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When MQTT Engine is configured to process historical events by writing them to the Tag instead of directly to the Historian, MQTT Transmission must be configured to flush history in-order before live data resumes.

This is to ensure that when the Edge side client comes back online and flushes history, MQTT Engine will receive the oldest historical events first (in order) before any live tag changes events.

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If the tag change rate at the Edge is faster than the Flush Period ( the period to wait in milliseconds between publishes when flushing messages upon reestablishing communication), these events will be written and subsequently flushed from the history store. This this can cause a build up of data in history store(s) and preventing prevent the publishing of live data. 

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If we see that history store is flushing a small amount of events (much less that the Flush Quantity which is the maximum number of tags to publish in a single message upon reestablishing communication), then the history is flushing too slowly to fully drain the History Store when new data is being inserted while in-order flushing is in progress.

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