Distributed Upcalls: A Mechanism for Layering Asynchronous Abstractions

It is common to use servers to provide access to facilities in a distributed system and to use remote procedure call semantics to access these servers. Procedure calls provide a synchronous interface to call downward through successive layers of abstraction, and remote procedure calls allow the layers to reside in different address spaces. Servers, however, often need the ability to initiate asynchronous and independent actions. Examples of this asynchrony are when a network server needs to signal to an upper layer in a protocol, or when a window manager server needs to respond to user input. Upcalls are a facility that allows a lower level of abstraction to pass information to a higher level of abstract in a clean way. We describe a facility for distributed upcalls that allows upcalls to cross address space boundaries. The complement of remote procedure calls for handling synchronous Server requests and distributed upcalls for handling asynchronous server activities provide a powerful tool for structuring servers. These facilities, together with the ability to dynamically load modules into a server, allow the user to arbitrarily place abstractions in the server or in the client. Distributed Upcalls have been built into a server Structuring system called CLAM, which is currently being used to support an extensible window management system. The CLAM system, including distributed upcalls, remote procedure call extensions to C++, dynamic loading, and basic window classes, is currently running under 4.3BSD UNIX on Microvax workstations.