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The history configuration for MQTT Engine tags can be setup as needed for your specific requirements as described in Ignition's Configuring Tag History 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.
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 affect.
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.
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:
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.
All historic data will be written to the configured Storage Provider, via the Historian, using the timestamp associated with the historical data.
The Edge side client can publish historic data either in-order (synchronously) or asynchronously.
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.
There are several reasons why you might need to write directly to the tag which include:
The configuration parameters required to write historical events to the 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 or reference tag) with these two settings required:
All historic data will be written to the tag using the timestamp associated with the historical data.
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 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.
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:
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.
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'.