Update objects contain the information necessary to send an update through the Relay system. Some of the fields of an Update object are values assigned by the client application and some by the Relay server. Client applications assign an object reference to the obj field to specify the object being updated and if the updated object has a unique name in the Relay system, the client application assigns the name to the objname field. The Relay server uses the objname field to assign names to new objects, and fills in the timestamp field to order the update with respect to other updates on the same object. A complete listing of Update.java is in Appendix A.
Because an Update object is transmitted between the Relay cache and server using object serialization, the Update class must implement the Serializable interface. Implementing Serializable is trivial because there are no methods to the interface. The Java compiler uses this interface as a flag to indicate special processing for object serialization. Serializing an Update object includes serializing the object referred to by the obj field. This is how the entire state of a modified object is sent with every update, regardless of the object's complexity. See [13] for the Java object serialization specification.