Patterns for Software Architectures

Software designers rely on informal patterns, or idioms, to describe the architectures of their software systems—the configurations of components that make up the systems. My purpose here is to reflect on the role these patterns play in software design. I am particularly interested in the ways that informal patterns shape the configurations. These patterns determine how separate parts are combined, or “woven together”. The resulting organization is often called the architecture of the system. Current programming languages do not support these patterns; indeed, the patterns address problems that lie outside the scope of conventional programming languages. This paper describes the character of these architectural patterns and the status of work on models and tools to support them. Design Patterns for Software Architectures The conventional view holds that software components interact by importing and exporting access rights to procedures, then calling the procedures (sometimes referred to as “invoking the methods”). However, software designers describe the interactions among components using quite a rich vocabulary of abstractions. Although the descriptions and the underlying vocabulary are imprecise and informal, designers nevertheless appear to communicate effectively. Even vocabulary that has been overloaded to the extent of becoming buzz-words, when used in context, carries content. For example: • "Camelot is based on the client-server model and uses remote procedure calls both locally and remotely to provide communication among applications and servers." [Spector 87] • "Abstraction layering and system decomposition provide the appearance of system uniformity to clients, yet allow Helix to accommodate a diversity of autonomous devices. The architecture encourages a client-server model for the structuring of applications." [Fridrich 85] • "We have chosen a distributed, object-oriented approach to managing information." [Linton 87] • "The easiest way to make the canonical sequential compiler into a concurrent compiler is to pipeline the execution of the compiler phases over a number of processors. ... A more effective way [is to] split the source code into many segments, which are concurrently processed through the various phases of compilation [by multiple