Understanding the Classical Waterfall Model in Software Engineering

The classical waterfall model is often called the "linear sequential model," so it is a method of developing software in a highly structured and phase-by-phase manner. In this sense, the name itself says that the structure flows in the form of waterfalls, where each phase cascades into the next, so it is "unidirectional." They are primarily used in projects with well-defined initial requirements, with minimal changes at the development time.

In this model, it is completed to the previous step before proceeding to the next one. The sequence of phases is well-defined and does not permit a re-visit of any previously accomplished stages, making it a non-iterative procedure.

The Classical Waterfall Model in Software Engineering features the following distinctive characteristics:

1. Linear Process: This is a very linear, sequential process. Once one phase is completed, the next one begins without modifying or revisiting the previous phase.

2. Structured Development: This model provides a clear structure with distinct phases, making it easy to understand and follow.

3. Emphasis on Documentation: Documentation is strictly emphasised in each phase, ensuring all the requirements and decisions are well documented and communicated.

4. Predictability: Every process phase has a predetermined timeline and cost, making it relatively predictable.

5. Minimal Client Interaction After Requirements: Client interaction is minimal once the requirements gathering is completed till the testing phase.

The classic waterfall model breaks down into six individual phases. There is a particular order for finishing each phase of classical waterfall model before beginning with the other. So, let us understand each phase briefly

1. Feasibility Study

The feasibility study checks if developing the product is technically and financially possible. It starts by analyzing the problem and gathering relevant information. Only the key requirements are considered, leaving out minor details. Feasibility studies include: 

Problem Definition

This involves identifying the main requirements of the customer without going into too much detail. It focuses on understanding the core problem that needs to be solved. 

Formulation Of The Different Strategies For Solving The Problem

Different ways to solve the problem are explored. This helps in identifying all possible approaches before choosing the best one.

Evaluation of The Different Solution Strategies

Each solution is analyzed for its pros and cons. This includes estimating costs, resources, and time required to implement each option, ensuring the best choice is made.

2. Requirement Gathering and Analysis

This is the first step of the waterfall model, where requirements are gathered from stakeholders, customers, or clients. This phase is crucial because any mistake in requirements gathering can create significant problems later. The main aim at this stage is to identify and document the exact specifications required before entering into design or development activities.

Requirement Gathering And Analysis

This involves detailed discussions with stakeholders to understand their needs and expectations. The gathered requirements are then analyzed for feasibility, consistency, and completeness, ensuring nothing is overlooked.

Requirement Specification

After analyzing the requirements, they are documented clearly and precisely. This helps developers and testers understand what needs to be built, avoiding any confusion later.

Software Requirement Specification (SRS)

The SRS document is created to list all functional and non-functional requirements in detail. It acts as a blueprint for the entire project, guiding the design, development, and testing phases.

Example: Suppose a customer requests a food-ordering mobile application. In the early stages, the features that meet this requirement are the display of menus, integration with payment gateways, and the requirement for login credentials.

3. System Design

Once the requirements are gathered, the next process is system design. This entails creating a blueprint for the system's architecture. These designs can then be divided into high-level design (HLD) and low-level design (LLD). HLD concerns the system architecture and its components, but LLD outlines the detailed aspects of each module.

SDD- Software Design Document

The SDD captures all design details, including system architecture, module breakdowns, and interactions between components. It serves as a reference throughout the development phase that keeps the design very accurate.

Example: For the food-ordering app, a system design would include a database structure for user profiles, order history, and menu items. It would define the UI elements at a module level and specify how the app would work.

4. Implementation 

After completing the system design, the development team begins to write actual code. In this stage, developers take up the design documents and convert them into code. It's usually the longest and most resource-consuming process of software development. Developers now start writing source code according to the design specifications created in the previous phase.

Example: Implementing the food app project will involve writing codes to create login functionality, managing a database to store orders, and integrating a payment gateway.

Code Snippet

# Example code for login functionality in Python
def user_login(username, password):
    if username == "user123" and password == "pass123":
        return "Login successful"
    else:
        return "Invalid credentials"

# Print the output for correct login
print(user_login("user123", "pass123"))

Output

Login successful

=== Code Execution Successful ===

In this code, the user_login() function takes a username and password and checks if they match predefined values.

5. Integration and Testing

At this stage, coding is finished. After the completion of software coding, integration is followed by testing. In integration, different application modules are combined and tested. This stage aims to find any bugs or faults that might not have been recognised during previous phases.

Alpha testing

This is the initial phase of testing conducted by the internal development team. It involves testing the software for major bugs and errors, so it works as expected in a controlled environment before external testing.

Beta testing

After alpha testing, beta testing is conducted with a select group of external users. These testers provide feedback on the software’s performance and usability, helping identify any remaining issues that need fixing before the final release.

Acceptance testing

This final phase of testing ensures that the software meets all the requirements specified by the client or stakeholders. It verifies that the product works in the real-world environment, and once it passes this stage, the software is considered ready for deployment.

Example: Testing might ensure that users can log in successfully, that their food order is processed correctly, and that the payment gateway works properly.

6. Deployment

After the successful test, the software will be ready for the production environment. In this phase, the application will be released to the end-users. Depending on the nature of the software, these applications can be deployed on a myriad of platforms, including web servers or even an app store.

Example: Deployment for the food-ordering app might involve launching it on Android and iOS stores and making it accessible to users.

7. Maintenance

Even after deployment, the software enters the maintenance phase. This phase fixes problems that may have arisen after deploying the software, such as bugs reported by the users or because of required updates on customer feedback. It is an ongoing process that ensures the software stays functional and updated.

Example: If users report issues with order processing in the food app, developers would address these problems and release updates to ensure smooth functionality.

Classical waterfall models are very important in software development history and process evolution. One of the earliest methodologies introduced took a step-by-step approach that brought much-needed clarity and predictability to project management. Its linear process ensures the completion of each phase before proceeding to the next one, which reduces the chances of errors and maintains focus on the project goals.

The main strength of the waterfall model is its high documentation, where all stages are documented in detail as a recorded trail. This provides a good paper trail for auditing the project while meeting regulatory compliance requirements.

It is also essential for projects with determined requirements and defined goals, where the change scope is likely minimal. In such cases, the waterfall model can be relied on for efficient resource management, tracking progress, and ensuring that all the agreed-upon milestones are met.

New methodologies have emerged, but the classical waterfall model is a fundamental concept for most industries, such as aerospace, defence, or healthcare, where rigid documentation and predictable processes make all the difference between success and failure.

The classical waterfall model in software engineering has been the cornerstone of software development for many decades. It is a structured and systematic approach to managing projects. Although it provides various benefits, it also has certain limitations. Thus, understanding its advantages and disadvantages will help decide whether it fits a particular project.

Advantages

1. Clear Structure and Documentation: One of the waterfall model's strongest advantages is its clear, linear structure. Since each phase is very well defined, it makes project management and tracking progress easier. Also, detailed documentation is created at each stage, ensuring comprehensive records that are handy for future reference, audits, or maintenance.
2. Predictability and Simplicity: The waterfall model is straightforward to understand and apply. It is very well suited for projects whose requirements are well understood. Predictable in nature, the model completes each phase of the process in a predetermined sequence, which will minimise confusion and ambiguity during development.
3. Easy to Manage: The fixed and sequential process made it easier for project managers to track work progress and allocate resources. Ending each phase with milestones gives them clear goals and deliverables, enabling them to monitor and account for individuals.
4. Suitable for Small Projects: The waterfall model is effective for small projects with a limited scope and minimal chances of change during development.

Disadvantages

1. Inflexibility: One of the classical waterfall model's biggest disadvantages is its inflexibility. Once a phase is completed, it is difficult and costly to change or revert to earlier stages; hence, such models are inappropriate for projects whose requirements change during development.
2. Late Testing: In the waterfall model, testing occurs only when the entire system is ready. Therefore, many problems are often found too late in the development process, causing increased costs and delays when errors occur.
3. Not Ideal for Complex Projects: The waterfall model is restrictive when dealing with large or complex projects where requirements may change or evolve. It presupposes that all requirements are known, which is far from the truth in dynamic environments.
4. High Risk of Failure: Since the model doesn't support continuous feedback or iteration, the project has a higher failure risk if the assumptions it starts with or its requirements are wrong.
5. 50% Efforts in the Maintenance Phase: In the waterfall model, the maintenance phase often takes up a significant portion of the overall effort. This is because any issues or bugs discovered post-deployment may require substantial changes. Since testing is done only at the end, fixing these problems can be time-consuming and resource-intensive.‍
6. No Parallelism: The waterfall model follows a strict linear progression where each phase must be completed before the next one begins. This lack of parallelism makes it difficult to manage large projects, as teams can't work on different phases simultaneously. Therefore, the absence of parallelism leads to longer development times and less flexibility.

Though the waterfall model has advantages, partially for simple, defined projects, it suffers from drawbacks that make it less applicable to more complex, dynamic projects that need flexibility and adaptability.

The classical waterfall model in software engineering is applied in many real software development projects, especially when the requirements are well understood, fixed, and unlikely to change during development. This makes it suitable for industries like aerospace, defence, and manufacturing, where projects are very structured, regulatory compliance is required, and clear and predictable outcomes are needed.

In many government and large-scale enterprise software projects, the scope and requirements are relatively stable and well-documented at the project's onset. Moreover, in industries requiring stringent quality control, such as healthcare software development, the classical waterfall model ensures that every phase is completed correctly before advancing, thereby minimising errors and regulatory compliance issues.

The model also applies to projects whose goals are simple, like updating software systems or small-scale systems that can be expected to cause minimal changes in development. This clear framework regarding documentation, tracking, and sign-off helps the project manager manage the entire development process. 

While not the most appropriate for evolving requirements or very fast iterations of a project, the classical waterfall is still applicable where controlled environments provide stable, predictable project needs.

Real-Life Example of Waterfall Model

Let’s consider a real-world scenario where we implement the waterfall model: developing an online grocery shopping app. This example will take you through each phase of the waterfall model, from requirements gathering to maintenance, demonstrating how it works step-by-step.

The example illustrates how each phase in the waterfall model is interconnected. While the model is structured and straightforward, it’s also important to note that making changes once the process has started can be costly and challenging. This makes it less flexible for projects with evolving requirements.

Product: Online Grocery Shopping App

1. Requirements Gathering and Analysis

The first step is to understand what the app needs to do and what users expect from it. This involves:

• Conducting user research to identify customer needs, preferences, and pain points.
• Defining key features and functionalities like product catalogue, search, shopping cart, checkout, payment options, delivery preferences, and order tracking.
• Creating user stories and use cases, which describe how users will interact with the app.
• Developing a detailed requirements document outlining the scope, technical specifications, and desired functionalities.

In this phase, all requirements are clearly documented and finalized, and no changes are expected as we move forward.

2. Design

Once the requirements are defined, the design phase begins. Here’s how we approach it:

• Creating wireframes and mockups to visualize the app’s layout and navigation. This helps stakeholders visualise the final product.
• Designing the user interface (UI) so it’s visually appealing and user-friendly.
• Developing user experience (UX) flows to ensure smooth interactions between the user and the app.
• Defining the technical architecture to plan out how the front-end and back-end will work together, along with system integrations.
• Creating a design document that developers will use to build the app so all details are captured.

At this stage, we have a clear blueprint of the app, ready for development.

3. Implementation and Coding

The development team now begins to write the code, turning the design into a functioning app. Tasks include:

• Writing the actual code using programming languages and frameworks.
• Integrating the front end with the back end ensures that user interactions (like browsing the catalogue or adding products to the cart) are connected to the database and servers.
• Developing APIs to enable communication with external services like payment gateways and delivery services.
• Implementing security measures to protect sensitive user data, such as payment details and personal information.

This is the longest phase, where the product starts taking shape.

4. Testing

Testing comes after the code is written, ensuring that everything works as expected. Key testing tasks include:

• Unit testing individual code components to make sure each part works on its own.
• Integration testing to check that different modules (e.g., the shopping cart and checkout) work well together.
• System testing to evaluate the overall system's behaviour under various conditions.
• User acceptance testing (UAT) with real users to gather feedback and identify any issues.
• Bug fixing based on the test results to ensure the app functions properly.

Only after passing these tests is the app ready for deployment.

5. Deployment

Once the app is fully tested, it’s time to release it to the public:

• Releasing the app to the app store or making it available for download through other platforms.
• Configuring servers and infrastructure to ensure smooth hosting and delivery of the app to users.
• Setting up monitoring tools to track app performance and usage.

Now, users can start using the app.

6. Maintenance

• After deployment, the app enters the maintenance phase, where ongoing tasks include:
• Addressing bugs or issues reported by users to maintain the app's quality.
• Releasing updates with new features or improvements to keep the app fresh and competitive.
• Regular maintenance and security checks to keep the app running smoothly and securely.
• Adapting to changing technologies and user preferences to ensure the app remains relevant over time.

To understand the difference between the classical waterfall model and the iterative waterfall model, it's crucial to recognize that both methodologies utilize a structured, sequential approach to software development. Nonetheless, one is characterized by rigidity and a lack of flexibility, whereas the other allows for adaptation and greater flexibility throughout the process. Here is a comparison table of the Classical Waterfall Model vs the Iterative Waterfall Model.

While the Waterfall model is widely used, it isn't the best fit for every project, especially those with evolving requirements. Thankfully, there are several alternative development models that offer more flexibility and adaptability. Each of these models provides a different approach to managing the phases of development and project life cycles. However, their best use case depends on the nature and complexity of the project. Let’s discuss briefly:

Agile Model

The Agile model is known for its flexibility and iterative approach. It focuses on delivering small, incremental improvements to the product in short development cycles, known as sprints. This method promotes collaboration with stakeholders and allows for regular feedback, making it ideal for projects with changing requirements. Agile encourages continuous improvement, so that the product evolves based on user feedback and changing market conditions.

Iterative Model

The Iterative model focuses on refining and improving the product through repeated cycles or iterations. It is different from Waterfall which is linear. The Iterative model allows development to revisit previous phases based on the results of each iteration. This method makes it easier to adapt to new requirements, as each iteration can incorporate feedback and improvements, making it suitable for projects that require frequent updates or have uncertain requirements.

Spiral Model

The Spiral model combines elements of both the Waterfall and Iterative models and it gives an emphasis on risk management. It divides the project into repeated cycles (spirals), where each spiral involves planning, design, development, and testing, along with risk assessment. This approach ensures that potential risks are addressed early and regularly throughout the project. The Spiral model is well-suited for complex projects where risks need constant evaluation.

V-Model (Verification and Validation Model)

The V-Model emphasises a strict validation and verification process during each phase of the development cycle. It is similar to the Waterfall model but with a focus on testing and quality assurance. In this model, each development stage has a corresponding testing phase so that the product is verified and validated at every step. This model is suitable for projects where quality and reliability are critical.

DevOps

DevOps is a methodology that focuses on collaboration between development and operations teams. It emphasizes continuous integration, continuous delivery, and automation to set up a robust development process. DevOps aims to shorten development cycles and improve product quality by fostering collaboration, automating repetitive tasks, and allowing for faster deployments. It is particularly useful for projects requiring frequent updates and rapid responses to customer feedback.

The classical waterfall model in software engineering has opened doors to several modern software development methodologies. Nowadays, it might not be feasible for large or dynamic projects with too much change; however, it is still a structured approach and quite simple for small-scale or medium-scale projects. Thus, one cannot understand the roots of modern-day software engineering if one does not understand the principles of the classical waterfall model.

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1.What is the classical waterfall model in software engineering? 

A waterfall model represents a classical approach to developing applications sequentially. The next step occurs only when the previous phase is completed. It is typically structured in phases: requirements gathering, design, coding, testing, and maintenance.

2.What are the main phases of the classical waterfall model? 

The major phases involved are Requirement Gathering, System Design, Implementation, Integration and Testing, Deployment, and Maintenance. This approach completes each phase before starting the next to ensure a structured project sequence.

3.How does the classical waterfall model differ from the iterative model? 

The classical waterfall model is linear and does not allow going back to previous phases. On the other hand, the iterative model allows multiple iterations, thus permitting feedback and adjustments at earlier stages of development, making it more flexible.

4.What are the advantages of the classical waterfall model? 

The main advantages are that it is simple, clear in documentation, and has defined project milestones. It's easy to manage. It works well for small-scale projects with fixed, well-understood requirements.

5.What are the disadvantages of the classical waterfall model? 

The main disadvantages are its rigidity and late testing. It is not easy to adapt to changes in requirements during development, so it is not very suitable for large, dynamic projects with changing needs.