Software Development Process Models

Steven J Zeil

Last modified: Dec 21, 2019


Each development organization has its own working process that it evolves for how it gets software development done. In this lesson, we look at the constituent steps that make up typical instances of these processes, and will survey some of the more common arrangments of those steps into a software development process model:

1 Software Development Process Models


Each development organization has its own working process that it evolves for how it gets software development done.

Although that might sound chaotic, in practice there is pretty broad consensus on what the constituent steps are that make up the entire process. So it becomes a matter of arranging those steps, of tweaking the details, and especially of settling on the relative emphasis and level of detail in these steps.

The arrangements that companies arrive at are seldom entirely innovative. Instead, they fall into a few standard patterns, which we will survey in this lesson.

A software development process model (SDPM), a.k.a., a software life-cycle model, is the process by which an organization develops software.

1.1 Common Activities

Although there are many models (in theory, one per development team), there is pretty broad agreement on what needs to go on during this process:

  1. Recognition of problem / need / opportunity
  2. Feasibility study
  3. Analysis of requirements
  4. Design of system
  5. Implementation
  6. Testing
  7. Deployment
  8. Maintenance

Different SDPMs will divide these activities among phases in different ways.

Let’s talk about a few of these in more detail.

1.1.1 Analysis of Requirements

“System” here is used in its generic sense: a collection of people, organizations, machines, and procedures for getting things done. There’s almost always an existing system, even if it is totally un-automated.

If you are a follower of Object-Oriented (OO) approaches, you have a deep conviction that studying and, ultimately, simulating an existing system is a fundamental principle of software development. OO developers never ask the question “Is it possible to build a system that does X?”. That’s because the existing system serves as an existence proof — they’re already doing X, so we start by understanding and then simulating what they are doing now.

Common Documents from Requirements Analysis

We’ll look at requirements in more detail in a later section.

1.1.2 Design

“Design” means deriving

a solution which satisfies the software requirements.

Commonly recognized subproblems include

A possible breakdown of design activities

You are probably pretty familiar already with procedures for doing high-level and low-level design. Architectural design, on the other hand, is something that is seldom worth worrying about in the scale of projects addressed within an academic semester.


The breakdown shown in this picture is probably more elaborate than you would have attempted, though the component ideas should, considered separately, be clear enough.

The diagram here suggests a fairly document-heavy process typical of Waterfall, our first process model.

1.1.3 Maintenance

Maintenance is another practice that seldom arises in academic projects. Normally, when you do an assignment for a course, you’re completely done with at the end of the semester. Keeping it working, adding new functionality, etc., is not a concern.

But you’ve certainly seen how operating systems, application programs, games, and many other software products are subject to an ongoing process of bug fixes, enhancements, and new releases.

Maintenance can have a number of forms:

2 Major Software Development Models

3 The Waterfall Model


The waterfall model gets its name from the fact that the first diagrams of this process illustrated it as a series of terraces over which a stream flowed, cascading down from one level to another. The point of this portrayal was that water always flows downhill — it can’t reverse itself. Similarly, the defining characteristic of the waterfall model is the irreversible forward progress from phase to phase.

Waterfall is often criticized as inflexible, in large part because of that irreversible forward motion. Many organizations, in practice, will do a kind of “waterfall with appeals”, allowing developers to revisit and revise decisions and documents from earlier phases after jumping through a number of deliberately restrictive hoops.

3.1 Verification & Validation

Most of the activities in Waterfall are familiar, with the possible exception of requirements analysis, which we will be looking at in more detail in a later lesson.

For now, I want to look at Verification and Validation (V&V).

Verification & Validation: assuring that a software system meets the users’ needs.

The principle objectives are:

3.1.1 What’s the Difference?

Verification is essentially looking for mistakes in our most recent bit of work by comparing what we have now to the most recent “official” document defining our system.

Validation is a return to first principles, comparing what we have now to what we (or our customers) originally wanted.

You might think that, in a process divided into steps, if we do each step “correctly”, then the entire sequence must be “correct”. In practice, though, the accumulation of small errors can lead to massive alterations over time. (That’s not just a matter for programmers.)

Most V&V activities mix verification and validation together to different degrees.


Industry figures of 1-3 faults per 100 statements are quite common.

Is testing verification or validation? A great deal depends on how we decide whether the test output is correct. If we do this by viewing the data ourselves and looking for things that jump out to our eyes as “wrong”, then we are doing mainly validation. On the other hand, if part of our design process was to set up a set of tests with files of their expected outputs, and we are simply comparing the actual output files to the expected output files, then we are doing more verification.

3.2 Testing throughout the Waterfall

3.2.1 Testing stages

3.2.2 Not just in one phase


Although the waterfall model shows V&V as a separate phase near the end, we know that some forms of V&V occur much earlier.

So this phase of the waterfall model really describes system and acceptance testing.

A Still-broader View

Even the “V&V V” does not capture the full context of V&V:

3.3 Advantages of Waterfall

3.4 Disadvantages of Waterfall

4 Iterative/Incremental Development


As a counter-reaction to what many believe to be an overly rigid waterfall model, there are a variety of incremental approaches that emphasize quick cycles of development, usually with earlier and more user-oriented validation.

There is a greater emphasis on producing intermediate versions, each adding a small amount of additional functionality. Some of these are releases, either external (released outside the team) or internal (seen only by the team), which may have been planned earlier.

What’s the difference between iterative and incremental?

Iterative versus Incremental Models

Incremental development is almost always iterative, but you can be iterative without being incremental.


4.1 Advantages

4.2 Disadvantages

5 The Spiral Model


1986, Boehm

An iterative approach with a focus on risk management

Spiral Phases

  1. Determine objectives, alternatives and constraints:
    • Define requirements
    • Alternatives (including, e.g., 3rd-party code) identified
    • Constraints defined
  2. Identify and resolve risks, evaluate alternatives:
    • Evaluate identified alternatives
    • Identify risks
    • Resolve risks
    • Produce prototype
  3. Develop and test
    • Analyze performance of prototype
    • Create & review design, code, test
  4. Plan next iteration
    • Often includes customer evaluation of prototype or current project iteration

5.1 Advantages of Spiral Model

5.2 Disadvantages of Spiral Model

6 Rational Unified Process

These three were already some of the biggest names in OOA&D before they decided to collaborate on a unified version of their previously distinctive approaches.

Their collaboration coincided with their being hired by Rational Corp., a major vendor of software development tools. Hence the “Rational” in RUP refers to the name of the company. It’s not bragging. They aren’t saying that this is a uniquely intellectual approach or that Waterfall, Spiral, et. al., are “irrational”.

6.1 Unified Model Phases


6.2 Unified Model Phases Continued


One task during Elaboration is to plan releases:


The term “increments” gets used a lot in different models. Sometimes it refers, as it does here, to the time period during which the next release of the software is developed. In other cases it refers to the next version of the software. In other cases it refers to the software release itself.

6.3 Key Concepts of the RUP

6.3.1 Common Workflows


For example, deep in the implementation phase of a Waterfall project, a programmer is assigned a function to implement.

That programmer will

But we aren’t in the analysis, design, or validation phases.

The diagram on the right is supposed to illustrate that, although the percentage of time devoted to the activities of analysis, design, implementation, and validation, none of those activites ever entirely go away and are, once and for all, done.

A process model may still use some of these same terms as the name for major phases, but that’s really a different sense of the terms. For example, the “Design” phase of the Waterfall is when the language in which we “implement” is the collection of notations and diagrams that we use for system design. But we still analyze, design, implement, and validate our Design decisions.


In the RUP, all progress is made as continual ADIV cycles


6.3.2 An Evolution of Models

RUP supports development via a series of models.

The most important of these are

Models Evolved

RUP embraces the

Object-Oriented philosophy

  • Every program is a simulation
  • The quality of a program’s design is proportional to how faithfully the objects and interactions in the program reflect those in the real world

6.4 Advantages of RUP

6.5 Disadvantages of RUP

7 Agile Methods

A modern variant of incremental development.

Agile development is

Emphasis Areas

Emphasis is on

We’ll look at Agile in more detail later in the semester, after we have learned more about these “best practices” that lie at the heart of the process.