What is an iteration

An iteration is a set period of time within a project in which you produce a stable, executable version of the product, together with any other supporting documentation, install scripts, or similar artifacts necessary to use this release. The executable is demonstrable, allowing the team to demonstrate true progress to stakeholders, and get feedback on how they are doing so that they can improve their understanding of what needs to be done and how to do it.

Each iteration builds upon the results of the previous iteration, and will produce a product increment one step closer to the final product. Iterations are timeboxed, meaning that the schedule for an iteration should be regarded as fixed, and the scope of the iteration's content actively managed to meet that schedule.

At each iteration, artifacts are updated. It is said that this is a bit like "growing" software. Instead of developing artifacts one after another, in a pipeline fashion, they are evolving across the cycle, although at different rates.

Iterative development is very disciplined: the iteration length is fixed, the objectives of iterations are carefully planned, the evaluation criteria are established when each iteration is planned, and the tasks and responsibilities of participants are well-defined. Additionally, objective measures of progress are captured. Some reworking takes place from one iteration to the next, but this too is done in a structured fashion.

Each iteration should address the most critical risks, and implement the highest-priority Work Items. This ensures that each iteration adds maximum stakeholder value, and reduces uncertainty. Iterative development is typically combined with frequent or continuous integration: as unit-tested components become available, they are integrated, then a build is produced and subjected to integration testing. In this way, the capability of the integrated software grows as the iteration proceeds, towards the goals set when the iteration was planned.

Regular builds, such as daily or more frequent builds, let you break down the integration and test issues and spread them across the development cycle. These issues have often been the downfall of large projects, because all of the problems were discovered at once during the single massive integration step, which occurred very late in the cycle, and where a single problem can halt the whole team.

What Problem Do Iterations Address?

Today’s software applications are too complex to allow you to sequentially define the requirements, come up with an architecture and design, perform an implementation, carry out testing, and get it all right. With waterfall development, you typically do not get meaningful feedback on what improvements can be made until it is so late in the project that it is too costly to make them.

By contrast, dividing the project into a series of time-boxed iterations allows you to deliver capabilities that can be assessed by stakeholders at the end of each iteration. This approach provides rapid and timely feedback loops, enabling issues to be addressed and improvements made at a lower cost, while budget and time still allow, and before the project has gone so far ahead that major rework is required.

Iteration Length

Iterations are typically 4 weeks long, although some teams will work with iterations as short as a week or as long as six weeks. For factors driving iteration length, see the following table.

Why Iterate?

The iterative approach has proven itself superior to the waterfall approach for a number of reasons. See [KRO05] for more details:

• You are more likely to build an application that addresses user needs, involve the customer in the development project, and implement and validate the capabilities deemed most essential in each iteration.
• Integration is not one “big bang” at the end of a project. Each iteration evolves executable code that is continuously integrated to enable rapid feedback and minimize later revision.
• Risks are usually discovered or addressed during early iterations. Implement the most essential capabilities partially, and demonstrate them to key stakeholders to make sure that you are on the right track.
• Your ability to work effectively is fine-tuned. During early iterations, team members are walking through all lifecycle activities, which ensures that they have the tools, skills, organizational structure, and so on to work effectively.
• Management has a way of making tactical changes to the product. Iterative development allows you to evolve partial implementations of the end product quickly, and use these for quick release of a reduced-scope product to counter a competitor's move.
• Reuse is facilitated. Discussions and reviews of the design in early iterations allow team members to spot potential opportunities for reuse, and then develop a mature common code for these opportunities in subsequent iterations.
• Defects can be found and corrected over several iterations. Flaws are detected in early iterations, rather than during a massive testing phase at the end.
• Project personnel are better used. An iterative process encourages widening the scope of expertise of the team members, allowing them to play many roles, and thus enabling a project manager to make better use of the available staff and simultaneously remove problematic handoffs.
• Team members learn along the way. The project members have several opportunities within a development cycle to learn from their mistakes and improve their skills from one iteration to another.
• The development process itself is improved and refined along the way. The end of iteration assessment includes a retrospective where the team identifies what can be improved in the next iteration in both the organization and the process.