Wonderware connector Store-Forward algorithm

A tag that is marked as in store-forward mode is moved into the store-forward queue and is processed separately from normal data collection. Within the store-forward processing, the tag is either in Currently-in-Store-Forward state or has moved to a Store-Forward-Transition state. Tags that are in the Currently-in-Store-Forward state use the following start and end times for the query:

• Start time of DateTime.Now – one millisecond

• End time of DateTime.Now.

Although the tag returns a bad quality and a Connection Loss quality detail (10), it remains in the Currently-in-Store-Forward state. The tag exits from this state into the Store-Forward-Transition state only if none of the queried data contains a quality detail of Connection-Loss (10).

When the data no longer contains the Connection-Loss (10) quality detail then the tag moves into the Store-Forward-Transition state. The start and end times for the transition queries are:

• Start time is the minimum last good update time of all tags grouped in the same query

• End time is the maximum transition time of all tags grouped in the same query (maximum transition is assigned at the time the tag moves to the Store-Forward-Transition state and fixed from then on)

The tag remains in Store-Forward-Transition state until the First-Store-Forward (248) quality detail is present in the queried data. When the data is present, the algorithm then waits for a number of values based on history for the span of the query (an exception to this is when the data is unchanging). When the expected values are met, or when the number of attempts exceeds an internal fallback maximum, then the algorithm watches for three subsequent queries that contain the same number of collection values using the same Store-Forward-Transition start and end times. When three subsequent polls contain the same number of collection values, the tag is moved out of store-forward queue and back into the normal data-collection queue.

The initial normal data collection (after moving from store-forward queue to normal queue) uses the following start and end times:

• The start time is the minimum last good update time of all tags that are grouped in the same query.

• The end time is DateTime.Now.

  Note: No data is written to PI Data Archive while the tag is in the store-forward queue. Because the data values for a tag are both out of order and incomplete (and sometimes contain unusual or invalid data) the data is not written to the PI system until the tag moves into the normal data collection queue.