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Extreme programming (XP) is a software development methodology which is intended to improve software quality and responsiveness to changing customer requirements. As a type of agile software development,it advocates frequent "releases" in short development cycles, which is intended to improve productivity and introduce checkpoints at which new customer requirements can be adopted.
Agile software development comprises various approaches to software development under which requirements and solutions evolve through the collaborative effort of self-organizing and cross-functional teams and their customer(s)/end user(s). It advocates adaptive planning, evolutionary development, early delivery, and continual improvement, and it encourages rapid and flexible response to change.
Other elements of extreme programming include: programming in pairs or doing extensive code review, unit testing of all code, avoiding programming of features until they are actually needed, a flat management structure, code simplicity and clarity, expecting changes in the customer's requirements as time passes and the problem is better understood, and frequent communication with the customer and among programmers.The methodology takes its name from the idea that the beneficial elements of traditional software engineering practices are taken to "extreme" levels. As an example, code reviews are considered a beneficial practice; taken to the extreme, code can be reviewed continuously, i.e. the practice of pair programming.
Pair programming is an agile software development technique in which two programmers work together at one workstation. One, the driver, writes code while the other, the observer or navigator, reviews each line of code as it is typed in. The two programmers switch roles frequently.
Code review is a software quality assurance activity in which one or several persons check a program mainly by viewing and reading parts of its source code, and they do so after implementation or as an interruption of implementation. At least one of the persons must not be the code's author. The persons performing the checking, excluding the author, are called "reviewers".
In computer programming, unit testing is a software testing method by which individual units of source code, sets of one or more computer program modules together with associated control data, usage procedures, and operating procedures, are tested to determine whether they are fit for use.
Kent Beck developed extreme programming during his work on the Chrysler Comprehensive Compensation System (C3) payroll project.Beck became the C3 project leader in March 1996. He began to refine the development methodology used in the project and wrote a book on the methodology (Extreme Programming Explained, published in October 1999). Chrysler cancelled the C3 project in February 2000, after seven years, when Daimler-Benz acquired the company.
Kent Beck is an American software engineer and the creator of extreme programming, a software development methodology that eschews rigid formal specification for a collaborative and iterative design process. Beck was one of the 17 original signatories of the Agile Manifesto, the founding document for agile software development. Extreme and Agile methods are closely associated with Test-Driven Development (TDD), of which Beck is perhaps the leading proponent.
The Chrysler Comprehensive Compensation System was a project in the Chrysler Corporation to replace several payroll applications with a single system. The new system was built using Smalltalk and GemStone. The software development techniques invented and employed on this project are of interest in the history of software engineering. C3 has been referenced in several books on the extreme programming (XP) methodology. The software went live in 1997 paying around ten thousand people. The project continued, intending to take on a larger proportion of the payroll but new development was stopped in 1999.
Contemporary business and science treat as a project any undertaking, carried out individually or collaboratively and possibly involving research or design, that is carefully planned to achieve a particular aim.
Many extreme-programming practices have been around for some time; the methodology takes "best practices" to extreme levels. For example, the "practice of test-first development, planning and writing tests before each micro-increment" was used as early as NASA's Project Mercury, in the early 1960s.To shorten the total development time, some formal test documents (such as for acceptance testing) have been developed in parallel with (or shortly before) the software being ready for testing. A NASA independent test group can write the test procedures, based on formal requirements and logical limits, before programmers write the software and integrate it with the hardware. XP takes this concept to the extreme level, writing automated tests (sometimes inside software modules) which validate the operation of even small sections of software coding, rather than only testing the larger features.
Project Mercury was the first human spaceflight program of the United States, running from 1958 through 1963. An early highlight of the Space Race, its goal was to put a man into Earth orbit and return him safely, ideally before the Soviet Union. Taken over from the US Air Force by the newly created civilian space agency NASA, it conducted twenty uncrewed developmental flights, and six successful flights by astronauts. The program, which took its name from Roman mythology, cost $2.2 billion adjusted for inflation. The astronauts were collectively known as the "Mercury Seven", and each spacecraft was given a name ending with a "7" by its pilot.
In engineering and its various subdisciplines, acceptance testing is a test conducted to determine if the requirements of a specification or contract are met. It may involve chemical tests, physical tests, or performance tests.
Two major influences shaped software development in the 1990s:
Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data, in the form of fields, and code, in the form of procedures. A feature of objects is an object's procedures that can access and often modify the data fields of the object with which they are associated. In OOP, computer programs are designed by making them out of objects that interact with one another. OOP languages are diverse, but the most popular ones are class-based, meaning that objects are instances of classes, which also determine their types.
Procedural programming is a programming paradigm, derived from structured programming, based on the concept of the procedure call. Procedures, also known as routines, subroutines, or functions, simply contain a series of computational steps to be carried out. Any given procedure might be called at any point during a program's execution, including by other procedures or itself. The first major procedural programming languages appeared circa 1957–1964, including Fortran, ALGOL, COBOL, PL/I and BASIC. Pascal and C were published circa 1970–1972.
Business is the activity of making one's living or making money by producing or buying and selling products. Simply put, it is "any activity or enterprise entered into for profit. It does not mean it is a company, a corporation, partnership, or have any such formal organization, but it can range from a street peddler to General Motors."
Rapidly changing requirements demanded shorter product life-cycles, and often clashed with traditional methods of software development.
The Chrysler Comprehensive Compensation System (C3) started in order to determine the best way to use object technologies, using the payroll systems at Chrysler as the object of research, with Smalltalk as the language and GemStone as the data access layer. Chrysler brought in Kent Beck,a prominent Smalltalk practitioner, to do performance tuning on the system, but his role expanded as he noted several problems with the development process. He took this opportunity to propose and implement some changes in development practices - based on his work with his frequent collaborator, Ward Cunningham. Beck describes the early conception of the methods:
The first time I was asked to lead a team, I asked them to do a little bit of the things I thought were sensible, like testing and reviews. The second time there was a lot more on the line. I thought, "Damn the torpedoes, at least this will make a good article," [and] asked the team to crank up all the knobs to 10 on the things I thought were essential and leave out everything else.
Beck invited Ron Jeffries to the project to help develop and refine these methods. Jeffries thereafter acted as a coach to instill the practices as habits in the C3 team.
Information about the principles and practices behind XP disseminated to the wider world through discussions on the original wiki, Cunningham's WikiWikiWeb. Various contributors discussed and expanded upon the ideas, and some spin-off methodologies resulted (see agile software development). Also, XP concepts have been explained[ by whom? ], for several years, using a hypertext system map on the XP website at http://www.extremeprogramming.org circa 1999.
Beck edited a series of books on XP, beginning with his own Extreme Programming Explained (1999, ISBN 0-201-61641-6), spreading his ideas to a much larger audience. Authors in the series went through various aspects attending XP and its practices. The series included a book critical of the practices.
XP generated significant interest among software communities in the late 1990s and early 2000s, seeing adoption in a number of environments radically different from its origins.
The high discipline required by the original practices often went by the wayside, causing some of these practices, such as those thought too rigid, to be deprecated or reduced, or even left unfinished, on individual sites. For example, the practice of end-of-day integration tests for a particular project could be changed to an end-of-week schedule, or simply reduced to testing on mutually agreed dates. Such a more relaxed schedule could avoid people feeling rushed to generate artificial stubs just to pass the end-of-day testing. A less-rigid schedule allows, instead, the development of complex features over a period of several days.
Meanwhile, other agile-development practices have not stood still, and as of 2019 [update] XP continues to evolve, assimilating more lessons from experiences in the field, to use other practices. In the second edition of Extreme Programming Explained (November 2004), five years after the first edition, Beck added more values and practices and differentiated between primary and corollary practices.
The Theory of Sustainable Software Development explains why extreme programming teams can thrive in spite of team disruptions. [ non-primary source needed ]
Extreme Programming Explained describes extreme programming as a software-development discipline that organizes people to produce higher-quality software more productively.
XP attempts to reduce the cost of changes in requirements by having multiple short development cycles, rather than a long one. In this doctrine, changes are a natural, inescapable and desirable aspect of software-development projects, and should be planned for, instead of attempting to define a stable set of requirements.
Extreme programming also introduces a number of basic values, principles and practices on top of the agile programming framework.
XP describes four basic activities that are performed within the software development process: coding, testing, listening, and designing. Each of those activities is described below.
The advocates of XP argue that the only truly important product of the system development process is code – software instructions that a computer can interpret. Without code, there is no working product.
Coding can be used to figure out the most suitable solution. Coding can also help to communicate thoughts about programming problems. A programmer dealing with a complex programming problem, or finding it hard to explain the solution to fellow programmers, might code it in a simplified manner and use the code to demonstrate what he or she means. Code, say the proponents of this position, is always clear and concise and cannot be interpreted in more than one way. Other programmers can give feedback on this code by also coding their thoughts.
Testing is central to extreme programming.Extreme programming's approach is that if a little testing can eliminate a few flaws, a lot of testing can eliminate many more flaws.
System-wide integration testing was encouraged, initially, as a daily end-of-day activity, for early detection of incompatible interfaces, to reconnect before the separate sections diverged widely from coherent functionality. However, system-wide integration testing has been reduced, to weekly, or less often, depending on the stability of the overall interfaces in the system.[ citation needed ]
Programmers must listen to what the customers need the system to do, what "business logic" is needed. They must understand these needs well enough to give the customer feedback about the technical aspects of how the problem might be solved, or cannot be solved. Communication between the customer and programmer is further addressed in the planning game .
From the point of view of simplicity, of course one could say that system development doesn't need more than coding, testing and listening. If those activities are performed well, the result should always be a system that works. In practice, this will not work. One can come a long way without designing but at a given time one will get stuck. The system becomes too complex and the dependencies within the system cease to be clear. One can avoid this by creating a design structure that organizes the logic in the system. Good design will avoid lots of dependencies within a system; this means that changing one part of the system will not affect other parts of the system.[ citation needed ]
Extreme programming initially recognized four values in 1999: communication, simplicity, feedback, and courage. A new value, respect, was added in the second edition of Extreme Programming Explained. Those five values are described below.
Building software systems requires communicating system requirements to the developers of the system. In formal software development methodologies, this task is accomplished through documentation. Extreme programming techniques can be viewed as methods for rapidly building and disseminating institutional knowledge among members of a development team. The goal is to give all developers a shared view of the system which matches the view held by the users of the system. To this end, extreme programming favors simple designs, common metaphors, collaboration of users and programmers, frequent verbal communication, and feedback.
Extreme programming encourages starting with the simplest solution. Extra functionality can then be added later. The difference between this approach and more conventional system development methods is the focus on designing and coding for the needs of today instead of those of tomorrow, next week, or next month. This is sometimes summed up as the "You aren't gonna need it" (YAGNI) approach.Proponents of XP acknowledge the disadvantage that this can sometimes entail more effort tomorrow to change the system; their claim is that this is more than compensated for by the advantage of not investing in possible future requirements that might change before they become relevant. Coding and designing for uncertain future requirements implies the risk of spending resources on something that might not be needed, while perhaps delaying crucial features. Related to the "communication" value, simplicity in design and coding should improve the quality of communication. A simple design with very simple code could be easily understood by most programmers in the team.
Within extreme programming, feedback relates to different dimensions of the system development:
Feedback is closely related to communication and simplicity. Flaws in the system are easily communicated by writing a unit test that proves a certain piece of code will break. The direct feedback from the system tells programmers to recode this part. A customer is able to test the system periodically according to the functional requirements, known as user stories .To quote Kent Beck, "Optimism is an occupational hazard of programming. Feedback is the treatment."
Several practices embody courage. One is the commandment to always design and code for today and not for tomorrow. This is an effort to avoid getting bogged down in design and requiring a lot of effort to implement anything else. Courage enables developers to feel comfortable with refactoring their code when necessary.This means reviewing the existing system and modifying it so that future changes can be implemented more easily. Another example of courage is knowing when to throw code away: courage to remove source code that is obsolete, no matter how much effort was used to create that source code. Also, courage means persistence: a programmer might be stuck on a complex problem for an entire day, then solve the problem quickly the next day, but only if they are persistent.
The respect value includes respect for others as well as self-respect. Programmers should never commit changes that break compilation, that make existing unit-tests fail, or that otherwise delay the work of their peers. Members respect their own work by always striving for high quality and seeking for the best design for the solution at hand through refactoring.
Adopting the four earlier values leads to respect gained from others in the team. Nobody on the team should feel unappreciated or ignored. This ensures a high level of motivation and encourages loyalty toward the team and toward the goal of the project. This value is dependent upon the other values, and is oriented toward teamwork.
The first version of rules for XP was published in 1999 by Don Wellsat the XP website. 29 rules are given in the categories of planning, managing, designing, coding, and testing. Planning, managing and designing are called out explicitly to counter claims that XP doesn't support those activities.
Another version of XP rules was proposed by Ken Auerin XP/Agile Universe 2003. He felt XP was defined by its rules, not its practices (which are subject to more variation and ambiguity). He defined two categories: "Rules of Engagement" which dictate the environment in which software development can take place effectively, and "Rules of Play" which define the minute-by-minute activities and rules within the framework of the Rules of Engagement.
Here are some of the rules (incomplete):
The principles that form the basis of XP are based on the values just described and are intended to foster decisions in a system development project. The principles are intended to be more concrete than the values and more easily translated to guidance in a practical situation.
Extreme programming sees feedback as most useful if it is done frequently and promptly. It stresses that minimal delay between an action and its feedback is critical to learning and making changes. Unlike traditional system development methods, contact with the customer occurs in more frequent iterations. The customer has clear insight into the system that is being developed, and can give feedback and steer the development as needed. With frequent feedback from the customer, a mistaken design decision made by the developer will be noticed and corrected quickly, before the developer spends much time implementing it.
Unit tests contribute to the rapid feedback principle. When writing code, running the unit test provides direct feedback as to how the system reacts to the changes made. This includes running not only the unit tests that test the developer's code, but running in addition all unit tests against all the software, using an automated process that can be initiated by a single command. That way, if the developer's changes cause a failure in some other portion of the system that the developer knows little or nothing about, the automated all-unit-test suite will reveal the failure immediately, alerting the developer of the incompatibility of their change with other parts of the system, and the necessity of removing or modifying their change. Under traditional development practices, the absence of an automated, comprehensive unit-test suite meant that such a code change, assumed harmless by the developer, would have been left in place, appearing only during integration testing – or worse, only in production; and determining which code change caused the problem, among all the changes made by all the developers during the weeks or even months previous to integration testing, was a formidable task.
This is about treating every problem as if its solution were "extremely simple". Traditional system development methods say to plan for the future and to code for reusability. Extreme programming rejects these ideas.
The advocates of extreme programming say that making big changes all at once does not work. Extreme programming applies incremental changes: for example, a system might have small releases every three weeks. When many little steps are made, the customer has more control over the development process and the system that is being developed.
The principle of embracing change is about not working against changes but embracing them. For instance, if at one of the iterative meetings it appears that the customer's requirements have changed dramatically, programmers are to embrace this and plan the new requirements for the next iteration.
Extreme programming has been described as having 12 practices, grouped into four areas:
The practices in XP have been heavily debated.Proponents of extreme programming claim that by having the on-site customer request changes informally, the process becomes flexible, and saves the cost of formal overhead. Critics of XP claim this can lead to costly rework and project scope creep beyond what was previously agreed or funded.
Change-control boards are a sign that there are potential conflicts in project objectives and constraints between multiple users. XP's expedited methods are somewhat dependent on programmers being able to assume a unified client viewpoint so the programmer can concentrate on coding, rather than documentation of compromise objectives and constraints. [ citation needed ]This also applies when multiple programming organizations are involved, particularly organizations which compete for shares of projects.
Other potentially controversial aspects of extreme programming include:
Critics have noted several potential drawbacks,including problems with unstable requirements, no documented compromises of user conflicts, and a lack of an overall design specification or document.
ThoughtWorks has claimed reasonable success on distributed XP projects with up to sixty people.[ citation needed ]
In 2004, industrial extreme programming (IXP)was introduced as an evolution of XP. It is intended to bring the ability to work in large and distributed teams. It now has 23 practices and flexible values.
In 2003, Matt Stephens and Doug Rosenberg published Extreme Programming Refactored: The Case Against XP, which questioned the value of the XP process and suggested ways in which it could be improved.This triggered a lengthy debate in articles, Internet newsgroups, and web-site chat areas. The core argument of the book is that XP's practices are interdependent but that few practical organizations are willing/able to adopt all the practices; therefore the entire process fails. The book also makes other criticisms, and it draws a likeness of XP's "collective ownership" model to socialism in a negative manner.
Certain aspects of XP have changed since the publication of Extreme Programming Refactored; in particular, XP now accommodates modifications to the practices as long as the required objectives are still met. XP also uses increasingly generic terms for processes. Some argue that these changes invalidate previous criticisms; others claim that this is simply watering the process down.
Other authors have tried to reconcile XP with the older methodologies in order to form a unified methodology. Some of these XP sought to replace, such as the waterfall methodology; example: Project Lifecycles: Waterfall, Rapid Application Development (RAD), and All That. JPMorgan Chase & Co. tried combining XP with the computer programming methods of capability maturity model integration (CMMI), and Six Sigma. They found that the three systems reinforced each other well, leading to better development, and did not mutually contradict.
Extreme programming's initial buzz and controversial tenets, such as pair programming and continuous design, have attracted particular criticisms, such as the ones coming from McBreenand Boehm and Turner, Matt Stephens and Doug Rosenberg. Many of the criticisms, however, are believed by Agile practitioners to be misunderstandings of agile development.
In particular, extreme programming has been reviewed and critiqued by Matt Stephens's and Doug Rosenberg's Extreme Programming Refactored.
Martin Fowler is a British software developer, author and international public speaker on software development, specialising in object-oriented analysis and design, UML, patterns, and agile software development methodologies, including extreme programming.
Code refactoring is the process of restructuring existing computer code—changing the factoring—without changing its external behavior. Refactoring is intended to improve nonfunctional attributes of the software. Advantages include improved code readability and reduced complexity; these can improve source-code maintainability and create a more expressive internal architecture or object model to improve extensibility.
In computer programming, a code smell is any characteristic in the source code of a program that possibly indicates a deeper problem. Determining what is and is not a code smell is subjective, and varies by language, developer, and development methodology.
Rapid-application development (RAD), also called rapid-application building (RAB), is both a general term, used to refer to adaptive software development approaches, as well as the name for Terry Barraclough's approach to rapid development. In general, RAD approaches to software development put less emphasis on planning and more emphasis on an adaptive process. Prototypes are often used in addition to or sometimes even in place of design specifications.
Software development is the process of conceiving, specifying, designing, programming, documenting, testing, and bug fixing involved in creating and maintaining applications, frameworks, or other software components. Software development is a process of writing and maintaining the source code, but in a broader sense, it includes all that is involved between the conception of the desired software through to the final manifestation of the software, sometimes in a planned and structured process. Therefore, software development may include research, new development, prototyping, modification, reuse, re-engineering, maintenance, or any other activities that result in software products.
Test-driven development (TDD) is a software development process that relies on the repetition of a very short development cycle: requirements are turned into very specific test cases, then the software is improved so that the tests pass. This is opposed to software development that allows software to be added that is not proven to meet requirements.
"You aren't gonna need it" (YAGNI) is a principle of extreme programming (XP) that states a programmer should not add functionality until deemed necessary. XP co-founder Ron Jeffries has written: "Always implement things when you actually need them, never when you just foresee that you need them." Other forms of the phrase include "You aren't going to need it" and "You ain't gonna need it".
In time management, timeboxing allocates a fixed time period, called a timebox, within which planned activity takes place. It is employed by several project management approaches and for personal time management.
In software engineering, continuous integration (CI) is the practice of merging all developers' working copies to a shared mainline several times a day. Grady Booch first proposed the term CI in his 1991 method, although he did not advocate integrating several times a day. Extreme programming (XP) adopted the concept of CI and did advocate integrating more than once per day – perhaps as many as tens of times per day.
Lean software development is a translation of lean manufacturing principles and practices to the software development domain. Adapted from the Toyota Production System, it is emerging with the support of a pro-lean subculture within the Agile community. Lean offers a solid conceptual framework, values and principles, as well as good practices, derived from experience, that support agile organizations.
Extreme programming (XP) is an agile software development methodology used to implement software projects. This article details the practices used in this methodology. Extreme programming has 12 practices, grouped into four areas, derived from the best practices of software engineering.
Coding conventions are a set of guidelines for a specific programming language that recommend programming style, practices, and methods for each aspect of a program written in that language. These conventions usually cover file organization, indentation, comments, declarations, statements, white space, naming conventions, programming practices, programming principles, programming rules of thumb, architectural best practices, etc. These are guidelines for software structural quality. Software programmers are highly recommended to follow these guidelines to help improve the readability of their source code and make software maintenance easier. Coding conventions are only applicable to the human maintainers and peer reviewers of a software project. Conventions may be formalized in a documented set of rules that an entire team or company follows, or may be as informal as the habitual coding practices of an individual. Coding conventions are not enforced by compilers.
Agile testing is a software testing practice that follows the principles of agile software development. Agile testing involves all members of a cross-functional agile team, with special expertise contributed by testers, to ensure delivering the business value desired by the customer at frequent intervals, working at a sustainable pace. Specification by example is used to capture examples of desired and undesired behavior and guide coding.
Matt Stephens is an author and software process expert based in London, UK. In January 2010 he founded independent book publisher Fingerpress UK Ltd, and in November 2014 he founded the Virtual Reality book discovery site Inkflash.
Specification by example (SBE) is a collaborative approach to defining requirements and business-oriented functional tests for software products based on capturing and illustrating requirements using realistic examples instead of abstract statements. It is applied in the context of agile software development methods, in particular behavior-driven development. This approach is particularly successful for managing requirements and functional tests on large-scale projects of significant domain and organisational complexity.
Acceptance test–driven development (ATDD) is a development methodology based on communication between the business customers, the developers, and the testers. ATDD encompasses many of the same practices as specification by example (SBE), behavior-driven development (BDD), example-driven development (EDD), and support-driven development also called story test–driven development (SDD). All these processes aid developers and testers in understanding the customer's needs prior to implementation and allow customers to be able to converse in their own domain language.
Evolutionary database design involves incremental improvements to the database schema so that it can be continuously updated with changes, reflecting the customer's requirements. People across the globe work on the same piece of software at the same time hence, there is a need for techniques that allow a smooth evolution of database as the design develops. Such methods utilize automated refactoring and continuous integration so that it supports agile methodologies for software development. These development techniques are applied on systems that are in pre-production stage as well on systems that have already been released. These techniques not only cover relevant changes in the database schema according to customer's changing needs, but also migration of modified data into the database and also customizing the database access code accordingly without changing the data semantics.
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