Rockwell Automation FactoryTalk Historian SE 3.0 UniInt Interface User Guide - Manual (Page 91)

UniInt Interface User Manual 85
Primary Interface
When an interface is installed in a failover configuration, only one copy will be actively
sending data from the data source to Historian and from Historian to the data source. This
copy of the interface is referred to as the primary interface.
Point
For the discussion concerning failover, an entity that exists on the data source is referred to
as a point. A point may also be referred to as a control point.
Tag
For the discussion concerning failover, an entity that exists on the Historian Server is
referred to as a tag or Historian tag. A tag may also be referred to as a control tag.
Thrashing
Thrashing is defined as two interfaces continually alternating between the primary and
backup roles. This condition must never be allowed to occur. A defective implementation of
failover can lead thrashing.
Warm Failover
In a warm failover configuration, the backup interface does not actively collect data. The
backup interface is able to load the list of Historian tags and is in a state ready to start
collecting data as soon as the primary interface fails or stops collecting data for any reason. If
the backup interface assumes the role of primary, it will start data collection and it will send
its data to Historian. There may be some data disruption.
Failover Scenarios
There are a number of conditions that cause a failover as described below. The assumption is
made in the following sections that the two interface copies are in a steady-state condition
and a change in state forces a failover.
Primary Interface Shutdown
This scenario refers to a graceful shutdown of the primary interface. The chain of events is as
follows.