Rockwell Automation FactoryTalk Historian SE 3.0 H2H Interface User Guide - Manual (Page 96)

UniInt Failover Configuration
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is interrupted. The updates to the Historian Server will be buffered by Bufserv or PIBufss in
this case.
In a hot failover configuration, each interface participating in the failover solution will queue
three 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 3 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 5 seconds will result in overlapping data between 0 and 15
seconds. The no data loss claim for hot failover 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 value. In normal operation, the
Heartbeat value on the shared file is incremented by UniInt from 1 - 15 and then wraps
around to a value of 1 again. UniInt increments the heartbeat value on the shared file every
failover update interval. The default failover update interval is 5 seconds. UniInt also reads
the heartbeat value 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 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
problems. This can be accomplished by setting the ActiveID tag on the Historian Server to
the ActiveID of the desired interface copy.
As previously mentioned, in a hot failover configuration 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 three failover update intervals. The data in the
queue is continuously updated to contain the most recent data. Data older than three update
intervals is discarded if the primary interface is in a good status as determined by the backup.
If the backup interface transitions to the primary, it will have data in its queue to send to
Historian. This queued data is sent to Historian using the same function calls that would have
been used had the interface been in a primary state when the function call was received from
UniInt. If UniInt receives data without a timestamp, the primary copy uses the current
Historian Time to timestamp data sent to Historian. Likewise, the backup copy timestamps
data it receives without a timestamp with the current Historian Time before queuing its data.
This preserves the accuracy of the timestamps.