Rockwell Automation FactoryTalk Historian SE 3.0 H2H Interface User Guide - Manual (Page 95)

FactoryTalk Historian To Historian Interface User Guide 89
points, then it cannot read any other data. There is no need for a backup communications path
between the control data and the interface.
When synchronizing through a shared file, however, we cannot assume that loss of control
information from the shared file implies that the primary interface is down. We must account
for the possible loss of the path to the shared file itself and provide an alternate control path
to determine the status of the primary interface. For this reason, if the shared file is
unreachable for any reason, the interfaces use the Historian Server as an alternate path to pass
control data.
When the backup interface does not receive updates from the shared file, it cannot tell
definitively why the primary is not updating the file, whether the path to the shared file is
down, whether the path to the data source is down, or whether the interface itself is having
problems. To resolve this uncertainty, the backup interface uses the path to the Historian
Server to determine the status of the primary interface. If the primary interface is still
communicating with the Historian Server, than failover to the backup is not required.
However, if the primary interface is not posting data to the Historian Server, then the backup
must initiate failover operations.
The primary interface also monitors the connection with the shared file to maintain the
integrity of the failover configuration. If the primary interface can read and write to the
shared file with no errors but the backup control information is not changing, then the backup
is experiencing some error condition. To determine exactly where the problem exists, the
primary interface uses the path to Historian to establish the status of the backup interface. For
example, if the backup interface controls indicate that it has been shutdown, it may have been
restarted and is now experiencing errors reading and writing to the shared file. Both primary
and backup interfaces must always check their status through Historian to determine if one or
the other is not updating the shared file and why.
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 value; monitoring the heartbeat value for the backup interface,
checking the active ID value, and checking the device status for the backup interface every
failover update interval on the shared file. Likewise, the backup interface is updating its
heartbeat value; monitoring the heartbeat value for the primary interface, checking the active
ID value, and checking the device status for the primary interface every failover update
interval on the shared file. As long as the heartbeat value for the primary interface indicates
that it is operating properly, the ActiveID has not changed, and the device status on the
primary interface is good, the backup interface will continue in this mode of operation.
An interface configured for hot failover will have the backup interface actively collecting and
queuing data but not sending that data to Historian. An interface for warm failover in the
backup role is not actively collecting data from the data source even though it may be
configured with Historian tags and may even have a good connection to the data source. An
interface configured for cold failover in the backup role is not connected to the data source
and upon initial startup will not have configured Historian tags.
The interaction between the interface and the shared file is fundamental to failover. The
discussion that follows only refers to the data written to the shared file. However, every value
written to the shared file is echoed to the tags on the Historian Server. Updating of the tags on
the Historian Server is assumed to take place unless communication with the Historian Server