To use histories, each station must contain a single history service that provides http access to all of the histories in a station. This service is responsible for creating the history database, as well as enabling the collection and storage of histories in the database. If you do not have the history service in your active station, you can add it by dragging and dropping a copy from the History Palette, as shown in Figure 127.
The history service manages histories in ways that are not always visible to the user, including the following:
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identifies histories
when the service is started (normally this is whenever the station is started) all existing histories are added to the service.
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life cycle management
handles history management functions, such as creation and deletion of individual histories.
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namespace
maintains the naming convention (namespace) for all histories.
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configuration
maintains a global default configuration for the histories.
The history extension manager and the history property sheet are two views of the history service that provide ways to work with history extensions. Refer to the following sections for more information about these views:
As the default view of the history service, the history extension manager shows all history extensions in the station and displays their current status. Using this view as both a management and navigational tool, you can double-click on any entry-row to go directly to the property sheet view of that extension. This table has the standard table features. Use the menu in the top right corner of the history table to modify the table view or to export the data in the view, as desired. Refer to Table controls and options for information about using the tabular view, including the menu.
You can also , , , or (starting in NiagaraAX-3.4) for any collection from this view by selecting the desired history extension in the table and using the History Ext Manager menu, popup menu, or toolbar icons, as described in the sections listed below:
If a history is disabled, its row is dimmed with a gray background in the history extension manager view.
The history service property sheet, shown in Figure 129, includes the following properties:
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Max Open Time
this is an editable parameter for setting the amount of time that the history database may remain open without being accessed before it is subject to being closed by the action. If the history is accessed more frequently than the Max Open Time, it is not closed when action is invoked.
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History Groupings
Starting in AX-3.5, this property is available for organizing histories according to history properties that you can designate and enable using the History Group component.
History service actions are available from the menu when you right-click the HistoryService node in the nav side bar pane. Available actions include:
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Save Db
this action initiates a save of all histories to the history database.
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Close unused histories
this action closes any histories that have not been accessed within the Max Open Time (this time is set in the history service property sheet).