1. A method for achieving exactly-once handling of a plurality of items, the method comprising steps of: for each item in the plurality of items that arrives at a destination site from another site, performing the steps of reading a sequence number associated with the item; determining whether the item has already been processed based on a comparison between the sequence number associated with the item and a persistently stored watermark value; and processing said item only if the item has not already been processed; as the items are processed at the destination site, based on the sequence numbers associated with said items, repeatedly updating said persistently stored watermark value to indicate which items have been processed; periodically sending a message from said destination site to said other site, wherein said message indicates which items have been processed at said destination site; and wherein the steps are performed by one or more computing devices.