Wednesday, September 23, 2009

Specification is Planning is Design is Coding is Approximation

All Phases of Software Development are Variations on a Theme
It is easy to think of specification, estimation, planning, design, implementation, and customer feedback as completely separate and unrelated activities. However, a useful way to think about them is as variations on a theme where the theme is producing a product which provides the best return on investment as measured by the ratio of revenues to cost. Each of the phases of the development life-cycle can be thought of as a combination of discovery, prediction, and design. The prediction comes into play when you make a decision. When you decide that you will schedule a particular requirement to be implemented in your product, you are predicting that the end result is something that will provide value to the customer. You have no way of being sure until the customer has actually used it as part of their normal course of business.

What does it matter if in fact it is true that all of these activities are variations on a theme? I believe it provides a conceptual framework that can help us make better decisions about the products we create and the way we go about creating them.

Requirements Gathering is Not Design
Requirements gathering is not the process of finding out what customers need, it is the process of understanding the customer. The problem with producing elaborate requirements is not just that requirements change over time or that people change their minds, it is that users can’t tell you what they want because they don’t know. Customers cannot tell you ahead of time what they will actually spend money on. At best, requirements are an approximation of what customers need and a best guess of what will provide them value at the time that they actually use what you implement for them.

Gathering requirements from customers and the market in general is much like removing compiler errors. For any particular module that you are working on, the first error is the most accurate and accuracy goes downhill rapidly from there. After you fix the first error, the results from your compiler or IDE will probably be very different. This is the same with customer requirements. For any particular functional area, the first thing customers ask for is most likely the most accurate. Once they see the new version which incorporates that feedback, what they want next is probably very different than what they would have asked for next before they saw the new version.

The most you can hope for is to gather information and ideas that are useful in producing a successful product. If you just build whatever customers ask for, then you are ceding design to them. Customers cannot design for you. If they can then they are not just customers they are also designers. It may be that in your situation your customer is also a designer, but then you should explicitly think of them that way. Otherwise, you run the risk of making the mistake of blending the process of gathering requirements and creating a design.

When you have multiple customers, it is especially true that the customer cannot tell you what they need. It is rare that you will be able to do everything that all customers asked for, let alone everything that any one customer asked for and thus you will have to predict which combination of requests that your customers asked for will produce the best result.

Specification is Design
When you write a Marketing Requirements Document, Product Requirements Document, Specification Document, or whatever combination of documents you create along these lines and whatever you call them, let's call that "specification." A specification is a design. It is a design because you are taking all of the feedback from the customer and saying, at a high level, "this is what will provide value to the customer." You are deciding and predicting that this specification rather than any other specification is the way to go forward. It is the same as deciding that this line of code vs that line of code is the right way to go.

Specification is the First Approximation
When working with software, even if you have clear requirements/specifications that you have validated with users, it is not always clear how you will implement them.The specification is an approximation in two ways. First, it is only an approximation of what users actually want. Second, it is only an approximation of the final result. This same statement holds true at every stage of the game, from specification to design, to plans, to estimates, to the code itself. From initial customer interaction to final implementation you are building approximation on top of approximation.

I'm not advocating against specification, estimation, or design. It is useful and even necessary to produce these, and you will often hit on solutions or eliminate problems much more quickly and much more economically than jumping right to implementation, but in the end they are still only an approximation of reality, an interlocking web of educated guesses.

Not until you actually attempt to create something real will you find out how close to reality all of these approximations are and the real costs and difficulties involved. The longer you take to turn these approximations into reality, the more likely it is that they will be inaccurate and the larger those inaccuracies will be.

Estimation is Design
Estimation, which is also another word for approximation, is part of design. If somebody gives you a requirement do you just shout out a random number as the estimate? The accuracy of your estimate depends on the amount of design that you put into it. You may not think of estimation as design, but it is. Let’s say that somebody says “I need a module which will calculate the sum of an array.” If you have such a module at hand, you will give an estimate that involves integrating the existing module into the overall product. Deciding to use that module is a design decision. If you don’t have such a module at hand, you will give an answer based on your prior experience doing the exact same thing. The decision to use an approach similar to a past experience is also a design decision. It is part of creating the final design.

On the other hand, if you are given a more complicated requirement, then you will probably break it down into sub-tasks and estimate those. That forces you to think about what it will really take to implement the requirement. The process of deciding what the subtasks are is actually design. The more design you do, the more accurate you can be. The accuracy of an estimation is on a spectrum from “wild guess” to delivery of product, from complete prediction to statement of fact, from “I believe it will take 10 days” to “it took 103 hours.” The other aspect here is how much is research/design and how much is counting. When well-known tasks are what is at hand, then it is a matter of counting. If you know you need to change 20 dialogs and each change will take 10 minutes then you know the task will take 200 minutes. That’s simple counting. If you need to produce something that has never been done before, then it is a research project and you won’t have accuracy until you have removed all of the uncertainty.

Planning is Design
The specific set of features that go into a product is in fact a design. By selecting that set, you are deciding (predicting) that this is the optimal set of features to achieve a chosen goal. While it is true that you may just be choosing a set of work items that sum up to the available time for a release, that is still design, but perhaps not the optimal design.

Design is Design
Obviously, the activities that we associate with “the design process” are part of product design. The design that you have prior to implementation may be a fully fleshed out design document with lots of illustrative figures, it may be in the form of UML, or it might be entirely in your head. As with requirements and estimation, the design is also an approximation. The design that you do prior to implementation is only an approximation because during implementation you have still more design to do and you will discover information that you didn’t have during your initial design.

Implementation is Design
During implementation you are doing two kinds of design: micro-design and re-design. Micro-design is all of the little decisions you make while coding. Will you use a vector or a doubly-linked list? Will you specify an index or let the RDBMS do it for you on the fly? Re-design happens when you find out that once it came time to implement something, an unexpected problem arose. For instance, the design specifies the use of a particular third-party library, but in reality that library doesn’t provide the functionality that it claimed to provide. Another example is that the implementation works fine, but doesn’t provide the expected performance and no amount of tweaking seems to be doing the trick.

Final Validation of the Design
Finally, when you deliver new features to customers, you discover how close your approximations were. You discover what customers like and don’t like, what they use and what they don’t use, what they will pay for and what they will not pay for, what provides value and what does not provide value. This information can then be used to determine what you will put more effort into and what you will discontinue. The tighter you can make the loop from discovery to delivery, the faster you can produce the product that your customers really value.

Next: Software Development is a Search Algorithm


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