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Currently history configuration for MQTT Engine tags is not persistent and it may be required to delete MQTT Engine tags in certain cases. One example is UDTs which are being propagated from the Edge and the UDT definition on the Edge has been updated. This requires one to delete all instances of the UDT under MQTT Engine and delete the corresponding UDT definition so that it can be recreated/updated at MQTT Engine for the Edge side changes to take affectReference Understanding how tag changes at the Edge affect MQTT Engine for more details.
The recommended way of configuring history for MQTT Engine tags is to use reference tags to indirectly reference MQTT Engine tags. This allows the history configuration to be persisted on the reference tag when the underlying MQTT Engine tags are deleted.
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MQTT Engine has two ways to process historical events and insert the historical data into Ignition's Tag Historian module:
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Writing historical events directly to tag is required if there are:
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The configuration parameters required to write historical events directly to the database, via the Historian, bypassing the Tag are shown below.
Under the MQTT Engine Settings General Tab, navigate to the Miscellaneous Settings and ensure Store Historical events is selected
History must be enabled on the tag (either directly on the MQTT Engine tag or reference tag) but the only property used from the tag history configuration is the Storage Provider.
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All other properties such as Sample Mode, Max Time Between Samples, Deadband etc are ignored when writing historic data |
The Edge side client can publish historic data either in-order (synchronously) or asynchronously.
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If you are using MQTT Transmission as the Edge side client, under the MQTT Transmission Settings for your transmitter, navigate to the History Settings to disable the In-Order History configuration parameter. Enabling the In-Order History for MQTT Transmission is unnecessary and will result in a waste of resources. |
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There are several reasons why you might need to write directly to the tag which include:
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The configuration parameters required to write historical events to the Tag tag instead of directly to the Historian are detailed below.
Under the MQTT Engine Settings General Tab, navigate to the Miscellaneous Settings and ensure Store Historical events is de-selected
History must be enabled on the tag (either directly on the MQTT Engine tag and/or reference tag) with these two three settings required:
All historic data will be written to the tag using the timestamp associated with the historical data.
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Deadband Style must be set to Discrete for all tag datatypes. This ensures that a change is registered any time the value moves +/- the specified amount from the last stored value. If left at the default Auto setting, Ignition will determine the deadband style based on the tag datatype. See this Ignition Configuring Tag History Deadband and Analog Compression for details on how the Analog and Discrete Deadband Styles differ. |
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All other properties such as Max Time Between Samples , Deadband etc are ignored when writing historic datais greater than 0, inserts at this sample time are not honored during historical data flushes because the tag updates for historical data occur too quickly. For example the Max Time Between Samples is 5 Seconds. Whilst the edge node is offline, the edge node tag has updates every 10 seconds. On reconnect, all these historical change events are published and written to the MQTT Engine tag within milliseconds. As a result the Max Time Between Samples does not trigger. |
If you are not using reference tags, the Edge side client must publish historic data in-order (synchronously) before live data resumes. This This is because because Ignition will ignore writes to the tag if the timestamp on the tag change is older that the current value.
If you are using reference tags and have the Ignition Allow Back-fill Data disabled on the tag provider that will contain the reference tags, the Edge side client must publish historic data in-order (synchronously) before live data resumes. This is because because Ignition will ignore writes to the tag if the timestamp on the tag change is older that the current value.
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If you are using MQTT Transmission as the Edge side client, under the MQTT Transmission Settings for your transmitter, navigate to the History Settings to select or deselect the In-Order History configuration parameter. As shown below, when the MQTT Transmission client comes back online and flushes history, it will flush the oldest historical events first (in order) before sending live Tag changes events to Engine. |
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