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Agile software development is a conceptual framework for undertaking software engineering projects. There are a number of agile software development methodologies.

Most agile methods attempt to minimize risk by developing software in short timeboxes, called iterations, which typically last one to four weeks. Each iteration is like a miniature software project of its own, and includes all the tasks necessary to release the mini-increment of new functionality: planning, requirements analysis, design, coding, testing, and documentation. While an iteration may not add enough functionality to warrant releasing the product, an agile software project intends to be capable of releasing new software at the end of every iteration. At the end of each iteration, the team reevaluates project priorities.

Agile methods emphasize realtime communication, preferably face-to-face, over written documents. Most agile teams are located in a bullpen and include all the people necessary to finish software. At a minimum, this includes programmers and their "customers". (Customers are the people who define the product. They may be product managers, business analysts, or actual customers.) The bullpen may also include testers, interaction designers, technical writers, and management.

Agile methods also emphasize working software as the primary measure of progress. Combined with the preference for face-to-face communication, agile methods produce very little written documentation relative to other methods. This has resulted in criticism of agile methods as being undisciplined hacking.


Contents

History

Agile software development evolved in the mid 1990s as part of the reaction against "heavyweight" methods, like the Rational Unified Process (RUP). The processes originated from those methods were seen as bureaucratic, slow, demeaning, and contradicted the ways that software engineers actually work.

Initially, agile methods were called "lightweight methods." In 2001, prominent members of the community met (see "The Agile Manifesto,) and adopted the name "agile methods." Later, some of these people formed the Agile Alliance [1], a non-profit organization that promotes agile development.

Early agile methods--created prior to 2000--include Scrum, Crystal Clear, Extreme Programming (XP), Adaptive Software Development, and Dynamic Systems Development Method (DSDM).

Extreme Programming, while it may not have been the first agile method, unarguably established the popularity of agile methods. Extreme Programming was created by Kent Beck in 1996 as a way to rescue the struggling Chrysler Comprehensive Compensation (C3) project. The methodology was refined by Ron Jeffries' full-time coaching and public discussion on Ward Cunningham's Portland Pattern Repository. In 2000, Kent Beck published the first book on Extreme Programming [2]. Elements of Extreme Programming appear to be based on Scrum and Ward Cunningham's Episodes pattern language. DSDM is considered the first established agile method throughout Europe.

The Agile Manifesto

Agile methodologies are a family of methodologies, not a single approach to software development. In 2001, 17 prominent figures in the field of agile development (then called "light-weight methodologies") came together to discuss the unifying theme of their methodologies. They created the "Agile Manifesto" [5], widely regarded as the canonical definition of agile development:

Manifesto for Agile Software Development
We are uncovering better ways of developing software by doing it and helping others do it. Through this work we have come to value:
Individuals and interactions over processes and tools
Working software over comprehensive documentation
Customer collaboration over contract negotiation
Responding to change over following a plan
That is, while there is value in the items on the right, we value the items on the left more.
Kent Beck, Mike Beedle, Arie van Bennekum, Alistair Cockburn, Ward Cunningham, Martin Fowler, James Grenning, Jim Highsmith, Andrew Hunt, Ron Jeffries, Jon Kern, Brian Marick, Robert C. Martin, Steve Mellor, Ken Schwaber, Jeff Sutherland, Dave Thomas
© 2001, the above authors
this declaration may be freely copied in any form, but only in its entirety through this notice.

The Agile Manifesto is accompanied by the Principles behind the Agile Manifesto, a complete list of agile principles.

Comparison with other types of methodologies

Agile methods are often characterized as being at the opposite end of a spectrum from "plan-driven" or "disciplined" methodologies. This distinction is misleading, as it implies that agile methods are "unplanned" or "undisciplined." A more accurate distinction is to say that methods exist on a continuum from "adaptive" to "predictive." Agile methods exist on the "adaptive" side of this continuum.

Adaptive methods focus on adapting quickly to changing realities. When the needs of a project change, an adaptive team changes with it. An adaptive team will have difficulty describing exactly what will happen in the future. The further away a date is, the more vague an adaptive method will be about what will happen on that date. An adaptive team can report exactly what tasks are being done next week, but only which features are planned for next month. When asked about a release six months from now, an adaptive team may only be able to report the mission statement for the release, or a statement of expected value vs. cost.

Predictive methods, in contrast, focus on planning the future in detail. A predictive team can report exactly what features and tasks are planned for the entire length of the development process. Predictive teams have difficulty changing direction. The plan is typically optimized for the original destination and changing direction can cause completed work to be thrown away and done over differently. Predictive teams will often institute a change control board to ensure that only the most valuable changes are considered.

Specific comparisons to other families of methodologies:

Iterative development

Most agile methods share iterative development's emphasis on building releasable software in short time periods. Agile methods differ from iterative methods in that their time period is measured in weeks rather than months. Most agile methods also differ by treating their time period as a strict timebox rather than a planned goal.

The waterfall model

Agile development has less in common with the waterfall model. The waterfall model is the most predictive of the methodologies, stepping through requirements analysis, design, coding, and testing in a strict pre-planned sequence, partially completing every feature at each stage. The waterfall model produces releasable software at the very end of the cycle, a time period typically extending from several months to several years. Agile methods, in contrast, produce completely developed features (but a very small set subset of the total) every few weeks.

Some agile teams use the waterfall model on a small scale, repeating the entire waterfall cycle in every iteration. Other teams, most notably Extreme Programming teams, work on activities simultaneously, so that there are no distinguishable phases.

Cowboy coding

Another "method" in common use is cowboy coding. Cowboy coding is the absence of a defined method: team members do whatever they feel is right. Agile development's frequent reevaluation of plans, emphasis on face-to-face communication, and relatively sparse use of documents sometimes causes people to confuse it with cowboy coding. Agile teams, however, do follow defined (and often very disciplined and rigorous) processes, distinguishing agile approaches from cowboy coding. Cowboy coding is relying mostly on the build-and-fix approach of software development.

When to use agile methods

Agile development has been widely documented as working well for small (<10 developers) collocated teams. Agile development is particularly indicated for teams facing unpredictable or rapidly changing requirements. While there are experience reports of teams succeeding with agile development outside of these parameters, there are too few experiences reported to this day to draw firm conclusions.

Agile development's applicability to the following scenarios is open to question:

  • Large scale development efforts (>20 developers)
  • Distributed development efforts (non-collocated teams)
  • Mission- and life-critical efforts
  • Command-and-control company cultures
  • Low requirements change
  • Junior developers

Agile home ground:

  • Low criticality
  • Senior developers
  • High requirements change
  • Small number of developers
  • Culture that thrives on chaos

By analyzing the project against these home grounds, the risk of using an agile or plan-driven method can be determined.

Agile methodologies

Some of well-known agile software development methodologies include

  • Extreme Programming (XP)
  • Scrum [2]
  • Adaptive Software Development (ASD) [3]
  • Crystal Clear]] and Other Crystal Methodologies [4]
  • Dynamic Systems Development Method (DSDM) [5]
  • Feature Driven Development [6]
  • Lean software development [7]

Other methodologies include

  • Agile documentation
  • Agile ICONIX
  • Microsoft Solutions Framework (MSF)
  • Agile Data [8]
  • Agile Modeling [9]

References

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