interfaces are running with IF1 in the primary role.
T+2.5	IF2 reads the active ID point and IF1’s heartbeat point. Both are good, so IF2 discards data in its queue older than time 0.5.
T+2.8	Event A: the active ID point on the data source is changed from 1 to 2.
T+3	Event B: IF1 reads the active ID point and IF2’s heartbeat point. IF2’s heartbeat is good and the active ID has changed to a 2, so IF1 immediately transitions to backup.
T+3.5	Event C: IF2 notices that the active ID point on the data source is 2 and immediately transitions to primary. The data in its queue and any new data are sent to PI.
Overlap data from time 0.5 to 3.

Figure 4: Timing chart for a manual failover to the backup interface.

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