Points versus Histories
Cloud proxy points and cloud history exports use the same
Sentience message structure to send data to the cloud. The two mechanisms push data into the same database. Therefore, you should choose
whether to export station data using histories or points based upon the nature of the data and its usage. Differences between
the way point and history information is collected and sent to
Sentience may influence your design.
Cloud proxy points use a batch snapshot approach to export point data. At assigned intervals, three batch update threads collect
a current snapshot of each source point's value and status. They add these data to the list of batch updates, which are subject
to tuning rules that determine if a point update should be sent. At last, they send these values to
Sentience. If the update interval is 5 minutes, the driver sends the value once every 5 minutes. If the point experiences a value change
in between update intervals, an update is not reported, only the value at the 5 minute interval is sent. This is analogous
to an interval history extension in
Niagara.
Cloud proxy points can be put into a COV (change of value) mode, where the driver sends every point value and status change
to
Sentience, which is subject to change tolerance (refer to ”Configuring Tuning Policy”). However, this is inefficient. Instead of sending
thousands of points per message as is possible with batch updates, the driver sends every point change as an individual message.
Due to limitations with the
Sentience IoT Hub, it is recommended to not use more than 20 COV points at a time, and only for debug and diagnostic purposes. It is
not recommended to use COV as a normal operation mechanism.
NOTE: Retrieving real-time data for
Sentience Connected Application developers: To obtain point values, use the CloudMultiPointReadCommand to request multiple point values. For pushing values,use the CloudMultiPointWriteCommand to send multiple values to the station at once.
CloudHistoryExport can export all histories in the database, including histories imported from other stations (for example, using drivers like
BACnet, OBIX, and NiagaraDriver) to
Sentience. The history extension that generates the records or imports them from the foreign device governs the collection frequency
for the history. CloudHistoryExport retrieves all records that have not yet been sent for a history, and includes them in the update message sent to
Sentience.
NOTE: Capturing every point change: The best way to push every point change to
Sentience without missing any value changes would be to add a
Niagara CovHistoryExt to the point, so that every point change is captured to a
Niagara history. Then use a CloudHistoryExport to export the history records in bulk.
It is important to note that JACE platforms have a limit to the number of histories they can contain. This is an inherent OS limit due to the number of allowed
file descriptors, one of which is consumed by each history. While the exact number you might be able to reach in an individual
station depends on several variables, the general limit is no more than 6000 histories included in a history database resident
on a JACE-8000. This limit does not apply to Windows-based
Supervisor stations, nor to Linux-based
Supervisor. There is no defined limit on the maximum number of cloud proxy points, although you are still subject to space limitations
and to driver licensing restrictions.