Introduction

Software architecture is a concept that is easy to understand, and that most engineers intuitively feel, especially with a little experience, but it is hard to precisely define. In particular, it is difficult to draw a sharp line between design and architecture -- architecture is one aspect of design that concentrates on some specific features.

In An Introduction to Software Architecture, David Garlan and Mary Shaw suggest that software architecture is a level of design concerned with issues: "Beyond the algorithms and data structures of the computation; designing and specifying the overall system structure emerges as a new kind of problem. Structural issues include gross organization and global control structure; protocols for communication, synchronization, and data access; assignment of functionality to design elements; physical distribution; composition of design elements; scaling and performance; and selection among design alternatives." [GAR93]

But there is more to architecture than just structure; the IEEE Working Group on Architecture defines it as "the highest-level concept of a system in its environment" [IEP1471]. It also encompasses the "fit" with system integrity, with economical constraints, with aesthetic concerns, and with style. It is not limited to an inward focus, but takes into consideration the system as a whole in its user environment and its development environment -- an outward focus.

In this process, the architecture of a software system (at a given point) is the organization or structure of the system's significant components interacting through interfaces, with components composed of successively smaller components and interfaces.

Architecture Description

To speak and reason about software architecture, you must first define an architectural representation, a way of describing important aspects of an architecture. This description is captured in the Artifact: Architecture Notebook.

Architectural views

The software architecture can be represented in multiple architectural views. Each architectural view addresses some specific set of concerns, specific to stakeholders in the development process: users, designers, managers, system engineers, maintainers, and so on.

The views capture the major structural design decisions by showing how the software architecture is decomposed into components, and how components are connected by connectors to produce useful forms [DEW92]. These design choices must be tied to the requirements -- functional and supplementary -- and other constraints. But these choices in turn put further constraints on the requirements, and on future design decisions at a lower level.

Architectural Focus

Although the views above could represent the whole design of a system, the architecture concerns itself only with some specific aspects:

• The structure of the model: the organizational patterns, for example, Layering
• The essential elements: critical use cases, main classes, common mechanisms, and so on, as opposed to all the elements present in the model
• A few key scenarios showing the main control flows throughout the system
• The services, to capture modularity, optional features, product-line aspects

In essence, architectural views are abstractions (or simplifications) of the entire design, in which important characteristics are made more visible by leaving details aside. These characteristics are important when reasoning about:

• System evolution: going to the next development cycle
• Reuse of the architecture, or parts of it, in the context of a product line
• Assessing supplementary qualities, such as performance, availability, portability, and safety
• Assigning development work to teams or subcontractors
• Decisions about including off-the-shelf components
• Insertion in a wider system

Architectural Patterns

Architectural patterns are ready-made forms that solve recurring architectural problems. An architectural framework or an architectural infrastructure (middleware) is a set of components on which you can build a certain kind of architecture. Many of the major architectural difficulties should be resolved in the framework or in the infrastructure, usually targeted to a specific domain: command and control, MIS, control system, and so on.

Examples of Architectural Patterns

[BUS96] groups architectural patterns according to the characteristics of the systems in which they are most applicable, with one category dealing with more general structuring issues. The following table shows the categories presented in [BUS96] and the patterns they contain.

Refer to [BUS96] for a complete description of these patterns.

Architectural Style

A software architecture, or simply an architectural view, may have an attribute called architectural style, which reduces the set of possible forms to choose from, and imposes a certain degree of uniformity to the architecture. The style may be defined by a set of patterns, or by the choice of specific components or connectors as the basic building blocks.