A.1. Introduction

Much of the current work in distributed, object-oriented systems is based on the assumption that objects form a single ontological class. This class consists of all entities that can be fully described by the specification of the set of interfaces supported by the object and the semantics of the operations in those interfaces. The class includes objects that share a single address space, objects that are in separate address spaces on the same machine, and objects that are in separate address spaces on different machines (with, perhaps, different architectures). On the view that all objects are essentially the same kind of entity, these differences in relative location are merely an aspect of the implementation of the object. Indeed, the location of an object may change over time, as an object migrates from one machine to another or the implementation of the object changes.

It is the thesis of this note that this unified view of objects is mistaken. There are fundamental differences between the interactions of distributed objects and the interactions of non-distributed objects. Further, work in distributed object-oriented systems that is based on a model that ignores or denies these differences is doomed to failure, and could easily lead to an industry-wide rejection of the notion of distributed object-based systems.

A.1.1. Terminology

In what follows, we will talk about local and distributed computing. By local computing (local object invocation, etc.), we mean programs that are confined to a single address space. In contrast, we will use the term distributed computing (remote object invocation, etc.) to refer to programs that make calls to other address spaces, possibly on another machine. In the case of distributed computing, nothing is known about the recipient of the call (other than that it supports a particular interface). For example, the client of such a distributed object does not know the hardware architecture on which the recipient of the call is running, or the language in which the recipient was implemented.

Given the above characterizations of “local” and “distributed” computing, the categories are not exhaustive. There is a middle ground, in which calls are made from one address space to another but in which some characteristics of the called object are known. An important class of this sort consists of calls from one address space to another on the same machine; we will discuss these later in the paper.