Since taking down the old Clicktricity website, I have had a number of requests to re-publish the agile principles.
Agile Development Core Values
The two most important core values of Agile Development are Communication and Simplicity.
It is recognised that poor communication in software teams is one of the root causes of failures within projects – whether it be schedule slips, botched requirements, misunderstandings, faulty development assumptions, and the like. Extreme Programming mitigates this by stressing good communication between all project stakeholders – customers, team members, and project managers (or “coaches”) – on a consistent basis. A representative from the customer should be available as much as possible to answer questions and clarify project requirements. Programmers often work simultaneously in pairs, with each programmer reviewing the other’s work.
One of the most common XP slogans is “YAGNI” or ‘You Aren’t Going to Need It’
The principal of YAGNI is that you shouldn’t develop any code that will only be used by a feature that is needed tomorrow. This does not exclude development of flexibility, but it should be kept in check and only functionality that is required now is actually developed.
Agile Development Principles
User stories are short, simple descriptions of required functionality that are written by the customer, rather than our interpretation of what the customer wants. The descriptions should only contain sufficient detail to allow development of an accurate estimate for planning purposes, development of unit tests, and are used instead of large unwieldy requirements documents. When we develop and implement the user stories, the developers will work face-to-face with the customer to determine the specific detailed requirements.
The Planning phase of development is performed in two parts – firstly the “user stories” described above are elicited from the customer. Secondly, the development team estimates development effort for each story. The customer then decides on priorities and agrees what user stories will be implemented in the next release. This defines the project schedule.
The planning phase determines small units of functionality that make good business sense and can be released into the customer’s environment early in the project. This is critical to getting valuable feedback in time to have an impact on the system’s development.
The overwhelming aim is to make things as simple as possible. The only code being worked on should be the code that is absolutely necessary to implement the latest user stories, and no more (see YAGNI, as described previously). The drive for simplicity leads to continual refactoring of code, as described below.
Testing, and in particular writing test specifications from the user stories before coding is another essential component of this technique. Traditional development techniques that follow a “code first, test later” technique are prone to only testing a limited part of the development. In addition it is common that as schedules become tight – thorough testing is often dropped in the interest of expediency, leading to low quality, error prone code. Our technique is to define and develop automated tests before coding ensuring that untested code cannot be released.
In pair programming, one member of the pair will write code while at the same time another programmer is critiquing the work at hand, and at the same time offering insight as to the next step as well as exposing trivial defects with the code. In tests, pair programming has been shown to be 30% more productive when quality is taken into account, than the combined productivity of two developers working separately.
Once it becomes apparent to the development team that the system design, a module or piece of individual code is becoming too complex, the code is refactored. The refactoring process is one by which the system functionality remains stationary – all tests that pass prior to refactoring should pass after refactoring is completed – but the code base becomes greatly simplified. This may involve eliminating shared code, redesigning model hierarchies and relationships, or simply renaming variables to fit a particular metaphor.
A standard working week with no overtime is strictly adhered to, based on the assertion that development teams are able to produce high-quality product when they are comfortable and not overly-exerted. This principle serves to complement the idea of both pair programming and communication amongst the development team and their customer.
It is not enough to have only occasional access to a customer and a customer representative should be continuously available to the development team. This ensures that all unanticipated questions or requirements can be immediately resolved and eliminates the need to produce exhaustive and definitive (and probably still incomplete and non-definitive) specifications at the beginning of the project. This also allows the customer to interact with the development and help in the evolution of the product. Maximizing customer availability makes it possible to minimize the time spent on preliminary planning and focus on development, and maximize feedback, thereby increasing the value delivered to the customer.
The entire development team agrees to maintain a common set of rules concerning the maintenance and creation of new code. These standards greatly simplify communication through common naming conventions and generate shared ownership and responsibility among all developers.
For more information on the principles of Extreme Programming, please see