1. A shared library system embodied in a computer to support dynamic on-line replacement of a shared software library in the computer while preserving application program compatibility, the system comprising: an implementation library including at least one routine, the implementation library being loadable, the implementation library providing a service; an interface library including at least one proxy interface to each routine in the implementation library, the proxy interface being linkable to an application program for accessing the corresponding routine; and a service registry including at least one entry for mapping the interface library to its corresponding implementation library, so that the implementation library can be modified while the system is executing the application program.