Extreme Programming
Encyclopedia
Extreme programming 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 (timeboxing), which is intended to improve productivity and introduce checkpoints where new customer requirements can be adopted.
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, simplicity and clarity in code, 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, on the theory that if a little is good, more is better.
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.
during his work on the Chrysler Comprehensive Compensation System
(C3) payroll project. Beck became the C3 project leader
in March 1996 and began to refine the development method used in the project and wrote a book on the method (in October 1999, Extreme Programming Explained was published).
Chrysler cancelled the C3 project in February 2000, after the company was acquired by Daimler-Benz.
Although extreme programming itself is relatively new, many of its practices have been around for some time; the methodology, after all, 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 (or shortly before) the software is ready for testing. A NASA independent test group can write the test procedures, based on formal requirements and logical limits, before the software has been written and integrated with the hardware. In XP, this concept is taken to the extreme level by writing automated tests (perhaps inside of software modules) which validate the operation of even small sections of software coding, rather than only testing the larger features. Some other XP practices, such as refactoring
, modularity
, bottom-up design
, and incremental design
were described by Leo Brodie in his book published in 1984.
replaced procedural programming
as the programming paradigm favored by some in the industry; externally, the rise of the Internet and the dot-com boom emphasized speed-to-market and company-growth as competitive business factors. Rapidly-changing requirements demanded shorter product life-cycles
, and were often incompatible with traditional methods of software development.
The Chrysler Comprehensive Compensation System
was 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
. They brought in Kent Beck
, a prominent Smalltalk practitioner, to do performance tuning
on the system, but his role expanded as he noted several problems they were having with their development process. He took this opportunity to propose and implement some changes in their practices based on his work with his frequent collaborator, Ward Cunningham
. Beck describes the early conception of the methods:
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 was 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, 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, yet very receptive, audience. Authors in the series went through various aspects attending XP and its practices. The series included a book that was critical of the practices.
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 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, for some complex features to be more fully developed over a several-day period. However, some level of periodic integration testing can detect groups of people working in non-compatible, tangent efforts before too much work is invested in divergent, wrong directions.
Meanwhile, other agile development practices have not stood still, and XP is still evolving, assimilating more lessons from experiences in the field, to use other practices. In the second edition of Extreme Programming Explained, Beck added more values and practices and differentiated between primary and corollary practices.
XP attempts to reduce the cost of changes in requirements by having multiple short development cycles, rather than one 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.
Coding can also 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 and finding it hard to explain the solution to fellow programmers might code it 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.
A "testathon" is an event when programmers meet to do collaborative test writing, a kind of brainstorming relative to software testing.
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.
(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. 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.
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."
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, if only they are persistent.
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 very dependent upon the other values, and is very much oriented toward people in a team.
Another version of XP rules was proposed by Ken Auer in 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.
Unit tests also contribute to the rapid feedback principle. When writing code, the unit test provides direct feedback as to how the system reacts to the changes one has made. If, for instance, the changes affect a part of the system that is not in the scope of the programmer who made them, that programmer will not notice the flaw. There is a large chance that this bug will appear when the system is in production.
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.
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 methodology is 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. 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:
has claimed reasonable success on distributed XP projects with up to sixty people.
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. As it is a new member of the Agile family, there is not enough data to prove its usability, however it claims to be an answer to what it sees as XP's imperfections.
in a negative manner.
Certain aspects of XP have changed since the book Extreme Programming Refactored (2003) was published; 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.
RDP Practice is a technique for tailoring extreme programming. This practice was initially proposed as a long research paper in a workshop organized by Philippe Kruchten and Steve Adolph( See APSO workshop at ICSE 2008) and yet it is the only proposed and applicable method for customizing XP. The valuable concepts behind RDP practice, in a short time provided the rationale for applicability of it in industries. RDP Practice tries to customize XP by relying on technique XP Rules.
Other authors have tried to reconcile XP with the older methods in order to form a unified methodology. Some of these XP sought to replace, such as the waterfall
method; example: Project Lifecycles: Waterfall, Rapid Application Development, and All That. JPMorgan Chase & Co.
tried combining XP with the computer programming methodologies 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.
and continuous design
, have attracted particular criticisms, such as the ones coming from McBreen and Boehm and Turner. Many of the criticisms, however, are believed by Agile practitioners to be misunderstandings of agile development.
In particular, extreme programming is reviewed and critiqued by Matt Stephens's and Doug Rosenberg's Extreme Programming Refactored.
Criticisms include:
Software development methodology
A software development methodology or system development methodology in software engineering is a framework that is used to structure, plan, and control the process of developing an information system.- History :...
which is intended to improve software quality and responsiveness to changing customer requirements. As a type of agile software development
Agile software development
Agile software development is a group of software development methodologies based on iterative and incremental development, where requirements and solutions evolve through collaboration between self-organizing, cross-functional teams...
,
it advocates frequent "releases" in short development cycles (timeboxing), which is intended to improve productivity and introduce checkpoints where new customer requirements can be adopted.
Other elements of extreme programming include: programming in pairs
Pair programming
Pair programming is an agile software development technique in which two programmers work together at one workstation. One, the driver, types in code while the other, the observer , reviews each line of code as it is typed in...
or doing extensive code review
Code review
Code review is systematic examination of computer source code. It is intended to find and fix mistakes overlooked in the initial development phase, improving both the overall quality of software and the developers' skills...
, unit testing of all code, avoiding programming of features until they are actually needed, a flat management structure, simplicity and clarity in code, 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, on the theory that if a little is good, more is better.
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.
History
Extreme programming was created by Kent BeckKent Beck
Kent Beck is an American software engineer and the creator of the Extreme Programming and Test Driven Development software development methodologies. Beck was one of the 17 original signatories of the Agile Manifesto in 2001....
during his work on the Chrysler Comprehensive Compensation System
Chrysler Comprehensive Compensation System
The Chrysler Comprehensive Compensation System was a project in the Chrysler Corporation to replace several payroll applications with a single system. The project began with the work of The new system was built using Smalltalk and GemStone...
(C3) payroll project. Beck became the C3 project leader
Project management
Project management is the discipline of planning, organizing, securing, and managing resources to achieve specific goals. A project is a temporary endeavor with a defined beginning and end , undertaken to meet unique goals and objectives, typically to bring about beneficial change or added value...
in March 1996 and began to refine the development method used in the project and wrote a book on the method (in October 1999, Extreme Programming Explained was published).
Chrysler cancelled the C3 project in February 2000, after the company was acquired by Daimler-Benz.
Although extreme programming itself is relatively new, many of its practices have been around for some time; the methodology, after all, 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
Project Mercury
In January 1960 NASA awarded Western Electric Company a contract for the Mercury tracking network. The value of the contract was over $33 million. Also in January, McDonnell delivered the first production-type Mercury spacecraft, less than a year after award of the formal contract. On February 12,...
, in the early 1960s . To shorten the total development time, some formal test documents (such as for acceptance testing) have been developed in parallel (or shortly before) the software is ready for testing. A NASA independent test group can write the test procedures, based on formal requirements and logical limits, before the software has been written and integrated with the hardware. In XP, this concept is taken to the extreme level by writing automated tests (perhaps inside of software modules) which validate the operation of even small sections of software coding, rather than only testing the larger features. Some other XP practices, such as refactoring
Refactoring
Code refactoring is "disciplined technique for restructuring an existing body of code, altering its internal structure without changing its external behavior", undertaken in order to improve some of the nonfunctional attributes of the software....
, modularity
Modularity (programming)
Modular programming is a software design technique that increases the extent to which software is composed of separate, interchangeable components called modules by breaking down program functions into modules, each of which accomplishes one function and contains everything necessary to accomplish...
, bottom-up design
Top-down and bottom-up design
Top–down and bottom–up are strategies of information processing and knowledge ordering, mostly involving software, but also other humanistic and scientific theories . In practice, they can be seen as a style of thinking and teaching...
, and incremental design
Iterative and incremental development
Iterative and Incremental development is at the liver of a cyclic software development process developed in response to the weaknesses of the waterfall model...
were described by Leo Brodie in his book published in 1984.
Origins
Software development in the 1990s was shaped by two major influences: internally, object-oriented programmingObject-oriented programming
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
replaced procedural programming
Procedural programming
Procedural programming can sometimes be used as a synonym for imperative programming , but can also refer to a programming paradigm, derived from structured programming, based upon the concept of the procedure call...
as the programming paradigm favored by some in the industry; externally, the rise of the Internet and the dot-com boom emphasized speed-to-market and company-growth as competitive business factors. Rapidly-changing requirements demanded shorter product life-cycles
Product life cycle management
Product life-cycle management is the succession of strategies used by business management as a product goes through its life-cycle. The conditions in which a product is sold changes over time and must be managed as it moves through its succession of stages.Product life-cycle Like human beings,...
, and were often incompatible with traditional methods of software development.
The Chrysler Comprehensive Compensation System
Chrysler Comprehensive Compensation System
The Chrysler Comprehensive Compensation System was a project in the Chrysler Corporation to replace several payroll applications with a single system. The project began with the work of The new system was built using Smalltalk and GemStone...
was 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
Smalltalk
Smalltalk is an object-oriented, dynamically typed, reflective programming language. Smalltalk was created as the language to underpin the "new world" of computing exemplified by "human–computer symbiosis." It was designed and created in part for educational use, more so for constructionist...
as the language and GemStone
Gemstone Database Management System
GemStone is a proprietary application framework that was first available for Smalltalk as an object database.GemStone Systems was founded in 1982 as Servio Logic, and then became GemStone Systems, Inc in 1995. GemStone developed its first prototype in 1982, and shipped its first product in 1986. ...
as the data access layer
Data access layer
A data access layer is a layer of a computer program which provides simplified access to data stored in persistent storage of some kind, such as an entity-relational database....
. They brought in Kent Beck
Kent Beck
Kent Beck is an American software engineer and the creator of the Extreme Programming and Test Driven Development software development methodologies. Beck was one of the 17 original signatories of the Agile Manifesto in 2001....
, a prominent Smalltalk practitioner, to do performance tuning
Performance tuning
Performance tuning is the improvement of system performance. This is typically a computer application, but the same methods can be applied to economic markets, bureaucracies or other complex systems. The motivation for such activity is called a performance problem, which can be real or anticipated....
on the system, but his role expanded as he noted several problems they were having with their development process. He took this opportunity to propose and implement some changes in their practices based on his work with his frequent collaborator, Ward Cunningham
Ward Cunningham
Howard G. "Ward" Cunningham is an American computer programmer who developed the first wiki. A pioneer in both design patterns and Extreme Programming, he started programming the software WikiWikiWeb in 1994 and installed it on the website of his software consultancy, Cunningham & Cunningham , on...
. Beck describes the early conception of the methods:
Beck invited Ron Jeffries
Ron Jeffries
Ron Jeffries is one of the 3 founders of the Extreme Programming software development methodology circa 1996, along with Kent Beck and Ward Cunningham. He was from 1996, an XP coach on the Chrysler Comprehensive Compensation System project, which was where XP was invented. He is an author of...
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 was disseminated to the wider world through discussions on the original Wiki
Wiki
A wiki is a website that allows the creation and editing of any number of interlinked web pages via a web browser using a simplified markup language or a WYSIWYG text editor. Wikis are typically powered by wiki software and are often used collaboratively by multiple users. Examples include...
, Cunningham's WikiWikiWeb
WikiWikiWeb
WikiWikiWeb is a term that has been used to refer to four things: the first wiki, or user-editable website, launched on 25 March 1995 by Ward Cunningham as part of the Portland Pattern Repository ; the Perl-based application that was used to run it, also developed by Cunningham, which was the first...
. Various contributors discussed and expanded upon the ideas, and some spin-off methodologies resulted (see agile software development
Agile software development
Agile software development is a group of software development methodologies based on iterative and incremental development, where requirements and solutions evolve through collaboration between self-organizing, cross-functional teams...
). Also, XP concepts have been explained, for several years, using a hypertext
Hypertext
Hypertext is text displayed on a computer or other electronic device with references to other text that the reader can immediately access, usually by a mouse click or keypress sequence. Apart from running text, hypertext may contain tables, images and other presentational devices. Hypertext is the...
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, yet very receptive, audience. Authors in the series went through various aspects attending XP and its practices. The series included a book that was critical of the practices.
Current state
XP created quite a buzz 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 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, for some complex features to be more fully developed over a several-day period. However, some level of periodic integration testing can detect groups of people working in non-compatible, tangent efforts before too much work is invested in divergent, wrong directions.
Meanwhile, other agile development practices have not stood still, and XP is still evolving, assimilating more lessons from experiences in the field, to use other practices. In the second edition of Extreme Programming Explained, Beck added more values and practices and differentiated between primary and corollary practices.
Goals
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 one 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.
Activities
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.Coding
The advocates of XP argue that the only truly important product of the system development process is code – software instructions a computer can interpret. Without code, there is no working product.Coding can also 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 and finding it hard to explain the solution to fellow programmers might code it 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
Extreme programming's approach is that if a little testing can eliminate a few flaws, a lot of testing can eliminate many more flaws.- Unit testUnit testIn computer programming, unit testing is a method by which individual units of source code are tested to determine if they are fit for use.A unit is the smallest testable part of an application. In procedural programming a unit could be an entire module but is more commonly an individual function...
s determine whether a given feature works as intended. A programmer writes as many automated tests as they can think of that might "break" the code; if all tests run successfully, then the coding is complete. Every piece of code that is written is tested before moving on to the next feature. - Acceptance testAcceptance testIn engineering and its various subdisciplines, acceptance testing is a test conducted to determine if the requirements of a specification or contract are met...
s verify that the requirements as understood by the programmers satisfy the customer's actual requirements. These occur in the exploration phase of release planning.
A "testathon" is an event when programmers meet to do collaborative test writing, a kind of brainstorming relative to software testing.
Listening
Programmers must listen to what the customers need the system to do, what "business logic"Business logic
Business logic, or domain logic, is a non-technical term generally used to describe the functional algorithms that handle information exchange between a database and a user interface.- Scope of business logic :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.
Designing
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.Values
Extreme Programming initially recognized four values in 1999. A new value was added in the second edition of Extreme Programming Explained. The five values are:Communication
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.Simplicity
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 ain't gonna need it"You Ain't Gonna Need It
"You ain't gonna need it" is the principle in extreme programming that programmers should not add functionality until it is necessary...
(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. 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.
Feedback
Within extreme programming, feedback relates to different dimensions of the system development:- Feedback from the system: by writing unit testUnit testIn computer programming, unit testing is a method by which individual units of source code are tested to determine if they are fit for use.A unit is the smallest testable part of an application. In procedural programming a unit could be an entire module but is more commonly an individual function...
s, or running periodic integration tests, the programmers have direct feedback from the state of the system after implementing changes. - Feedback from the customer: The functional tests (aka acceptance tests) are written by the customer and the testers. They will get concrete feedback about the current state of their system. This review is planned once in every two or three weeks so the customer can easily steer the development.
- Feedback from the team: When customers come up with new requirements in the planning game the team directly gives an estimation of the time that it will take to implement.
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
User story
In computer programming a user story is one or more sentences in the everyday or business language of the end user that captures what the user wants to achieve. User stories are used with Agile software development methodologies for the basis of what features that can be implemented...
. To quote Kent Beck
Kent Beck
Kent Beck is an American software engineer and the creator of the Extreme Programming and Test Driven Development software development methodologies. Beck was one of the 17 original signatories of the Agile Manifesto in 2001....
, "Optimism is an occupational hazard of programming. Feedback is the treatment."
Courage
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 refactoringRefactoring
Code refactoring is "disciplined technique for restructuring an existing body of code, altering its internal structure without changing its external behavior", undertaken in order to improve some of the nonfunctional attributes of the software....
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, if only they are persistent.
Respect
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 very dependent upon the other values, and is very much oriented toward people in a team.
Rules
The first version of rules for XP was published in 1999 by Don Wells at 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 Auer in 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.
Principles
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.Feedback
Extreme programming sees feedback as most useful if it is done rapidly and expresses that the time 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. He or she can give feedback and steer the development as needed.Unit tests also contribute to the rapid feedback principle. When writing code, the unit test provides direct feedback as to how the system reacts to the changes one has made. If, for instance, the changes affect a part of the system that is not in the scope of the programmer who made them, that programmer will not notice the flaw. There is a large chance that this bug will appear when the system is in production.
Assuming simplicity
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.
Embracing change
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.Practices
Extreme programming has been described as having 12 practices, grouped into four areas:Fine scale feedback
- Pair programmingPair programmingPair programming is an agile software development technique in which two programmers work together at one workstation. One, the driver, types in code while the other, the observer , reviews each line of code as it is typed in...
- Planning game
- Test-driven developmentTest-driven developmentTest-driven development is a software development process that relies on the repetition of a very short development cycle: first the developer writes a failing automated test case that defines a desired improvement or new function, then produces code to pass that test and finally refactors the new...
- Whole team
Continuous process
- Continuous integrationContinuous integrationIn software engineering, continuous integration implements continuous processes of applying quality control — small pieces of effort, applied frequently...
- RefactoringRefactoringCode refactoring is "disciplined technique for restructuring an existing body of code, altering its internal structure without changing its external behavior", undertaken in order to improve some of the nonfunctional attributes of the software....
or design improvement - Small releases
Shared understanding
- Coding standards
- Collective code ownership
- Simple design
- System metaphor
Coding
- The customer is always available
- Code the Unit testUnit testIn computer programming, unit testing is a method by which individual units of source code are tested to determine if they are fit for use.A unit is the smallest testable part of an application. In procedural programming a unit could be an entire module but is more commonly an individual function...
first - Only one pair integrates code at a time
- Leave Optimization till last
- No OvertimeOvertimeOvertime is the amount of time someone works beyond normal working hours. Normal hours may be determined in several ways:*by custom ,*by practices of a given trade or profession,*by legislation,...
Testing
- All code must have Unit tests
- All code must pass all Unit tests before it can be released.
- When a BugSoftware bugA software bug is the common term used to describe an error, flaw, mistake, failure, or fault in a computer program or system that produces an incorrect or unexpected result, or causes it to behave in unintended ways. Most bugs arise from mistakes and errors made by people in either a program's...
is found tests are created before the bug is addressed (a bug is not an error in logic, it is a test you forgot to write) - Acceptance tests are run often and the results are published
Controversial aspects
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 reworkRework
Rework is the term for the refinishing operation or repair of an electronic printed circuit board assembly. Mass processing techniques are not applicable to single device repair and/or replacement, so specialized techniques are required to replace defective components – most notably area array...
and project scope creep
Scope creep
Scope Creep in project management refers to uncontrolled changes or continuous growth in a project's scope. This phenomenon can occur when the scope of a project is not properly defined, documented, or controlled...
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 methodology is 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. 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:
- Requirements are expressed as automated acceptance tests rather than specification documents.
- Requirements are defined incrementally, rather than trying to get them all in advance.
- Software developers are usually required to work in pairs.
- There is no Big Design Up FrontBig Design Up FrontBig Design Up Front is a term for any software development approach in which the program's design is to be completed and perfected before that program's implementation is started...
. Most of the design activity takes place on the fly and incrementally, starting with "the simplest thing that could possibly work" and adding complexity only when it's required by failing tests. Critics compare this to "debugging a system into appearance" and fear this will result in more re-design effort than only re-designing when requirements change. - A customer representativeCustomer representativeA customer representative is an individual who represents a community that intends to purchase a product. The term is most often applied to a representative of a company who works closely with a producer or developer to clarify specifications for a product or service. The term is used in software...
is attached to the projectProjectA project in business and science is typically defined as a collaborative enterprise, frequently involving research or design, that is carefully planned to achieve a particular aim. Projects can be further defined as temporary rather than permanent social systems that are constituted by teams...
. This role can become a single-point-of-failure for the project, and some people have found it to be a source of stress. Also, there is the danger of micro-management by a non-technical representative trying to dictate the use of technical software features and architecture. - Dependence upon all other aspects of XP: "XP is like a ring of poisonous snakes, daisy-chained together. All it takes is for one of them to wriggle loose, and you've got a very angry, poisonous snake heading your way."
Scalability
Historically, XP only works on teams of twelve or fewer people. One way to circumvent this limitation is to break up the project into smaller pieces and the team into smaller groups. It has been claimed that XP has been used successfully on teams of over a hundred developers. ThoughtWorksThoughtWorks
ThoughtWorks is a privately owned global IT consultancy that delivers custom software, software tools, consulting, and transformation services to Global 1000 companies. It has a products division, ThoughtWorks Studios, which creates and markets software development and project management applications...
has claimed reasonable success on distributed XP projects with up to sixty people.
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. As it is a new member of the Agile family, there is not enough data to prove its usability, however it claims to be an answer to what it sees as XP's imperfections.
Severability and responses
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 socialismSocialism
Socialism is an economic system characterized by social ownership of the means of production and cooperative management of the economy; or a political philosophy advocating such a system. "Social ownership" may refer to any one of, or a combination of, the following: cooperative enterprises,...
in a negative manner.
Certain aspects of XP have changed since the book Extreme Programming Refactored (2003) was published; 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.
RDP Practice is a technique for tailoring extreme programming. This practice was initially proposed as a long research paper in a workshop organized by Philippe Kruchten and Steve Adolph( See APSO workshop at ICSE 2008) and yet it is the only proposed and applicable method for customizing XP. The valuable concepts behind RDP practice, in a short time provided the rationale for applicability of it in industries. RDP Practice tries to customize XP by relying on technique XP Rules.
Other authors have tried to reconcile XP with the older methods in order to form a unified methodology. Some of these XP sought to replace, such as the waterfall
Waterfall model
The waterfall model is a sequential design process, often used in software development processes, in which progress is seen as flowing steadily downwards through the phases of Conception, Initiation, Analysis, Design, Construction, Testing, Production/Implementation and Maintenance.The waterfall...
method; example: Project Lifecycles: Waterfall, Rapid Application Development, and All That. JPMorgan Chase & Co.
JPMorgan Chase & Co.
JPMorgan Chase & Co. is an American multinational banking corporation of securities, investments and retail. It is the largest bank in the United States by assets and market capitalization.It is a major provider of financial services, with assets of $2 trillion and according to Forbes magazine is...
tried combining XP with the computer programming methodologies of Capability Maturity Model Integration
Capability Maturity Model Integration
Capability Maturity Model Integration is a process improvement approach whose goal is to help organizations improve their performance. CMMI can be used to guide process improvement across a project, a division, or an entire organization...
(CMMI), and Six Sigma
Six Sigma
Six Sigma is a business management strategy originally developed by Motorola, USA in 1986. , it is widely used in many sectors of industry.Six Sigma seeks to improve the quality of process outputs by identifying and removing the causes of defects and minimizing variability in manufacturing and...
. They found that the three systems reinforced each other well, leading to better development, and did not mutually contradict.
Criticism
Extreme programming's initial buzz and controversial tenets, such as pair programmingPair programming
Pair programming is an agile software development technique in which two programmers work together at one workstation. One, the driver, types in code while the other, the observer , reviews each line of code as it is typed in...
and continuous design
Continuous design
Continuous design is a software development practice of creating and modifying the design of a system as it is developed, rather than specifying the system completely before development starts, or in bursts at the beginning of each iteration...
, have attracted particular criticisms, such as the ones coming from McBreen and Boehm and Turner. Many of the criticisms, however, are believed by Agile practitioners to be misunderstandings of agile development.
In particular, extreme programming is reviewed and critiqued by Matt Stephens's and Doug Rosenberg's Extreme Programming Refactored.
Criticisms include:
- A methodology is only as effective as the people involved, Agile does not solve this
- Often used as a means to bleed money from customers through lack of defining a deliverable
- Lack of structure and necessary documentation
- Only works with senior-level developers
- Incorporates insufficient software design
- Requires meetings at frequent intervals at enormous expense to customers
- Requires too much cultural change to adopt
- Can lead to more difficult contractual negotiations
- Can be very inefficient—if the requirements for one area of code change through various iterations, the same programming may need to be done several times over. Whereas if a plan were there to be followed, a single area of code is expected to be written once.
- Impossible to develop realistic estimates of work effort needed to provide a quote, because at the beginning of the project no one knows the entire scope/requirements
- Can increase the risk of scope creepScope creepScope Creep in project management refers to uncontrolled changes or continuous growth in a project's scope. This phenomenon can occur when the scope of a project is not properly defined, documented, or controlled...
due to the lack of detailed requirements documentation - Agile is feature driven; non-functional quality attributes are hard to be placed as user storiesUser storyIn computer programming a user story is one or more sentences in the everyday or business language of the end user that captures what the user wants to achieve. User stories are used with Agile software development methodologies for the basis of what features that can be implemented...
See also
- Software engineeringSoftware engineeringSoftware Engineering is the application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software, and the study of these approaches; that is, the application of engineering to software...
- Software craftsmanshipSoftware CraftsmanshipSoftware craftsmanship is an approach to software development that emphasizes the coding skills of the software developers themselves. It is a response by software developers to the perceived ills of the mainstream software industry, including the prioritization of financial concerns over developer...
- Agile software developmentAgile software developmentAgile software development is a group of software development methodologies based on iterative and incremental development, where requirements and solutions evolve through collaboration between self-organizing, cross-functional teams...
- Extreme project managementExtreme project managementExtreme project management refers to a method of managing very complex and very uncertain projects.Extreme project management differs from traditional project management mainly in its open, elastic and undeterministic approach. The main focus of XPM is on the human side of project management...
- Extreme programming practicesExtreme Programming PracticesExtreme programming is a popular agile software development methodology used to implement software projects. This article details the practices used in this methodology...
- Pair programmingPair programmingPair programming is an agile software development technique in which two programmers work together at one workstation. One, the driver, types in code while the other, the observer , reviews each line of code as it is typed in...
- KaizenKaizen, Japanese for "improvement", or "change for the better" refers to philosophy or practices that focus upon continuous improvement of processes in manufacturing, engineering, game development, and business management. It has been applied in healthcare, psychotherapy, life-coaching, government,...
- List of software development philosophies
- Scrum (development)Scrum (development)Scrum is an iterative, incremental framework for project management often seen in agile software development, a type of software engineering....
- Continuous obsolescenceContinuous obsolescenceContinuous obsolescence or perpetual revolution is a phenomenon where industry trends, or other items that do not immediately correspond to technical needs, mandate a continual readaptation of a system; such work does not increase the usefulness of the system, but is required for the system to...
Further reading
- Ken Auer and Roy MillerRoy MillerRoy Miller may refer to:*Roy Andrew Miller, linguist*Roy Miller , Costa Rican footballer*Roy Miller *Roy Miller...
. Extreme Programming Applied: Playing To Win, Addison–Wesley. - Kent BeckKent BeckKent Beck is an American software engineer and the creator of the Extreme Programming and Test Driven Development software development methodologies. Beck was one of the 17 original signatories of the Agile Manifesto in 2001....
: Extreme Programming Explained: Embrace Change, Addison–Wesley. - Kent BeckKent BeckKent Beck is an American software engineer and the creator of the Extreme Programming and Test Driven Development software development methodologies. Beck was one of the 17 original signatories of the Agile Manifesto in 2001....
and Martin FowlerMartin Fowler-Online presentations:* at RailsConf 2006* at JAOO 2006* at QCon London 2007 * at QCon London 2008 * at ThoughtWorks Quarterly Technology Briefing, October 2008...
: Planning Extreme Programming, Addison–Wesley. - Kent BeckKent BeckKent Beck is an American software engineer and the creator of the Extreme Programming and Test Driven Development software development methodologies. Beck was one of the 17 original signatories of the Agile Manifesto in 2001....
and Cynthia Andres. Extreme Programming Explained: Embrace Change, Second Edition, Addison–Wesley. - Alistair CockburnAlistair CockburnAlistair Cockburn is one of the initiators of the agile movement in software development, helping write theManifesto for Agile Software Development in 2001 and the agile PM Declaration of Interdependence in 2005...
: Agile Software Development, Addison–Wesley. - Martin FowlerMartin Fowler-Online presentations:* at RailsConf 2006* at JAOO 2006* at QCon London 2007 * at QCon London 2008 * at ThoughtWorks Quarterly Technology Briefing, October 2008...
: Refactoring: Improving the Design of Existing Code, Addison–Wesley. - Harvey Herela (2005). Case Study: The Chrysler Comprehensive Compensation System. Galen Lab, U.C. Irvine.
- Jim HighsmithJim HighsmithJames A. Highsmith III is an American software engineer and author of books in the field of software development methodology. He is the creator of Adaptive Software Development, described in his 1999 book "Adaptive Software Development", and winner of the 2000 Jolt Award, and the Stevens Award in...
. Agile Software Development Ecosystems, Addison–Wesley. - Ron JeffriesRon JeffriesRon Jeffries is one of the 3 founders of the Extreme Programming software development methodology circa 1996, along with Kent Beck and Ward Cunningham. He was from 1996, an XP coach on the Chrysler Comprehensive Compensation System project, which was where XP was invented. He is an author of...
, Ann Anderson and Chet Hendrickson (2000), Extreme Programming Installed, Addison–Wesley. - Mehdi Mirakhorli (2008). RDP technique: a practice to customize xp, International Conference on Software Engineering, Proceedings of the 2008 international workshop on Scrutinizing agile practices or shoot-out at the agile corral, Leipzig, Germany 2008, Pages 23–32.
- Craig LarmanCraig LarmanCraig Larman is a Canadian computer scientist specializing in Iterative and incremental development, Agile software development, Object-oriented analysis, Object-oriented design, and agile modeling. He is the author of several texts.-Biography:...
& V. Basili (2003). "Iterative and Incremental Development: A Brief History", Computer (IEEE Computer Society) 36 (6): 47–56. - Matt StephensMatt StephensMatt Stephens is an author and software process expert based in London, UK. In January 2010 he founded independent book publisher Fingerpress UK Ltd....
and Doug Rosenberg (2003). Extreme Programming Refactored: The Case Against XP, Apress. - Waldner, JB. (2008). "Nanocomputers and Swarm Intelligence". In: ISTE, 225–256.
External links
- Extreme Programming
- A gentle introduction
- Industrial eXtreme Programming
- XP magazine
- Problems and Solutions to XP implementation
- Using an Agile Software Process with Offshore Development – ThoughtWorksThoughtWorksThoughtWorks is a privately owned global IT consultancy that delivers custom software, software tools, consulting, and transformation services to Global 1000 companies. It has a products division, ThoughtWorks Studios, which creates and markets software development and project management applications...
' experiences with implementing XP in large distributed projects