Coerce digital state sets

At any time you can only configure a digital PI point with one digital state set. Typically, you never need to change digital state sets. When you do change a digital state set, the inconsistency for that PI point over time can lead to confusion and unintended side effects.

However, in the rare situation when you need to change the digital state set configured for a PI point, the piarchss utility provides an option that coerces data records to use the current digital state set for those records, without exception. By coercing data, you can maintain consistent data structures even after changing a digital state set for a particular point.

PI Server 2018 SP3 Patch 3, which includes Data Archive SP3 Patch 2, was the first release giving you the option to coerce digital states. In earlier releases coercion always occurred during reprocessing.

Data storage behavior

When you create a digital point, its digital states are stored in archive records as sets of states and offsets. This approach is a compact, efficient way to store state information. The digital state set number is registered in every archive record of a digital point.

When you change a digital state set of a digital point, a new archive record is created with the new digital state set in the record header. Archive records with the earlier digital state set in the header hold events with a timestamp before the digital state set change. Archive records with the current digital state set in the header hold events occurring after the digital state set change.

Coerced behavior

Beginning with the PI Server 2018 SP3 Patch 3 release, Data Archive adds the option to apply the currently configured digital state set for each point to all the record headers within an archive file. The offset value in all digital events is preserved but the digital state set for each point in the record header of the archive record is forced to reflect the current digital state set. This forced action is called coercion.

For example, if data is stored for a point with a set of On, Off, this data amounts to 0 and 1. Later, if the point is coerced to use a set with Open, Close, the header reflects the current digital state set but the old data displays as Open and Close. This behavior could be desirable in some cases and confusing in others. Before coercing digital state sets, be sure you understand the consequences and verify that you want to apply the current digital state set to record headers within an archive file.

See Specify archives to be coerced for details on how to coerce digital state sets.

Behavior in earlier releases

When reprocessing archives in releases earlier than PI Server 2018 SP3 Patch 3, archive records were always coerced. Newly created archive records had the current digital state set information. Archive records dating from before the digital state set change were reprocessed so their record headers contained the current digital state set information.