The history configuration for MQTT Engine tags can be configured 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.

History Configuration Persistence  

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 Historical Event Processing

MQTT Engine has two ways to process historical events and insert the historical data into Ignition's Tag Historian module:

• Writing historical events directly to the database, via the Historian, bypassing the tag
• Writing historical events directly to the tag

Writing historical events directly to the database, via the Historian, bypassing the tag

The configuration parameters required to write historical events directly to the database, via the Historian, bypassing the Tag are shown below.

MQTT Engine

Under the MQTT Engine Settings General Tab, navigate to the Miscellaneous Settings and ensure Store Historical events is selected

Tag History Configuration

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.

History Configuration Examples

Sample Mode = On Change, Min Time Between Samples = 0, Max Time Between Samples = 0

• Real-time data is written to the storage provider on each tag change event whilst the tag quality is Good
• Historical data is written to the storage provider using the timestamp associated with the historical data 
  • Handles historical metrics flushed from the Edge that have very high resolution; e.g., the historical metrics have timestamps 1 ms apart for a single tag

Sample Mode = On Change, Min Time Between Samples = 0, Max Time Between Samples = 5

• Real-time data is written to the storage provider on each tag change event whilst the tag quality is Good
• Real-time data is written to the storage provider on the Max Time Between Samples interval if no tag change events whilst the tag quality is Good
• Historical data is written to the storage provider using the timestamp associated with the historical data 
  • Max Time Between Samples inserts will not be inserted into the middle of historical data
  • Handles historical metrics flushed from the Edge that have very high resolution; e.g., the historical metrics have timestamps 1 ms apart for a single tag

Sample Mode = Periodic, Max Time Between Samples = 0, Sample Rate = 5

• Real-time data is written to the storage provider on the Sample Rate whilst the tag quality is Good
• Historical data is written to the storage provider using the timestamp associated with the historical data

Writing historical events directly to tag

There are several reasons why you might need to write directly to the tag which include:

• To have Tag Events scripts fire when applicable
• If indirectly referencing MQTT Engine tags
• To have alarms triggered when applicable

The configuration parameters required to write historical events to the Tag instead of directly to the Historian are detailed below. 

MQTT Engine

Under the MQTT Engine Settings General Tab, navigate to the Miscellaneous Settings and ensure Store Historical events is de-selected

Tag History Configuration

History must be enabled on the tag with these two settings required:

• Sample Mode set to On Change
• Min Time Between Samples set to 0
  • This is to account for historical metrics with high resolution timestamp eg. timestamps 1 ms apart for a single tag.

All historic data will be written to the tag using the timestamp associated with the historical data.

Examples

Back-fill support

Historical back-fill support was added in Ignition 8.1.4 which allows Reference Tags in Ignition to be updated even if history arrives out of order. See the following document for details : MQTT History Back-Fill with Reference Tags

If using an Ignition release before v8.1.4, in order for the historical tag changes at the MQTT Engine tag to propagate to the referencing tag, MQTT Engine must be configured to write historical data directly to its tags. See the Writing historical events directly to tag section for configuration details.