Agile chooses to do things in small increments with minimal planning, rather than long-term planning. Iterations are short time frames (known as 'timeboxes') which typically lasts from two to four weeks. Each iteration is worked on by a team through a full software development cycle, including planning, requirements analysis, design, coding, unit testing,and acceptance testing when a working product is demonstrated to stakeholders. This helps to minimize the overall risk, and allows the project to adapt to changes more quickly. Documentation is produced as required by stakeholders. An iteration may not add enough functionality to warrant releasing the product to market, but the goal is to have an available release (with minimal bugs) at the end of each iteration. Multiple iterations may be required to release a product or new features.
Team composition in an agile project is usually cross-functional and self-organizing without consideration for any existing corporate hierarchy or the corporate roles of team members. Team members normally take responsibility for tasks that deliver the functionality of an iteration. They decide for themselves how they will execute during an iteration.
Agile methods emphasize face-to-face communication over written documents. Most agile teams are located in a single open office to facilitate such communication. Team size is typically small (5-9 people) to help make team communication and team collaboration easier. Larger development efforts may be delivered by multiple teams working toward a common goal or different parts of an effort. This may also require a coordination of priorities across teams.
No matter what development disciplines are required, each agile team will contain a customer representative. This person is appointed by stakeholders to act on their behalf and makes a personal commitment to being available for developers to answer mid-iteration problem-domain questions. At the end of each iteration, stakeholders and the customer representative review progress and re-evaluate priorities with a view to optimizing the return on investment and ensuring alignment with customer needs and company goals.
Most agile methodologies include a routine and formal daily face-to-face communication among team members. This specifically includes the customer representative and any interested stakeholders as observers. In a brief session, team members report to each other what they did yesterday, what they intend to do today, and what their roadblocks are. This standing face-to-face communication prevents problems being hidden.
Agile methods emphasize working software as the primary measure of progress. Combined with the preference for face-to-face communication, agile methods usually produce less written documentation than other methods. In an agile project, documentation and other project artifacts all rank equally with working product. Stakeholders are encouraged to prioritize them with other iteration outcomes based exclusively on business value perceived at the beginning of the iteration.
Specific tools and techniques such as continuous integration, automated or xUnit test, pair programming, test driven development, design patterns, domain-driven design, code refactoring and other techniques are often used to improve quality and enhance project agility.
An adaptive software development process was introduced in a paper by Edmonds (1974). Notable early Agile methods include Scrum (1986), Crystal Clear, Extreme Programming (1996), Adaptive Software Development, Feature Driven Development, and Dynamic Systems Development Method (DSDM) (1995). These are typically referred to as Agile Methodologies since the Agile Manifesto was published in 2001.
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.
Some of the principles behind the Agile Manifesto are:
The manifesto spawned a movement in the software industry known as agile software development.
The functioning principles of Agile can be found in lean manufacturing and six sigma. These concepts include error proofing, eliminating waste, creating flow, adding customer value, and empowering workers. The concepts were first formally espoused in: the 14 principles of the Toyota Way, the two pillars of the Toyota Production System (Just-in-time and smart automation), the 5S methodology, and Deming’s 14 points. These have been summarized in the 7 points of lean software development.
Adaptive methods focus on adapting quickly to changing realities. When the needs of a project change, an adaptive team changes as well. 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.
Agile methods have much in common with the "Rapid Application Development" techniques from the 1980/90s as espoused by James Martin and others.
Most agile methods share other iterative and incremental development methods' emphasis on building releasable software in short time periods. Agile development differs from other development models: in this model time periods are measured in weeks rather than months and work is performed in a highly collaborative manner. Most agile methods also differ by treating their time period as a strict timebox.
The main problem with the waterfall model is the inflexible division of a project into separate stages, so that commitments are made early on, and it is difficult to react to changes in requirements. Iterations are expensive. This means that the waterfall model is likely to be unsuitable if requirements are not well understood or are likely to change in the course of the project. Agile methods, in contrast, produce completely developed and tested features (but a very small subset of the whole) every few weeks. The emphasis is on obtaining the smallest workable piece of functionality to deliver business value early, and continually improving it/adding further functionality throughout the life of the project.
In this respect, agile critics may assert that these features are not placed in context of the overall project, concluding that, if the sponsors of the project are concerned about completing certain goals with a defined timeline or budget, agile may not be appropriate. Proponents of Agile development counter that adaptations of Scrum show how agile methods are augmented to produce and continuously improve a strategic plan.
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.
As with all development methods, the skill and experience of the users determine the degree of success and/or abuse of such activity. The more rigid controls systematically embedded within a process offer stronger levels of accountability of the users. The degradation of well-intended procedures can lead to activities often categorized as cowboy coding.
Large scale agile software development remains an active research area.
Agile development has been widely documented (see Experience Reports, below, as well as Beck pg. 157, and Boehm and Turner pg. 55-57) as working well for small (<10 developers) co-located teams.
Some things that can negatively impact the success of an agile project are:
Several successful large scale agile projects have been documented. BT has had several hundred developers situated in the UK, Ireland and India working collaboratively on projects and using Agile methods. While questions undoubtedly still arise about the suitability of some Agile methods to certain project types, it would appear that scale or geography, by themselves, are not necessarily barriers to success.
Barry Boehm and Richard Turner suggest that risk analysis be used to choose between adaptive ("agile") and predictive ("plan-driven") methods. The authors suggest that each side of the continuum has its own home ground as follows:
Agile home ground:
Plan-driven home ground:
A process or capability in which human agents through responsive changes in, and dynamic interplays between contexts, intentions, and method fragments determine a system development approach for a specific project situation.
Potentially, almost all agile methods are suitable for method tailoring. Even the DSDM method is being used for this purpose and has been successfully tailored in a CMM context. Situation-appropriateness can be considered as a distinguishing characteristic between agile methods and traditional software development methods, with the latter being relatively much more rigid and prescriptive. The practical implication is that agile methods allow project teams to adapt working practices according to the needs of individual projects. Practices are concrete activities and products that are part of a method framework. At a more extreme level, the philosophy behind the method, consisting of a number of principles, could be adapted (Aydin, 2004).
Extreme Programming (XP) makes the need for method adaptation explicit. One of the fundamental ideas of XP is that no one process fits every project, but rather that practices should be tailored to the needs of individual projects. Although partial adoption of XP practices, as suggested by Beck, has been reported on several occasions. A tailoring practice is proposed by Mehdi Mirakhorli which provides sufficient roadmap and guidline for adapting all the practices. RDP Practice is designed for customizing XP. This practice first time proposed as a long research paper in APSO workshop at ICSE 2008 conference and yet it is the only proposed and applicable method for customizing XP. Although it is specifically a solution for XP, this practice has the capability of extending to other methodologies. At first glance, this practice seems to be in the category of static method adaptation but experiences with RDP Practice says that it can be treated like dynamic method adaptation. The distinction between static method adaptation and dynamic method adaptation is subtle. The key assumption behind static method adaptation is that the project context is given at the start of a project and remains fixed during project execution. The result is a static definition of the project context. Given such a definition, route maps can be used in order to determine which structured method fragments should be used for that particular project, based on predefined sets of criteria. Dynamic method adaptation, in contrast, assumes that projects are situated in an emergent context. An emergent context implies that a project has to deal with emergent factors that affect relevant conditions but are not predictable. This also means that a project context is not fixed, but changing during project execution. In such a case prescriptive route maps are not appropriate. The practical implication of dynamic method adaptation is that project managers often have to modify structured fragments or even innovate new fragments, during the execution of a project (Aydin et al, 2005).
PRINCE2 has been suggested as a suitable, complementary project management system.
A number of project management tools are specifically aimed at agile development. They are designed to help plan, track, analyse and integrate work. These tools play an important role in agile development, as a means of Knowledge Management.
Common features include: Version control integration, progress tracking, easy work allocation, integrated release and iteration planning, discussion forums, and reporting and tracking of software defects.
Note: although these are often considered methodologies, they are not. They are just techniques used in different methodologies.
While such approaches have been proposed to measure agility, the practical application of such metrics has yet to be seen.
In particular, Extreme Programming is reviewed and critiqued by Matt Stephens's and Doug Rosenberg's Extreme Programming Refactored.
The criticisms regarding insufficient software design and lack of documentation are addressed by the Agile Modeling method, which can easily be tailored into agile processes.
Agile software development has been criticized because it may not bring about all of the claimed benefits when programmers of average ability use this method.
It is argued that the meaning of Agile is ambiguous and being inappropriately applied to a very wide range of approaches like Six Sigma and CMMi. It is also argued that "Agile", "evolutionary", and "lean" (as in Lean software development) do not mean the same thing in practice, even though they are all lumped under the banner of "Agile" - possibly for marketing purposes.
Proponents also argue that process-oriented methods, especially methods that rely on repeatable results and that incrementally reduce waste and process variation like Six Sigma, have a tendency to limit an organisation's adaptive capacity (their "slack"), making them less able to respond to discontinuous change - i.e., less agile. It is proposed that "agile", "lean" and "evolutionary" are strategies that need to be properly understood and appropriately applied to any specific context. That is, there is a time to be "agile", a time to be "lean" and a time to be "evolutionary".
Some commentators propose a model of post-Agilism that is effectively constructive anarchy, in that teams should be self-organising to the point where even the core values of the Agile movement are considered too prescriptive, and that teams should simply "do whatever works for them".
As of 2006, experience reports have been or will be presented at the following conferences: