The PreviousWatermark element represents the watermark of the latest event that was successfully communicated to the client for the subscription.
The following sections describe attributes, child elements, and parent elements.
Contains information about the subscription and the events that have occurred since the last notification.
The PreviousWatermark property is useful to the client in determining the last successful notification. For example, if a subscription has three events with watermarks 1, 2, and 3, and the next notification is sent with a PreviousWatermark value of 3, the client can compare this value to the Watermark value of the last notification received. This enables the client to ensure the continuity of events.
For push clients, the PreviousWatermark is compared to the local, client-side last known watermark. If the values are different, the client has missed an event notification and should reestablish a subscription by using the latest local watermark. For example, if a push client receives three events for a subscription with watermarks 1, 2, and 3, and the next notification comes with a PreviousWatermark value of 5, the client has missed at least one notification and should create a new subscription, passing a 3 as the watermark.
In the case of a pull client, the value of PreviousWatermark will be the same as the Watermark included by the client in the GetEvents call.
The schema that describes this element is located in the EWS virtual directory of the computer that is running Microsoft Exchange Server 2007 that has the Client Access server role installed.