Refresh forces a refresh for all active conditions and inactive, unacknowledged conditions whose event notifications match the filter of the event subscription.
Clients will often need to get the current condition information from the server, particularly at client startup, e.g. for a current alarm summary. The OPC AE server meets this requirement by resending the most recent event notifications that match the filter in the event subscription and that are related to active and/or unacknowledged conditions. The client can then derive the current condition status from the "refreshed" event notifications. When the client needs a refreshed list of active conditions, it will request a refresh from the server. The server will send event notifications to that specific client indicating that they are refreshed instead of original event notifications. Since the client only needs to get the current state information for conditions, only condition events will be refreshed. Note: Refresh is not a general "replay" capability since the server is not required to maintain an event history. Refresh is only meant for updating the client's state information for active or unacknowledged conditions. There may be differences between original and refresh event notifications. Specifically, since some attribute information available at the time of the original event notification may be unavailable at the time of the refresh, some attributes in the refresh may be null. This method is applicable to condition-related events only. Notifications for simple events and tracking events are not returned, even if they would match the filter of the event subscription.