Process historian

A time-series database storing plant measurements over long periods.

A historian collects tag values from control systems and stores them for trending, reporting and analysis. It sits downstream of control, so reading from it usually adds no load to the process network, which often makes it the lowest-risk source of data for analytics.

The constraints are resolution and retention. Many historians apply compression on the way in, discarding points judged not to add information, and once a signal has been stored as a one-minute average the underlying detail cannot be recovered.

Retention is worth setting deliberately rather than accepting the default. Questions about degradation and about contractual performance are retrospective by nature, and the useful retention period is the life of the guarantee or the tenor of the debt rather than whatever the system was configured with at commissioning.