CE Reading (5); ＜11＞~＜19＞; Great Principles of Computing, Peter Denning. (P.Denning, 2003)

[문교식]

<11> How do other fields express their principles? Physicists use terms like photons, electrons, quarks, quantum wave function, relativity, and energy conservation. Astronomers use terms like planets, stars, galaxies, Hubble shift, and black holes. Thermodynamicists uses terms like entropy, first law, second law and Carnot cycle. Biologists use terms like phylogeny, ontogeny, DNA, and enzymes. Each of these terms is actually the title of a story! The principles of a field are actually a set of interwoven stories about the structure and behavior of field elements. They are the names of chapters in books about the field [4,5,7].

<12> These principle-stories seek to make simple the complex history of a complex area. They tell history, showing how the principle evolved and grew in acceptance over time. They name the main contributors. They chronicle feats of heroes and failures of knaves. They lay out obstructions and how they were overcome. They explain how the principle works and how it affects everything else. The game is to define many terms in terms of a few terms and to logically derive many statements from a few statements.

<13> Astronomy, thermodynamics, and physics use the term mechanics for the part of their fields dealing with the behavior and structure of components. For example, Celestial Mechanics deals with the motions of heavenly bodies; Statistical Mechanics with the macro behavior of physical systems comprising large numbers of small particles; Quantum Mechanics with wave behaviors of subatomic particles; Rigid-Body Mechanics with the balance of forces within and between connected objects. I adopt this term for computing.

<14> Computing Mechanics deals with the structure and operation of computations. It does so with stories for algorithm, Turing machine, grammar, message entropy, process, protocol stack, naming, caching, machine learning, virtual machine, and more. I found I could group the stories into the five categories of computation, communication, coordination, automation, and recollection (see Table 2). Every core technology expresses all five in its own way.

<15> The lines between these categories are blurry. For example, the Internet protocol stack is an element of both communication and coordination; naming and caching are both elements of communication and recollection. Therefore, I found it better to view the categories as windows into computing mechanics (see Figure 1). Although the view through the edges of windows overlap, the view through the centers is distinctive.

<16> Design

Computing Mechanics does not exhaust all the principles of our field. Computing professionals follow principles of design that enable them to harness mechanics in the service of users and customers. Five concerns drive the design principles:

• Simplicity: Various forms of abstraction and structure that overcome the apparent complexity of the applications.

• Performance: predicting throughput, response time, bottlenecks, capacity planning.

• Reliability: redundancy, recovery, checkpoint, integrity, system trust.

• Evolvability: adapting to changes in function and scale.

• Security: access control, secrecy, privacy, authentication, integrity, safety.

<17> The design principles themselves include abstraction, information hiding, modules, separate compilation, packages, version control, divide-and-conquer, functional levels, layering, hierarchy, separation of concerns, reuse, encapsulation, interfaces, and virtual machines. These principles are conventions that we collectively have found to lead consistently to dependable and useful programs, systems, and applications. These conventions are practiced within constraints of cost, schedule, compatibility, and usability.

<18> Design is not the same in computing as it is in other fields. In computing we design abstract objects that perform actions. Other fields use abstraction to explain or to organize tangible objects. Since design tells us about arrangements of basic components, design sits above mechanics in our picture of the field.

<19> Might we call Computing Mechanics the "science" of computing and the Design Principles the "art"? I think not. There is good science and engineering and much art at all levels-mechanics, design, and applications.

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