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Determining the settings for the MQTT Transmission History Store requires understanding the unique system properties at each Edge Node. There There are a number of factors involved in determining how large the tag capacity much history data can be stored including but not limited to system resources such as CPU, RAM (especially when using 'In-Memory'), Disk IOPS (if using 'Disk-Backed'), the nominal tag change rate (e.g. number of tags changing per second in the system), the flush rate, bandwidth availability, whether flushing in order vs asynchronously, etc.
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Large Tag Capacities
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How to determine these settings
Testing is the best way to determine your settings for your Edge Gateway(s) and the recommended approach is to:
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.
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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. |
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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. |
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The History Store values used in the video were selected to easily show the process for testing. It would be expected that a much higher Flush Quantity would be required in a live environment to stop a build up of data in history store preventing the publishing of live data. |
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