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Determining the settings for the MQTT Transmission History Store requires understanding the unique system properties at each Edge Node.

These include the number of tags at the Edge Node and Device levels, the frequency at which these tags are changing, the required period for data storage and the RAM or disk based storage available on the Edge hardware.

Testing is the best way to determine your settings for your Edge Gateway(s) and the recommended approach is to:

Configure a History Store with large Edge Node Tag Capacity and Device Tag Capacity
Disconnect for a set time period
Monitor the live updates for the count of tag change events and memory used from the Transmission Info History Store metrics
Reconnect and monitor the data flushing to ensure that the Flush settings (quantity and period) are able to handle the current tag changes that continue to build. Whilst the history flush is in progress, all new change events are written to the history store until it has been completely flushed. If the tag change rate at the Edge is faster than the MQTT Transmission Flush Period this can cause a build up of data in history store(s) and prevent the publishing of live data.
Extrapolate based on the test

For ease of control we recommend setting a Primary Host ID in both MQTT Engine and in the MQTT Transmission.

Removing the Primary Host ID from MQTT Engine will simulate the primary backend application going offline and cause MQTT Transmission to store data to the History Store.

Restoring the Primary Host ID to MQTT Engine will simulate that the primary backend application is online and cause MQTT Transmission to flush the stored data.

In this example, I have a History Store set up with 30,000 tags for both Edge Node and Device Tag Capacity

MQTT Modules for Ignition 8.x > Determining the settings for an MQTT Transmission History Store > History Store Settings 2022-07-07_13-39-58.png

At the Edge, I have 10 tags associated with the Edge Node, 20 tags associated with Device 1 and 8 tags associated with Device 2. Whilst fully connected, the Transmission Info > History Store folder looks as below:

MQTT Modules for Ignition 8.x > Determining the settings for an MQTT Transmission History Store > History Store Not Connected 2022-07-07_13-43-09.png

Once I remove the Primary Host ID from MQTT engine, we can see the metrics update real time

MQTT Modules for Ignition 8.x > Determining the settings for an MQTT Transmission History Store > History store Take 2 2022-07-07_13-31-45.png