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The history configuration for MQTT Engine tags can be configured setup as needed for your specific requirements as described in Ignition's Configuring Tag History and and How the Tag Historian System Works documents.
There are however some caveats regarding tag history configuration persistence and handling of historical event data that are detailed in this document.
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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.
However, when using MQTT Store and Forward where historical events will be published, there are some caveats to be aware of.
Reference 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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The reference tag must be a Reference tag only. Derived, Expression and OPC tags (expose MQTT Engine tag provider through OPCUA server) will not properly store history when MQTT Engine tags are updated with historical data at a high rate of speed. This is a limitation within the Ignition platform and may be addressed in a future release. |
Alternatively you can use scripting to reapply the history configuration directly to MQTT Engine tags on demand
MQTT Engine has two ways to process historical events and insert the historical data into Ignition's Tag Historian module Table of Contents
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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 MQTT Engine tag but the only property used from the tag history configuration is the Storage Provider.
All historic data will be written to the configured Storage Provider, via the Historian, using the timestamp associated with the historical data.
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All other properties such as Sample Mode, Max Time Between Samples, Deadband etc are ignored when writing historic data |
There are several reasons why you might need to write directly to the tag which include:
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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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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 but the two (either directly on the MQTT Engine tag and/or reference tag) with these three settings required are:
All historic data will be written to Storage Provider, via the Historian, 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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When All other properties such asMax Time Between Samples , Deadband etc are ignored when writing historic data |
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is 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 is 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
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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 enabled on the tag provider that will contain the reference tags, the Edge side client can publish historic data either synchronously or asynchronously. Using each of these different flush methods has the following implications:
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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 and ensure that to select or deselect the In-Order History is selectedconfiguration parameter. This ensures that As shown below, when the Edge side 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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Starting in Ignition 8.1.4 Ignition supports a feature to 'back-fill' historical data for tag providers.
Normally if a tag is updated with a value that is older than the current value, Ignition simply throws it away. In doing so, no tag change event will ever fire and the tag will not be updated. The back-fill option allows the value to be stored to the historical database if history is enabled on that tag. However even with this option enabled, it is important to note that no tag change event will fire. So, tag change event scripts, transaction groups, and any other subsystem that relies on tag change events will not be notified of the event if it is older than the current tag value (i.e. is historical).
To enable this option, browse to the Ignition Gateway Web UI → Config → Tag Providers → Realtime. Select 'edit' for the Tag Provider that will contain Reference Tags that will point to MQTT Engine tags. Scroll to the bottom and click the 'Show advanced properties' checkbox. This will show the 'Allow Back-fill Data' option. Set it to true as shown below and then click 'Save Changes'.