About Recovery Mode

Normally, the Server-to-Server collector operates in a real-time mode. A real-time mode is when the collector is polling data or has subscribed to events and triggers calculations based on these events occurring in real-time. Messages are also sent as they occur. Recovery mode allows you to recover tag and alarm data when the connection between the collector and the source server is re-established. After a connection loss, the configuration settings for the Server-to-Server collector determine how much tag and alarm data is recovered and if messages are included in the recovery.

When Does Recovery Occur?

Recovery mode executes:

When the collector is started.
When the collector is resumed after a pause.
When there is an on-the-fly change (similar to a pause and resume). Only tags in the new tag configuration are recovered.
When there has not been a collector stop and start, but the connection to the source Historian is restored.

What Happens When Recovery Occurs?

In recovery mode, after connecting to the source Historian, the collector will:

Set up subscriptions for all alarms and trigger tags.
Perform recovery in chronological order (oldest to newest).
Perform message recovery, if enabled.
Begin polling and processing subscriptions in real-time mode.

The following items are recovered:

Event-based tags: This includes the data from the last write time until now. The system retrieves all tags.
Messages: The system checks for new messages and verifies errors. Once the system verifies a connection to the destination, it sends the messages one at a time.
Alarm data: This includes all alarm data from the last write time until now.

Note: Alarm recovery uses a different write time than tag recovery. Alarm recovery starts from the time of the last alarm is replicated to the destination.

If your formula contains tags not in the trigger list or dependencies exist among tags (for example, if a calculation tag is a trigger for another calculation tag), you might not recover all data.