HistoryQuery Object

The HistoryQuery object is used to setup and execute history filtering queries for the VHS service. It is designed to be used with other queries that have obtained a tag list, such as the PointFacQuery.

Within the calling script, the basic sequence of operations is as follows:

• Create the history filter object
• Start history filtering
• Obtain history by point tag and either a specific time or a time window

This object does not provide a full-featured query engine that utilizes filters. It is, however, modeled like the other query objects in the CxEventIF interface, with an asynchronous thread that can be created, started, polled, and canceled. Once the object starts the query engine, it will run continuously until the calling script cancels the filtering.

History can be retrieved based on a point tag and a time. The query returns the history entry information at that time or at the last history entry before that time. Alternatively, history can be retrieved for a given time window instead of at one particular time.

Declare the object using the following code:

Each method in this document will assume you have declared the HistoryQuery object using the following code:

More:

HistoryQuery Methods