public void setDocumentUpdate(DocumentUpdate upd) {
        if (upd == null) {
            throw new IllegalArgumentException("Document update can not be null.");
        }
        buffer = null;
        decoder = null;
        update = upd;
    }
Sets the document update to perform.

@param upd The document update to set.
documentapi/src/main/java/com/yahoo/documentapi/messagebus/protocol/UpdateDocumentMessage.java