The essence of waterfall software development is that complex software systems can be built in a sequential, phase-wise manner where all of the requirements are gathered at the beginning, all of the design is completed next, and finally the master design is implemented into production quality software. This approach holds that complex systems can be built in a single pass, without going back and revisiting requirements or design ideas in light of changing business or technology conditions. It was first introduced in an article written by Winston Royce in 1970, primarily intended for use in government projects.
Waterfall equates software development to a production line conveyor belt.
“Requirements analysts” compile the system specifications until they pass the finished requirements specification document to “software designers” who plan the software system and create diagrams documenting how the code should be written. The design diagrams are then passed to the “developers” who implement the code from the design
Under the waterfall approach, traditional IT managers have made valiant efforts to craft and adhere to large-scale development plans. These plans are typically laid out in advance of development projects using Gantt or M.P.P (Microsoft Project Plan) or PERT charts to map detailed tasks and dependencies for each member of the development group months or years down the line. However, studies of past software projects show that only 9% to 16% are considered on-time and on-budget.
Waterfall equates software development to a production line conveyor belt.
“Requirements analysts” compile the system specifications until they pass the finished requirements specification document to “software designers” who plan the software system and create diagrams documenting how the code should be written. The design diagrams are then passed to the “developers” who implement the code from the design
Under the waterfall approach, traditional IT managers have made valiant efforts to craft and adhere to large-scale development plans. These plans are typically laid out in advance of development projects using Gantt or M.P.P (Microsoft Project Plan) or PERT charts to map detailed tasks and dependencies for each member of the development group months or years down the line. However, studies of past software projects show that only 9% to 16% are considered on-time and on-budget.
Up-front Requirements Analysis
What are requirements? From the stakeholder’s perspective, the requirements are the features and specifications of the system. Requirements define what developers are to build. For example, the system must have a web site with e-commerce capability that can handle 10,000 purchases per hour, or the system must be accessible 99.999% of the time.
One of the biggest problems with waterfall is that it assumes that all project requirements can be accurately gathered at the beginning of the project. In waterfall mode the first phase represents the requirements analysis phase of a software development project. Analysts slave for weeks or months compiling everything they can gleam about the proposed system into comprehensive
“Software Requirements Specification” (SRS) documents. Once finished, the SRS is sent over the fence to the designers while the requirements analysts go to work on the next project.
Imagine a scenario where you engage a software group to build a critical software system. Do you think you could provide every last detail the developers need to know right off the bat? I have yet to encounter such a customer and I am hard pressed to think I ever will. As a start, consider the areas that must be addressed:
What are requirements? From the stakeholder’s perspective, the requirements are the features and specifications of the system. Requirements define what developers are to build. For example, the system must have a web site with e-commerce capability that can handle 10,000 purchases per hour, or the system must be accessible 99.999% of the time.
One of the biggest problems with waterfall is that it assumes that all project requirements can be accurately gathered at the beginning of the project. In waterfall mode the first phase represents the requirements analysis phase of a software development project. Analysts slave for weeks or months compiling everything they can gleam about the proposed system into comprehensive
“Software Requirements Specification” (SRS) documents. Once finished, the SRS is sent over the fence to the designers while the requirements analysts go to work on the next project.
Imagine a scenario where you engage a software group to build a critical software system. Do you think you could provide every last detail the developers need to know right off the bat? I have yet to encounter such a customer and I am hard pressed to think I ever will. As a start, consider the areas that must be addressed:
- Business rules and exceptions
- Scalability and concurrent user support
- Browser or OS support
- User roles and restrictions
- User interface standards.
In fact, it is inevitable that attempts at up-front requirements specification will leave out some very important details simply because the stakeholders cannot tell developers everything about the system at the beginning of the project. This means that the requirements typically change outside of the requirements phase in the form of “change orders”, and in many waterfall projects this can be very costly. By virtue of a requirements change, the intricately planned design can be affected dramatically, which will in turn affect any implementation and test strategies. The cost of change in a waterfall project increases exponentially over time because the developer is forced to make any and all project decisions at the beginning of the project.
What if your business needs are still emerging and certain aspects of the system are rapidly changing or cannot be defined yet? Business climates and objectives often changrapidly, especially in today’s age of instant information. Can you afford to lock your business into a rigid long-term project where the cost of change grows exponentially? For example, a national test preparation organization commissioned my company to build a simulator for an upcoming standardized test. Since the test itself had not been released yet, the types of questions that would appear on the test were unknown when we started development. But the system had to be done shortly after the tests were released. Markets are forcing the software development community to respond with flexible development plans that flatten the cost of change.
People need to see and feel something before they really know what they want. The “I’ll Know it When I See It” (IKIWISI) law says that software development customers can better describe what they really want after seeing and trying working, functional software. I often use a “drawing” analogy to help explain this effect. Although I’m a terrible artist, when I draw a picture I need to see the drawing as I progress. If I tried to close my eyes and draw the same picture, it would prove far less successful. But this is what waterfall asks customers to do: specify the entire system without having a chance to periodically see the progress and make adjustments to the requirements as needed. Waterfall is an “over the fence” approach; the requirements are solicited from the user and some time later the finished product is presented to the user. This is entirely unnatural because customers find it difficult to specify software perfectly without seeing it evolve and progress.
The problem of undefined, changing, and emerging requirements presents a very large challenge to waterfall projects because by definition all requirements must be captured up-front at the risk of costly changes later.
What if your business needs are still emerging and certain aspects of the system are rapidly changing or cannot be defined yet? Business climates and objectives often changrapidly, especially in today’s age of instant information. Can you afford to lock your business into a rigid long-term project where the cost of change grows exponentially? For example, a national test preparation organization commissioned my company to build a simulator for an upcoming standardized test. Since the test itself had not been released yet, the types of questions that would appear on the test were unknown when we started development. But the system had to be done shortly after the tests were released. Markets are forcing the software development community to respond with flexible development plans that flatten the cost of change.
People need to see and feel something before they really know what they want. The “I’ll Know it When I See It” (IKIWISI) law says that software development customers can better describe what they really want after seeing and trying working, functional software. I often use a “drawing” analogy to help explain this effect. Although I’m a terrible artist, when I draw a picture I need to see the drawing as I progress. If I tried to close my eyes and draw the same picture, it would prove far less successful. But this is what waterfall asks customers to do: specify the entire system without having a chance to periodically see the progress and make adjustments to the requirements as needed. Waterfall is an “over the fence” approach; the requirements are solicited from the user and some time later the finished product is presented to the user. This is entirely unnatural because customers find it difficult to specify software perfectly without seeing it evolve and progress.
The problem of undefined, changing, and emerging requirements presents a very large challenge to waterfall projects because by definition all requirements must be captured up-front at the risk of costly changes later.
The waterfall methodology assumes that up-front planning is enough to take into account all variables that could impact the development process. In fact, waterfall projects allocate copious effort detailing every possible risk, mitigation plan, and contingency. But is it possible to predict any and all variables that could possibly affect a software project? The empirical
answer is “no” considering the limited success of waterfall projects.
answer is “no” considering the limited success of waterfall projects.
Waterfall therefore equates software development to an assembly line; defined processes can be established that, when used sequentially, result in a successful project each time. The first step is X, the second is Y, and the result is always Z.
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