Starting in NiagaraAX-3.4, the On Demand history feature enables polling of both local history sources and Niagara History imports for live data when displaying history chart or history table views. This feature works in addition to, and does not replace, a standard polling schedule. For example, you would typically still have your History Import descriptors scheduled to archive at some daily interval (such as every night), even though you might be displaying On Demand history views.
The On Demand feature provides a 'Live Updates' toggle button (check box in Hx view) on both the history chart and history table views. When you click the 'Live Updates' button, it initiates a history subscription that finds the source of the history and subscribes to that source component. If the source is a history import descriptor, then that descriptor starts polling at a frequency defined by its On Demand Poll Frequency property (described below). If the source is a local History Ext, the history chart or table updates whenever the history extension appends a new record to the history.
When the On Demand History is active, a timestamp appears in the view’s status bar (lower corner) to indicate the time of
the last successful update. This does not apply to Hx views.
Since histories eligible for on demand support are sourced directly from a local history extension or at a history import descriptor, On Demand History availability and update rate are both affected by parameters described below and shown in Figure 162:
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On Demand History availability is affected by the following factors:
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On Demand History is not available if the source component is not accessible. Since histories can be categorized (for permissions levels) independently from their source components, it may be possible for a user to have access to a history, but not have access to the source component. In this case, the On Demand History feature is disabled (button is dimmed on the History Chart or Table view). Also, if the source component is no longer in existence (has been deleted) the “Live Updates” toggle button is dimmed (inaccessible).
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On Demand History is not available if the source history import descriptor’s On Demand Poll Enabled property is set to
false(see Figure 161). On Demand Polling may be disabled to limit bandwidth usage. -
If the On Demand Poll Scheduler property is disabled (or the driver network does not have it included in a HistoryNetworkExt)
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On Demand History rates are affected by the following property settings (see Figure 162):
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On Demand Poll Scheduler (sources using import descriptor)
This property is located under the NiagaraNetwork History Policies property. You can set standard polling rate values for a Fast Rate, Normal Rate, and a Slow Rate in the fields displayed in the property sheet view. These rates are available as selection options for individual Niagara History Import descriptors under the NiagaraNetwork History Device Extensions.
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On Demand Poll Frequency (sources using import descriptor)
This property is located under the Niagara History Import property. Select one of the three options (defined in the On Demand Poll Scheduler):
Fast,Normal, orSlow. -
History extension update interval
If the On Demand history is being sourced from a local history extension, then the On Demand chart or table view updates whenever the local history extension appends a new record to the history. This function reflects the presence of a (Last Record) property starting in NiagaraAX-3.4.
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