History:
Early implementation of lightweight methods include Scrum (1995), Crystal Clear, Extreme Programming (1996), Adaptive Software Development, Feature Driven Development, and Dynamic Systems Development Method (DSDM) (1995). These are now typically referred to as agile methodologies, after the Agile Manifesto published in 2001.
Agile Manifesto:
Agile Manifesto reads, in its entirety, as follows: 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 pla
That is, while there is value in the items on the right, we value the items on the left more.
Twelve principles underlie the Agile Manifesto, including:
Customer satisfaction by rapid delivery of useful software
Welcome changing requirements, even late in development
Working software is delivered frequently (weeks rather than months)
Working software is the principal measure of progress
Sustainable development, able to maintain a constant pace
Close, daily co-operation between business people and developers
Face-to-face conversation is the best form of communication (co-location)
Projects are built around motivated individuals, who should be trusted
Continuous attention to technical excellence and good design
Simplicity
Self-organizing teams
Regular adaptation to changing circumstances
Characteristics:
Agile methods break tasks into small increments with minimal planning, and do not directly involve long-term planning. Iterations are short time frames that typically last from one to four weeks. Each iteration involves a team working 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 minimizes overall risk and allows the project to adapt to changes quickly. Stakeholders produce documentation as required. An iteration may not add enough functionality to warrant a market release, 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 an iteration requires. They decide individually how to meet an iteration's requirements.
Most agile implementations use 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 the previous day, what they intend to do today, and what their roadblocks are. This face-to-face communication exposes problems as they arise.
Agile methods emphasize face-to-face communication over written documents.
Team size is typically small (5-9 people) to simplify team communication and team collaboration.
Agile development emphasizes working software as the primary measure of progress.
Larger development efforts may be delivered by multiple teams working toward a common goal.
When a team works in different locations, they maintain daily contact through videoconferencing, voice, e-mail, etc.
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 (ROI) and ensuring alignment with customer needs and company goals.
Comparison with other methods:
Agile methods are sometimes characterized as being at the opposite end of the spectrum from "plan-driven" or "disciplined" methods; agile teams may, however, employ highly disciplined formal methods. A more accurate distinction is that methods exist on a continuum from "adaptive" to "predictive". Agile methods lie 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 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 cannot 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 require completed work to be started over. Predictive teams will often institute a change control board to ensure that only the most valuable changes are considered.
Formal methods, in contrast to adaptive and predictive methods, focus on computer science theory with a wide array of types of provers. A formal method attempts to prove the absence of errors with some level of determinism. Some formal methods are based on model checking and provide counter examples for code that cannot be proven. Generally, mathematical models (often supported through special languages see SPIN model checker) map to assertions about requirements. Formal methods are dependent on a tool driven approach, and may be combined with other development approaches. Some provers do not easily scale. Like agile methods, manifestos relevant to high integrity software have been proposed in Crosstalk.
Agile methods have much in common with the "Rapid Application Development" techniques from the 1980/90s as espoused by James Martin and others.