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The mythical man-month essays on software engineering


the mythical man-month essays on software engineering

They are shown in Fig. One can program and debug in a high-level language. Til take that desk you 're at for a few days and build you a set-up to protect you against such things. What is the main function, the reason for the pro- gram? Vehicle Machines and Data Services Simulators. Jock is Lord High Everything Else. Sussenguth, "A the mythical man-month essays on software engineering formal description of System/360 IBM Systems Journal 3, 3 (1964. 'la o o a 43 c3 a 6 Passing the Word He'll sit here and he'll say, 'Do this! Index accounting, 132 administrator, 33 advancement, dual ladder of, 119 Algol, 34, 181, 188 Algol, 68, 44, 64 allocation, dynamic core, 57 APL (A Programming Language 64, 98, 136, 175 architect, 37, 54, 62, 66, 79 architecture, 44, 143 archive. This, then, is how 10 people might contribute in well- differentiated and specialized roles on a programming team built on the surgical model. Mills's Proposal 33 The administrator.

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If, in addition, the whole interface is referenced only by symbolic names, the declaration can be changed by adding or inserting new variables with only recompi- lation, not alteration, of the using program. Program documentation is notoriously poor, and the mythical man-month essays on software engineering its maintenance is worse. Second, the technology changes when one changes machines or working language, so tool lifetime is short. Many criticisms have been made, and additional control structures, such as an n-way branch (the so-called case statement) for distinguishing among many contingencies, and a disaster bail-out (GO TO abnormal END) are very convenient. It is not clear how much of the planning effort, or effort in machine support, writing, and the like, is included. With 10 people, documents can simply be num- bered. The utilization looked something like Fig. The same techniques can be used in Algol and even Fortran. Optimism 15 The pervasiveness of optimism among programmers deserves more than a flip analysis.


Photography Section of the Natural History Museum of Los Angeles. The Programming Systems Product One occasionally reads newspaper accounts of how two program- mers in a remodeled garage have built an important program that surpasses the best efforts of large teams. Hence plan to throw one away; you will, anyhow. This then is the problem with the small, sharp team concept: it is too slow for really big systems. Portman, C, private communication. But this naive statement of the alternatives avoids the hard problem how does one build large systems on a meaningful schedule? As a bound, let them be seven times as productive as medi- ocre programmers in both programming and documentation, be- cause they are sharp. (Most, but not all, engineering changes fix defects in the realization or implementation, rather than the mythical man-month essays on software engineering the architecture, and so are invisible to the user.) Two Steps Forward and One Step Back 121 Program maintenance involves no cleaning, lubrication, or re- pair of deterioration. But false scheduling to match the patron's desired date is much more common in our discipline than elsewhere in engineer- ing. Three workers require three times as much pairwise intercommunication as two; four require six times as much as two.


The first is to reduce the role conflict and inspire sharing of status. In most projects, the first system built is barely usable. Reducing the role conflict. 189 190 Index channel, 45 chief programmer, 32, 180 clerk, program, 33 Clingen,. First, a computer faciliti/. P., "Research in the management of computer programming the mythical man-month essays on software engineering Report SP-2059, System Development Corp., Santa Monica, 1965. Conceptual integrity edit To make a user-friendly system, the system must have conceptual integrity, which can only be achieved by separating architecture from implementation. As the system comes up, the component builders will from time to time appear, bearing hot new versions of their pieces faster, smaller, more complete, or putatively less buggy. It is one of the first documents generated in propos- ing a new product, and the last document finished. The definition will be cleaner and the discipline tighter if at least two implementations are built initially. But it is not. Show the relationship to the book algorithm: a) changes b) specialization c) representation. How does one estimate?


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The manager of architecture, the manager of control program implementation, and I were threshing out the plan, schedule, and division of responsibilities. This man masters all the com- mon tools and is able to instruct his client-boss in their use. In practice, actual (as opposed to formal) authority is acquired from the very momentum of accomplishment. This precept, too, is obvious, but optimism and laziness tempt us to violate. If it fails, admit it frankly and try another. So it is possible to put 200 people on a problem and face the problem of coordinating only 20 minds, those of the surgeons. 23rd ACM Conf., 1968,. After all, precision is the stock in trade, the raison d'etre of formal notations. Second, the repairer is usually not the man who wrote the code, and often he is a junior programmer or trainee. Copyright 1975 by Addison-Wesley Publishing Company, Inc. E., "How do committees invent?" Dfl/flmfl/7, 14, 4 (April, 1968. To supplement Sophocles, no one enjoys bearing bad news, either, so it gets softened without any real intent to deceive.


If the prices are above those postulated, a disastrous spiral begins, and all hands must struggle to break. When a child has learned this architecture, he can tell time as easily from a wristwatch as from a church tower. How, then, shall teams communicate with one another? It illustrates results reported from a study done by Nanus and Farr at System Development Corpo- ration. As to the last question, it is the easiest. Opposed to these one has a formidable set of disadvantages. A report showing milestones and actual completions is the key document. For a fuller discussion, see Brooks,. All testimony indicates that one can do what he needs to do, but that it takes work to find out how, and one may occasionally need unlovely artifices. Sjcq 25 (Spring, 1964. But a written program has another face, that which tells its story to the human user.


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And these experts turn out to be very useful and very widely consulted. We found two levels of these to be useful. M., viii, 160 pilot plant, 116 Pisano,., 127 PL/C compiler, 47, 180 PL/I, 32, 47, 64, 66, 93, 135, 172, 181, 184, 185, 188 planning, 20 Plans and Controls team, 160 playpen, 133, 149 policed system, 65 Pomeroy,. How well was their project equipped with the prerequisites for success? The programmed computer has all the fasci- nation of the pinball machine or the jukebox mechanism, carried to the ultimate. Ershov considers this not only a woe, but also a part of the joy. He is responsible for maintaining all the technical records of the team in a programming-product library. Indeed, an artist's What Does the Implementer the mythical man-month essays on software engineering Do While Waiting?


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The next woe is that designing grand concepts is fun; finding nitty little bugs is just work. His observations on hidden pitfalls are also useful. Technical prose is almost immortal. Lehman and Belady offer evidence that quanta should be very large and widely spaced or else very small and frequent. Algol compiler would require six months, regardless of the number of workers involved (it required longer). It is shown in Fig. One can't be sure. The manager can do two things to help his team make good space-time trade-offs. Neverthe- less there are finer-grained answers that deserve attention. It is therefore very important to allow enough system test time in the original schedule. For example, a system may include a sort program which isn't finished yet.


Yet simultaneously we are moving also toward on-line storage of prose documents for access and for updating via computerized text-editing. From previous experience we projected how many hours of S/360 time we would need, and began to acquire early machines from factory produc- 130 Sharp Tools tion. How about the technical director? This effect, which I have seen many times, is confirmed. He devises or adapts the tools he will need, especially the record- keeping system, including the design automation system. The document itself serves as a check list, a status control, and a data base for his reporting. Within just this group the ratios between best and worst performances averaged about 10:1 on productivity measurements and an amazing 5:1 on program speed and space measurements! Em going to have a switch placed on your desk that will operate the mythical man-month essays on software engineering a sealed recorder at my desk. Notes and References 185 Corbato,.


The architect or team of architects should develop an idea of what the system should do and make sure that this vision is understood by the rest of the team. C., 185 Hezel,. Human beings are not accustomed to being perfect, and few areas of human activity demand. The architect of a system, like the architect of a building, is the user's agent. The inadequacies of the tree approximation give rise to staff groups, task forces, committees, and even the matrix-type organization used in many engineering laboratories. The barriers are sociological, and they must be fought with constant vigilance. He depends upon other people's programs. O., 185 Hetzel,. It seems appropriate, however, to offer this little book, reflecting essentially a personal view.


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Well, what about the classical objections to such a tool? He must disci- pline himself not to act on problems his managers can solve, and never to act on problems when he is explicitly reviewing status. 1, report prepared for the.S. B., 188 Watson,. Sci., 14, 1 (Sept., 1967. These two ways are represented by the boundaries in the diagram. When we undertook to emulate the IBM 1401 on System/360, for example, it developed that there were 30 different 'curios' side effects of supposedly invalid operations that had come into widespread use and had to be considered as part of the definition.


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Time after time, the careful product tester will find places where the word didn't get passed, where the design deci- sions were not properly understood or accurately implemented. Microfiche has its drawbacks. Thus it is that writing, experimentation, 'working out' are essential disciplines for the theoretician. Then, too, the implementation will sometimes give unex- pected and unplanned answers when sharp questions are asked, and the de facto definition will often be found to be inelegant in these particulars precisely because they have never received any thought. As to speed, optimizing compilers now produce some code that is faster than most programmer's handwritten code, Further- more, one can usually solve speed problems by replacing from one to five percent of a compiler-generated program by handwritten substitute after the former is fully debugged. A second notion is to use space and format as much as possible to improve readability and show subordination and nesting. How does the architect avoid the second-system effect? Yes, although naively impossible. (Poor Richard 87 Franklin,. " Then the Lord came down to look at the city and tower which human beings had built. The argument is pubhshed in Iverson,. This is a collection of interact- ing programs, coordinated in function and disciplined in format, so that the assemblage constitutes an entire facility for large tasks. This process in itself added to the total size and slowed execution down.


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Some- times the strategic breakthrough will be a new algorithm, such as the Cooley-Tukey Fast Fourier Transform or the substitution of an n log n sort for an « set of comparisons. How would one do it today? 1335 Scala, New York/Pirenze and Foto Alinari, Firenze 127 128 Sharp Tools Even at this late date, many programming projects are still oper- ated like machine shops so far as tools are concerned. The formal description of PL/I uses new notions of abstract syntax, and it is adequately described. It requires language, and a language policy must be laid down. This idea is known. "Specifications signed by architects and implementers "source coding 100 percent complete, keypunched, entered into disk library "debugged version passes all test cases." These con- crete milestones demark the vague phases of planning, coding, debugging. A terminal, controlled only by program, would be asso- ciated with each program being debugged. To cover repartitioning and system test effects, one would have to add still other men. So all the dirt gets swept under the rug. With a blend of software engineering facts and thought-provoking opinions, Fred Brooks offers insight for anyone managing complex projects. The manual was vague on some points?


Fourth, the design can be tested at each of its refinement steps, so testing can start earlier and focus on the proper level of detail at each step. One must consider the OS/360 documentation as a li- brary or an encyclopedia, not a set of mandatory texts. It is better to have a system omit certain anomalous features and improvements, but to reflect one set of design ideas, than to have one that contains many good but independent and uncoordinated ideas. It is just different creative work. An intermediate step called the pilot plant is necessary to give experience in scaling quantities up and in operating in nonprotec- tive environments. / is the zenith of a style, the work of artists who had understood and assimilated all their predecessors ' successes, in complete possession of the techniques of their times, but using them without indiscreet display nor gratuitous feats of skill. 36 The Surgical Team I Secretary Fig. Where did they lack? H., 88, 182 multics, 93, 136, 146, 183, 185 multiple implementations, 68 Nanus,.,.8, 182 Naur,., 64 Needham,. The Mythical Man Month (Anniversary Edition with four new chapters.).


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How much more vital is the documentation for a public pro- gram, whose user is remote from the mythical man-month essays on software engineering the author in both time and space! Harlan Mills says, in a private communication, "h4y experience begins to tell me that in production programming the person to put at the terminal is the secretary. Compiler and assembler vehicles. W., 47, 180 Cooley,. Strachey, C, 'Review of Planning a Computer System Comp. This effect limits the maximum efficient system that can be generated from the modules of a small system. These costs in turn deter- mine prices. He divides such systems according to interactions among programmers (and- system parts) and finds productivities as follows: -4 Very few interactions 10,000 instructions per man-year Some interactions 5,000 Many interactions 1,500 The man-years do not include support and system test.


Yes, the pyramidal or conical structure is inherently stable and spreads the compressive load well. Milestones must be concrete, specific, measurable events, defined with knife-edge sharpness. These uncertainties aside, the numbers describe the real productivities achieved on a large system, using present-day programming techniques. Saltzer, "Evolutionary design of complex sys- tems. Finally, the use of an implementation as a formal definition is peculiarly susceptible to confusion as to whether the prose de- scription or the formal description is in fact the standard. With modern programming systems this gain does exceed the cost, but in recent years the ratio of gain to cost seems to have fallen as more and more complex functions have been added. Therefore let us begin by identifying the craft of system pro- gramming and the joys and woes inherent. This is as foolish as criticizing a Boeing 747 because it costs 27 million. Minutes Conferences and Courts 67 are kept and decisions are formally, promptly, and widely dis- seminated.


All during implementation, the system architects must maintain continual vigilance to ensure con- tinued system integrity. W., 179, 183 workbook, 75 The Mythical Man-Month : Essays on Software Engineering by Frederick. At Chapel Hill,. To modify a program. Offices have to be of equal size and appointment. Wirth advocates using as high-level a notation as is possible at each step, exposing the concepts and concealing the details until further refinement becomes necessary. A whole technology for doing this was worked out for the System/360 hardware design effort, and it is equally applicable to software projects. New York: Wiley, 1969, Chapter. Brooks' law, and is presented along with the second-system effect and advocacy the mythical man-month essays on software engineering of prototyping.


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That, it develops, was a better way to allocate and the mythical man-month essays on software engineering schedule. When in 1964 I became manager of Operating System/360, I found a programming world quite changed by the progress of the previous few years. The style must be precise, full, and accurately detailed. The principle that no man can serve two masters dictates that the authority structure be tree-like. Productivity is stated in terms of debugged words per man-year. The data showed no correlation whatsoever between experience and per- formance. The prod- uct-testing group then is the surrogate customer, specialized for finding flaws. What are the alternatives facing the manager?


Less and less effort is spent on fixing the mythical man-month essays on software engineering original design flaws; more and more is spent on fixing flaws introduced by earlier fixes. The output listings go back to him to be filed and in- dexed. N., 44 Moore,. The second is that of formal separation and progression from the playpen, to integration, to release. The techniques of communication and organization demand from the manager much thought and as much experienced competence as the software technology itself.



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