About the real-time data window

The real-time data window is applicable only for store-and-forward data from remote IDASes that are not upgraded to Historian 2017.

The real-time "window" is the maximum delay, relative to current time of the server, in which data is considered real-time by storage. The real-time window can range from -30 seconds to +999 milliseconds of the current server time.

• For real-time data, the timestamp for the data value must fall within the time window.

• For late data, the timestamp for the data value can fall either inside or outside of the time window, depending on whether the late date setting is enabled for the topic. Note that late data can be processed very slowly if the timestamp falls outside of the real-time window.

• For non-streamed data, the data can have any timestamp.

The following rules apply when storing data with timestamps relative to the real-time window:

• If the late data setting for a tag topic is not enabled and the received data is more than 30 seconds late, the value is discarded, and a warning is logged. If the received data value is within 30 seconds of the server time, it is stored as received.

• If the late data setting for a tag topic is enabled and the received data value is within the real-time window, then the value is stored by the real-time storage service with no changes. If the received data value is outside of the real-time window, then the value is passed to the alternate storage services and stored without changes.

• If the late data setting for a tag topic is enabled, the received data value is stored in delta mode, even if the tag is configured for cyclic storage and the received data value is within the real-time window.

For more information on the late data setting for a topic, see IDAS Late Data Handling.

You can adjust the real-time window for "late" data topics by configuring the RealTimeWindow system parameter. The real-time window for regular real-time data (not "late") is fixed as 30 seconds. For more information, see Edit system parameters.

Adjusting the real-time window also has implications if you are using delta storage with a swinging door deadband. For more information, see Swinging Door Deadband for Delta Storage.

If the system does not have enough memory to adequately process real-time data, the window is adjusted internally. An appropriate message is logged. The value of the RealTimeWindow system parameter, however, remains unchanged.

Important: The real-time window is not intended to accommodate time synchronization problems between an IDAS and the Historian. It is imperative that you properly synchronize the IDAS and Historian. If the IDAS is sending data in a steady stream outside of the real-time window, it is likely there is a time synchronization problem. For more information, see Time synchronization for data acquisition.

If a data value is discarded because it did not fit the requirements of the real-time window, the Historian logs a warning message. Warning messages are logged at one intervals during the period when data is being discarded.