ABSTRACT
Software system design takes place at many levels, each with its own concerns. We learn from computer hardware design that each of these levels has its own elements and composition operators and its own notations, analysis tools, and design rules. From the 1960s through the 1980s software developers concentrated on the programming level. At this level, so-called higher level programming languages provide for the definitions of algorithms and data structures using the familiar programming language control statements, types, and procedures. Now we are turning our attention to the architectural level, in which patterns for organizing module-scale components guide software system design.
