The Waterfall Model

Estimated time: 25 minutes

Overview

Questions

• What is the Waterfall model?
• How is the Waterfall model used in practice?
• When is it appropriate to use the Waterfall model?

Objectives

• Understand the Waterfall model.
• Practice using the Waterfall model.
• Learn when the Waterfall model works well.

The Waterfall Model

---

The Waterfall model consists of various stages that follow sequentially – one after the other. The idea of having defined phases is to try to complete each phase before the next phase begins. This is generally referred to as the waterfall model, e.g., “Progress flows downwards through the phases, like a waterfall.” Each phase produces specific deliverables, which feed into the next phase with the assumption that the artifacts from the previous phases are actually correct.

Ideally, each phase will also identify any issues or flaws in the previous phase. When this happens, you are supposed to iterate between phases to fix any issues that arise.

Waterfall has either five or six stages, depending on the source, but for our purposes, we will consider the phases to be:

1. Requirements
2. Design
3. Implementation
4. Verification
5. Maintenance

Let’s Build a House

0. Split into 4 groups and grab a set of LEGOs ™️.
1. (6 min) Gather requirements from your customer. Ask important questions to help you determine what needs to be done to build their dream house!
2. (4 min) Determine your design. How can you satisfy the requirements with your LEGOs?
3. (8 min) Build the house!
4. (2 min) Show it to your customer and see how you did.

Instructor Note

Choose two people to act as customers (either two of the attendees or other instructors/TAs). Assign one of them as “Customer 1” and the other as “Customer 2”.

Show them the appropriate images:

1. Customer 1 - Box style house
2. Customer 2 - Cabin style house

Assign two groups to each customer. The groups will ask questions together but then separate to do their individual work. The intent of this is to see how two groups who gathered the same information may approach work differently.

Instructions to give to the customers:

Stage 1 - Waterfall Model: The attendees will break up into 4 groups - two groups assigned to each customer (you). You each will be given a picture of a house. WITHOUT SHOWING THEM THE PICTURE, you will describe the requirements for the house in the picture to them. Start really basic / high level / vague like customers sometimes do. “I want you to build me a house.” The attendees should then start asking you questions about the house: “How tall? How many windows and doors?” (etc. etc.) They have 6 minutes to gather the requirements. Then they aren’t allowed to talk to you again while they build the house. After 12 minutes of design and implementation, they show you their “completed” house. You provide feedback on what is wrong, what is right, etc.

Discussion

How was that experience? What did you like? What did you dislike?

The Waterfall Model in Real Life

---

It is very common in real life scenarios that there is a miscommunication of software requirements. These miscommunications may not even be identified until the software is demonstrated for clients or formal acceptance testing is completed. In traditional waterfall models, the client is often only involved in requirements phase and in acceptance-testing (or later phases).

If mistakes occur using the waterfall model, very significant amounts of work may need to be reviewed, revised, or even discarded.

What do you require?

“It’s 50x to 200x less expensive to get a requirement right in the first place than to fix a requirement after implementation.” - Boehm and Papaccio, 1988

The Waterfall model works well when:

• Software requirements are very clear, well understood, and unchanging
• Technical implementation details are also very well understood (generally considered more suited to manufacturing, but this is also changing with rapid prototyping!)

Inversely, waterfall model works terribly when these things aren’t present and can lead to greatly exceeding time and financial budgets. Waterfall-based models don’t handle lack of knowledge, or change, very effectively. Poorly understood or changing software requirements, technology, or architecture choices can negatively impact the performance of the waterfall-model. Though, it should be noted that a lack of software-engineering discipline virtually negates all approaches.

Key Points

• The Waterfall model consists of stages that happen sequentially, one after another.
• The Waterfall model is helpful when requirements are very clear and well understood.
• The Waterfall model is difficult if requirements and technologies are unclear or change frequently.