Recovery

This feature is unique to Calculation and Server-to-Server collectors. If the calculation engine is not running for a period of time, recovery makes it look like it was running. Recovery can also be used to fill in a hole of time where the collector was not able to communicate with the source archiver.

Recovery is applicable to both unsolicited and polled tags. Messages are also recovered. Comments are not recovered.

Normally, it is impossible to go back to the past and collect data. However, since these collectors are 'deriving' data instead of 'collecting' data, it is possible to recover past data, especially since the source of the derived data is archived in the Historian. It is important to understand that while recovery is possible in the calculation and Server-to-Server collectors, it only makes sense for certain types of calculation formulas.

Intended candidates for data recovery are formulas whose only inputs are Historian tags, since past data for these tags can be interpolated. Formulas that use data from external text files or from ADO via CreateObject will most likely not recover correct data because the inputs are not historized. If you are using these types of formulas, you should turn off recovery for the whole collector or insert VBScript code in the formula of individual tags to detect recovery. An example of this is given in the Historian documentation. A similar approach can be used to set a Max Recovery Time on a tag basis, overriding the collector wide setting.

Even calculation tags using only Historian tags as inputs have some caveats for recovery. If you are deriving calculated data from other calculated data, be sure to set up a trigger tag for each of the tags used in your formula. This way the tags will be processed in chain order. All tags are processed in time order.

The recovery logic is not intended to overcome polled collection overruns. If you configure too much collection, then you will get overruns.

You can control the amount of recovered data using Max Recovery Time configuration setting. You can turn off the recovery by setting it to zero.