Rockwell Automation FactoryTalk Historian SE 3.0 UniInt Interface User Guide - Manual (Page 92)

UniInt Failover Scheme
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 The primary interface is stopped.
 On exit, the primary interface writes a value of 0 to its heartbeat point and to the active
ID point.
 The backup interface reads the ActiveID. As soon as it detects a value of 0, it assumes
the role of the primary interface.
 The primary interface may stop just after the backup checked the value of the control
points on the data source. It will then take an additional failover update interval before
the backup realizes the primary has shut down. This will not result in a loss of data
because the backup has two failover update intervals of data in its queue. When the
backup transitions to the primary state, it sends its two intervals worth of queued data.
This will result in between one and two failover update intervals worth of overlapping
data (e.g., for the default failover update interval of 1 second, there will be up to 2
seconds of overlapping data).
Assuming Role of Primary Interface
 The interface in backup mode writes its failover ID to the active ID point.
 It waits two failover update intervals and then verifies that the ActiveID is the same as
its failover ID.
 If the ActiveID is equal to the other copy‘s failover ID, the interface remains in
backup mode. If the ActiveID is not the other copy‘s failover ID and it is not this
copy‘s failover ID, the interface sets the ActiveID to its failover ID and waits another
two failover update intervals. If the interface ActiveID remains the same, the backup
interface sends its queued data to Historian and starts operating as the primary interface.
The timing chart in Figure 2 shows the primary interface gracefully shutting down and the
backup interface failing over to assume the role as primary. Time increases from left to right
according to the scale at the bottom of the figure. Every number along the horizontal axis
represents one failover update interval. In the explanations that follow the charts, the failover
update interval is the default of one second. Key events in time are denoted by the labeled
arrows A, B, C, and D.