While software development is a very important topic, we will not delve very deep into this concept in the class. However, it is important to understand what software design is.
Over many years there have been several different software design models. One model, that seemed to catch on, was the waterfall model.
The Waterfall Model
The first formal description of this method is often cited as an article published by Winston W. Royce in 1970 although Royce did not use the term "waterfall" in this article. Royce presented this model as an example of a flawed, non-working model.
The basic principles are:
- Project is divided into sequential phases, with some overlap and splashback acceptable between phases.
- Emphasis is on planning, time schedules, target dates, budgets and implementation of an entire system at one time.
- Tight control is maintained over the life of the project via extensive written documentation, formal reviews, and approval/signoff by the user and information technology management occurring at the end of most phases before beginning the next phase. Written documentation is an explicit deliverable of each phase.
The waterfall model is a traditional engineering approach applied to software engineering. A strict waterfall approach discourages revisiting and revising any prior phase once it is complete. This "inflexibility" in a pure waterfall model has been a source of criticism by supporters of other more "flexible" models. It has been widely blamed for several large-scale government projects running over budget, over time and sometimes failing to deliver on requirements. Except when contractually required, the waterfall model has been largely superseded by more flexible and versatile methodologies developed specifically for software development.
Part of the problem with the waterfall model was that there was no process to stop and go back to a previous step if it was discovered that something wasn't quite right. Developers had to go all the way to the end of the process before they could return to the first step, and work all the way through to resolve issues. This can create long time frames for the development of software. At this point in history, software projects were long - 12 to 18 months - was not uncommon, even longer if the project got delayed.
As software development became more refined over time, and learning from previous models of development,various other models were introduced and used at various points in the entire history of software development.
Agile vs. waterfall
One of the differences between agile software development methods and waterfall is the approach to quality and testing. In the waterfall model, there is always a separate testing phase after a build phase; however, in agile software development testing is completed in the same iteration as programming.
Another difference is that traditional "waterfall" software development moves a project through various Software Development Lifecycle (SDLC) phases. One phase is completed in its entirety before moving on to the next phase.
Because testing is done in every iteration—which develops a small piece of the software—users can frequently use those new pieces of software and validate the value. After the users know the real value of the updated piece of software, they can make better decisions about the software's future. Having a value retrospective and software re-planning session in each iteration—Scrum typically has iterations of just two weeks—helps the team continuously adapt its plans so as to maximize the value it delivers. This follows a pattern similar to the PDCA cycle, as the work is planned, done, checked (in the review and retrospective), and any changes agreed are acted upon.
This iterative approach supports a product rather than a project mindset. This provides greater flexibility throughout the development process; whereas on projects the requirements are defined and locked down from the very beginning, making it difficult to change them later. Iterative product development allows the software to evolve in response to changes in business environment or market requirements.
Notice that in this model the sprint cycle is 1 to 4 weeks, not the months we saw with the waterfall model. The end date for the sprint and what will be delivered at the end of the sprint does NOT change. Also, there is a daily review, where changes can be made in order to stay on track to provide the agreed upon solution by the end of the sprint.
This model is used widely in the software industry today, because it is flexible, it is allows for quick changes, and a short time frame.