Rockwell Automation FactoryTalk Historian SE 3.0 UniInt Interface User Guide - Manual (Page 98)

UniInt Failover Scheme
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Primary Interface Loses Connection to Historian Server
Users may have criteria dependent upon continuous data flow to Historian. This failover
prevents the disruption of data flow to Historian that may cause user dependent operations to
fail when the connection to the Historian Server is lost. The following sequence of events is
taken by the primary and backup interface if the primary interface loses its connection to the
Historian Server.
 When the primary interface loses its connection to the Historian Server, it writes a value
incrementing between 17 – 31 and then back to 17 to its heartbeat point. UniInt will send
a signal to the state machine if any of the PI API functions return a value indicating that
Historian is unavailable.
 The backup interface reads a value of 17 – 31 on the primary‘s heartbeat point during its
next failover update interval.
 The backup transitions to a state indicating that the primary has lost its connection to the
Historian Server (PrimaryNOPI).
 The backup no longer discards queued data.
 At the next failover update interval, the backup attempts to read the snapshot of the
active ID tag from the Historian Server. If the read from the Historian Server is
successful, the interface transitions to the
AssumingControl state, updating the value
of the active ID point on the data source to its failover ID. At this point, the backup has
effectively taken over the role of primary.
 The old 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 role.
The newly designated backup interface begins to queue its data.
This scenario may result in up to 2 seconds of overlapping data depending on the timing. The
overlapping or duplicate data will not appear in Historian until the interface that lost its
connection to Historian reestablishes its communication to Historian and bufserv sends the
buffered data.
Following is a failover timing chart of the situation where the primary interface loses its
connection with the Historian Server.