Pass-through queries¶
Pass-through queries allow you to forward historical requests to OPC UA data sources. For this to work, every variable to be used in a passed-through query must be configured as described in the previous chapter. The functionality can be used for all kinds of historical requests. You can even combine local and passed-through historical data in a single call to e.g. webMI.data.queryFilter() or history.query(). The history list will also show local and passed-through historical data together in the table.
To be able to use pass-through queries, different requirements have to be met for the different kinds of requests:
- Raw values
the variable must be configured for pass-through of raw values (see above)
a correctly configured MirrorInput or MirrorInputOutput must exist
- Aggregated values
the variable must be configured for pass-through of aggregated values (see above)
an aggregate template must be assigned (see above)
a correctly configured MirrorInput or MirrorInputOutput must exist
- Alarms
Pass-through of historical alarms is closely tied to alarm mirroring. If you have configured alarm mirroring, queries are automatically forwarded to the data source for all alarm conditions known to atvise. Queries will not be forwarded for data sources where you have activated historize mirrored alarms.
- Other events
Other historical OPC UA events will be read from data sources, if you activate pass through event history in the data source configuration.
Hint
You can use the data source functions to directly read historical data from an OPC UA data source without the need to configure historization.