Rockwell Automation FactoryTalk Historian SE 3.0 UniInt Interface User Guide - Manual (Page 87)

UniInt Interface User Manual 81
Steady State Operation
Steady state operation is considered the normal operating condition. In this state, the primary
interface is actively collecting data and sending its data to Historian. The primary interface is
also updating its heartbeat point, monitoring the heartbeat point for the backup interface, and
checking the active ID point every failover update interval. The backup interface is actively
collecting and queuing data but not sending that data to Historian. It too is updating its
heartbeat point, monitoring the heartbeat point for the primary interface, and checking the
active ID point every failover update interval. As long as the heartbeat point for the primary
interface indicates that it is operating properly and the active ID point has not changed, the
backup interface will continue in this mode of operation.
The interaction of the control points is fundamental to failover. The discussion that follows
only refers to the data written to the control points on the data source. However, every value
written to the control points on the data source is echoed to the control tags on the Historian
Server. Updating of the control tags is assumed to take place unless communication with the
Historian Server is interrupted. The updates to the Historian Server will be buffered by
bufserv or BufSS in this case.
Each interface participating in the failover solution will queue two failover intervals worth of
data to prevent any data loss. When a failover occurs, there may be a period of overlapping
data for up to 2 intervals. The exact amount of overlap is determined by the timing and the
cause of the failover and may be different every time. Using the default update interval of 1
second will result in overlapping data between 0 and 2 seconds. The no data loss claim is
based on a single point of failure. If both interfaces have trouble collecting data for the same
period of time, data will be lost during that time.
As mentioned above, each interface has its own heartbeat point. In normal operation, the
value of the Heartbeat point on the data source is incremented by UniInt from 1 – 15 and
then wraps around to a value of 1 again. UniInt increments the heartbeat point on the data
source every failover update interval. The default failover update interval is 1 second. UniInt
also reads the value of the heartbeat point for the other interface copy participating in
failover every failover update interval. If the connection to the Historian Server is lost, the
value of the heartbeat point will be incremented from 17 – 31 and then wrap around to a
value of 17 again. Once the connection to the Historian Server is restored, the heartbeat
values will revert back to the 1 – 15 range. During a normal shutdown process, the heartbeat
value will be set to zero.
During steady state, the ActiveID will equal the value of the failover ID of the primary
interface. This value is set by UniInt when the interface enters the primary state and is not
updated again by the primary interface until it shuts down gracefully. During shutdown, the
primary interface will set the
ActiveID to zero before shutting down. The backup interface
has the ability to assume control as primary even if the current primary is not experiencing
problem. This can be accomplished by setting the ActiveID
control point on the data
source to the ActiveID of the desired interface copy.
As previously mentioned the backup interface actively collects data but does not send its data
to Historian. To eliminate any data loss during a failover, the backup interface queues data in
memory for two failover update intervals. The data in the queue is continuously updated to
contain the most recent data. Data older than two update intervals is discarded if the primary