Force 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, for things such as a current alarm summary. The OPC Event Server supports this requirement by resending the most recent event notifications which satisfy the filter in the event subscription and which 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 “refresh” 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 for updating the client’s state information for active or unacknowledged conditions. See section 2.6, Subscriptions to Event Notifications. In addition to the refresh indicator, there may be other 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. Refresh event notifications and original event notifications will not be mixed in the same invocation of the event callback, though refresh and original event callback invocations may be interleaved. Thus, it is the responsibility of the client to check time stamps on the event notifications and put them into the correct order, to ensure correct condition status is obtained. This method is applicable to condition-related events only. Notifications for simple events and tracking events are not returned, even if they would satisfy the filter of the event subscription. This method is applicable both when the subscription is active and when it is inactive (see the discussion of the active flag for the SetState method).

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