General issue of disconnected systems needs to be addressed in UA.

Larger topic is HA notifications.

Some of the bullet item actually do not apply to OPC UA historian.

1)Historical data for item was changed due to HistoryWrite() call
2)Underlying device supports buffering and historical data was retrieved independently from the realtime data for the period of time that was already received by the client
3)Reverse retrieval of the buffered realtime data (most recent values first) from the device
4)historical data was modified by external means

for Item 1) - a client could monitor for Audit events and detect when a HistoryWrite operation occurred and then perform a read if needed.
For Item 2) Assuming this type of data collection is not a HistoryWrite operation, then the client would have to rely on understand the gaps in data (i.e. the last stored data might be back in the pasted, or they might be a period of bad quality, but these case do not completely cover this case.
for Item 3) reverse retrieval is supported by the historian, this is just normal operation - no work would be needed.
for Item 4) if historical data is modified (even if not be the HistoryWrite), I would assume Audit event would be generate so again a client could easily detect this type of change.

The only issue that is not currently covered would be some sort of notification of a back fill operation (item 2). Maybe all that would be required is an event for this type of operation.

for reverse retrieval - this can be accomplished in the standard historian , since data can be asked for in any order

Added Audit event that indicate a bulk transfer completed

Agreed to changes edited in Web meeting.