Similar to Scrum, XP (Extreme Programming) starts the process by creating a backlog of work to perform during a sprint/iteration. XP creates the backlog by working with customers and creating user stories. In parallel with this work, the team performs an architectural spike, during which they experiment with the features to envision the initial architecture. XP classifies this work as the exploration phase.
The planning phase follows exploration. This phase focuses on identifying the most critical user stories and estimating when they can be implemented. Tasks are defined for each feature, to aid with estimating complexity. The team outlines an overall release schedule, with an understanding that a high level of uncertainty exists until the work begins. A release will have one to many iterations, which are typically 2- to 4- week construction windows.
When an iteration begins, the specific plan for the iteration is revisited. The team adds any new user stories and tasks that have been discovered since the overall release was outlined.
XP integrates customer testing into the development iteration. The customer is asked to identify the acceptance tests, and the team works to automate these tests so they can be run throughout the iteration.
The planning phase is followed by the productionizing phase, during which the code is certified for release. Certified means the code passes all customer tests plus nonfunctional requirements such as load testing, service-level requirements, and response time requirements. You can see an overview of XP in below figure.
Some of the characteristics of XP are as follows:
- Specific practice —Unlike Scrum, XP is specific about the practices that should be used during a software project. These practices include pair programming, TDD, continuous integration, refactoring, and collective code ownership.
- Modeling —XP teams frequently use modeling to better understand the tasks and architecture needed to support a user story.
- Simplicity —Teams perform the minimum work needed to meet requirements.
- Automation —Unit and functional tests are automated.
- Quality through testing —Features are tested constantly, and developers check each other’s code via pair programming.
These are some of XP’s strengths:
- Customer-focused (it’s all about user stories)
- Quality via frequent testing
- Constant focus on identifying and delivering the critical user stories
- High visibility on project status
- Great support for volatile requirements
It also has weaknesses:
- Need for team maturity —Practices such as pair programming and TDD require responsible developers, and they aren’t always easy to obtain.
- Dependency on testing —If developers know that significant testing will take place downstream, they may be less than diligent when they’re creating designs.
- Scalability —XP may not work well for large projects.
- Dependency on team member co-location —The team usually has a team room.
XP supports many of the critical goals of an agile process, such as dealing with volatile requirements and delivering prioritized, working software as soon as possible. XP also supports the principle of just enough, keeping with the lean philosophy of minimizing waste.
XP has sometimes been criticized for its lack of formality in system documentation and system design. In recent years this has changed, and XP teams now create the documentation needed to support a project’s customers.