Verifying Requirements Qua Requirements
VijaySankar 270000E5JQ Visits (6431)
We have with us today Bruce Powel Douglass. He doesn't need an intro for most of us -- Embedded Software Methodologist. Triathlete. Systems engineer. Contributor to UML and SysML specifications. Writer. Black Belt. Neuroscientist. Classical guitarist. High school dropout. Bruce Powel Douglass, who has a doctorate in neurocybernetics from the USD Medical School, has over 35 years of experience developing safety-critical real-time applications in a variety of hard real-time environments. He is the author of over 5700 book pages from a number of technical books including Real-Time UML, Real-Time UML Workshop for Embedded Systems, Real-Time Design Patterns, Doing Hard Time, Real-Time Agility, and Design Patterns for Embedded Systems in C. He is the Chief Evangelist at IBM Rational, where he is a thought leader in the systems space and consulting with and mentors IBM customers all over the world. He can be followed on Twitter @BruceDouglass. Papers and presentations are available at his Real-Time UML Yahoo technical group and from his IBM thought leader page.
The problems with poor requirements are legion and I don’t want to get into that in this limited space (see Managing Your Requirements 101 – A Refresher Part 1: What is requirements management and why is it important? ). What I want to talk about here is verification and validation of the requirements qua requirements rather than at the end of the project when you’re supposed to be done.
The usual thing is that requirements are reviewed by a bunch of people locked in a room until they are ready to either 1) gnaw off their own arm or 2) approve the requirements. Then the requirements are passed off to a development team – which may consist of many engineering disciplines and lots of engineers – for design and implementation. In parallel, a testing group writes the verification and validation (V&V) plan (including the test cases, test procedures and test fixtures) to ensure that the system conforms to the requirements and that the system meets the need. After implementation, significant problems tracing back to poor requirements require portions of the design and implementation are thrown away and redone, resulting in projects that are late and over budget. Did I get that about right?
The key problem with this workflow is that the design and implementation are started and perhaps even finished without any real assurance about the quality of the requirements. The actions that determine that the requirements are right are deferred until implementation is complete. That means that if the requirements are not right, the implementation (and corresponding design) must be thrown away and redone. Internal to the development effort, unit/developer and integration testing verify the system is being built properly and meets the requirements. Then at the end, the system verification testing provides a final check to make sure that the requirements are correctly addressed by the implementation.
During this development effort, problems with requirements do emerge – such as requirements that are incomplete, inconsistent, or incorrect. When such problems are identified, this kicks off a change request effort and an update to the requirements specification (at least in any reasonable process), resulting in the modification of the system design and implementation. But wouldn’t it be better to not have these defects in the first place? And even more important, wouldn’t it be useful to know that the implementing the requirements will truly result in a system that actually meets the customer’s needs?
There are two concerns I want to address here: ensuring that the requirements are “good” (complete, consistent, accurate, and correct) and that they reflect the customer’s needs. And I want to do this before design and implementation are underway.
It isn’t obvious
Imagine you’re building a house for your family. You contract an architect who comes back to you after 3 months with a 657 page specification with statements like:
My question to you is simple: is this the house you want to live in? How would you know? There might be 6500 requirements describing the house but it would almost impossible for any human to understand whether this is the house you want. For example:
What (real) architects do is they build models of the system that support the reasoning necessary to answer these questions. They don’t rely simply on hundreds or thousands of detailed textual statements about the house. Most systems that I’m involved with developing are considerably more complex than a house and have requirements that are both more technical and abstract.
Nevertheless, I still have the same basic need to be able to understand how the requirements fit together and reason about the emergent properties of the system. The problem of demonstrating that the implementation meets the stated requirements (“building the system right”) is called verification. The problem of showing that the solution meets the needs of the customer is called validation. Verification, in the presence of requirements defects, is an expensive proposition, largely due to the rework it entails. Implementation defects are generally easy and inexpensive to repair but the scope of the rework for requirements defects is usually far greater. Validation is potentially an even more expensive concern because not meeting the customer need is usually not discovered until the system is in their hands. Requirements defects are usually hundreds of times more expensive than implementation defects because the problems are introduced early, identified late in the project, and require you to throw away existing work, redesign and reimplement the solution, then integrate it into the system without breaking anything else.
A proposal: Verifiable and Validatable Requirements
The agile adage of “never be more than minutes away from demonstrating that the work you’re doing is right” applies to all work products, not just software source code. It’s easy to understand how you’d do that with source code (run and test it). But how do you do that with requirements?
The Core Concept
Premise: You can only verify things that run.
Conclusion: Build only things that run.
Solution: Build executable requirements models to support early requirements verification
If we can build models of the requirements, we can verify and validate them before handing them off to the design team. The way I recommend you do that is to
When problems are identified with the requirements during this functional use case analysis, they can be easily and inexpensively fixed before there is any design or implementation to throw away and redo.
Constructing the Executable Use Case
Some people are confused with the fundamental notion of using a state machine to represent requirements, thinking that state machines are inherently a design tool. State machines are just a behavioral specification, and requirements are really just statements of behavior in which we are trying to characterize the required inputoutput control and data transformations of the system. It’s a natural fit. Consider the set of user stories for a cardiac pacemaker
Of course, this is a simple model, but it actually runs which means that we can then verify that is correct and we can use it to support validation with the customer as well. We can examine different sequences of incoming events with different data values and look at the outcomes to confirm that they are what we expect.
Verifying the Requirements
For the verification of consistency of requirements, we must first decide what “inconsistent” means. I believe that inconsistent requirements manifest as incompatible outcomes in the same circumstance, such as when a traffic light would be Red because of one requirement but at the same time must also be Green to meet another. Since the execution of the requirements model has demonstrable outcomes, we can run the scenarios that represent the requirement and show through demonstration that all expected outcomes occur and that no undesired consequences arise. For the verification of completeness, we can first demonstrate – via trace links – that every requirement allocated to the use case is represented in at least one scenario as well as the normative state machine. Secondly, the precision of thought necessary to construct the model naturally raises questions during its creation. Have we considered what happens if the system is THIS state and then THAT occurs? What happens if THAT data is out of range? How quickly must THIS action occur? Have we created all of the scenarios and considered all of the operational variants? These questions will naturally occur to you as you construct the model and will result in the addition of new requirements or the correction of existing ones.
For correctness, I mean that the requirement specifies the proper outcome for a given situation. This is usually a combination of preconditions and a series of input-output event sequences resulting in a specified post-condition. With an executable use case model, we can show via test that for each situation, we have properly specified the output. We can do the same scenario with different data values to ensure that boundary values and potential singularities are properly addressed. We can change the execution order of incoming events to ensure that the specification properly handles all combinations of incoming events. Accuracy is a specific kind of correctness that has to do with quantitative outcomes rather than qualitative ones. For example, if the outcome is a control signal that is maintains an output that is proportional to an input (within an error range), we can run test cases to ensure that the specification actually achieves that. We can both execute the transformational logic in the requirements and formally (mathematically) analyze it as well if desired.
Be aware that this use case model is not the implementation. Even if the system use case model is functionally correct and executes properly, it is not operating on the desired delivery platform (hardware) and has not been optimized for cost, performance, reliability, safety, security, and other kinds of quality of service constraints. In fact, it has not been designed at all. All we’ve done is clearly and unambiguously state what a correctly designed system must do. This kind of model is known as a specification model and does not model the design.
Validating the Requirements
Validation refers to confirming that the system meets the needs of the customer. Systems that meet the requirements may fail to provide value to the customer because
The nice thing about the executable requirements model is that you can demonstrate what you’ve specified to the customer, not as pile of dead trees to be read over a period of weeks but instead as a representation that supports exploration, experimentation, and confirmation. You may have stated what will happen if the physician flips this switch, turns that knob, and then pushes that button, but what if the physician pushes the button first? What has been specified in that case? In a traditional requirements document, you’d have to search page by page looking for some indication as to what you specified would happen. With an executable requirements specification, you can simply say “I don’t know. Let’s try it and found out.” This means that the executable specification supports early validation of the requirements so that you can have a much higher confidence that the customer will be satisfied with the resulting product.
So does it really work?
I’ve consulted to hundreds of projects, almost all of which were in the “systems” space, such as aircraft, space craft, medical systems, telecommunications equipment, automobiles and the like. I’ve used this approach extensively with the Rational Rhapsody toolset for modeling and (usually) DOORS for managing the textual requirements. My personal experience is that it results in far higher quality in terms of requirements and a shorter development time with less rework and happier end customers. By way of a public example, I was involved in the development of the Eaton Hybrid Drivetrain project. We did this kind of use case functional analysis constructing executable use cases, and it identified many key requirements problems before they were discovered by downstream engineering. The resulting requirements specification was far more complete and correct after this work was done that in previous projects, meaning that the development team spent less time overall.
Building a set of requirements is both daunting and necessary. It is necessary because without it, projects will take longer – sometimes far longer – and cost more. Requirements defects are acknowledged to be the most expensive kind of defects because they are typically discovered late (when you’re supposed to be done) and require significant work to be thrown away and redone. It is a daunting task because text – while expressive – is ambiguous, vague and difficult to demonstrate its quality. However, by building executable requirements models, the quality of the requirements can be greatly improved at minimal cost and effort.
For more detail on the approach and specific techniques, you can find more information in my books Real-Time UML Workshop or Real-Time Agility