NEWS AT SEI
This article was originally published in News at SEI on: March 1, 1999
This article captures an exchange of ideas among members of the SEI technical
staff who work on different technical initiatives:
?Alan Christie is active in promoting the application of process and
collaboration technologies to make the practice of software development
?Sholom Cohen’s current research activities include object technology,
software product line practices, and product line introduction.
?David Gluch is investigating software engineering practices for dependably
upgrading systems, focusing on software verification and testing.
?Nancy Mead, guest editor of this release of SEI Interactive, was general chair
of the International Conference on Requirements Engineering last year. She
is currently involved in the study of survivable systems architectures.
?Mark Paulk was the "book boss" for Version 1.0 of the Capability Maturity
ModelÒ (CMMÒ) for Software and was the project leader during the
development of CMM Version 1.1. He is also actively involved with software
?Patrick Place is a member of the SEI technical staff in the COTS (commercial
off-the-shelf)-Based Systems Initiative.
The problem of requirements engineering
Bill Pollak (BP) (moderator): At the SEI, we look at requirements engineering from
the different perspectives of our technical initiatives. How can we improve requirements
engineering as part of our mission to improve software engineering practices and
Patrick Place (PP): I honestly believe that requirements are the root of all evil. A lot of
the problems that we face in our work with COTS (commercial off-the-shelf)-based
systems stem from requirements--this belief that you can do things the way you used to
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do them. I just heard someone say yesterday, “We had this great success in setting
requirements and in developing this particular system.” Their “success” was that they got
the system fielded. It’s an appalling system--it’s unmaintainable, it can’t be upgraded,
and it’s going to be a disaster. But they’re ready to do the same thing again because
they’ve had a success!
You almost wish that the system had not been fielded. But I think if we want to be
software engineers, we have to consider the entire cycle, the entire life of our systems
from inception to final turnoff, cradle to grave. And that involves figuring out what that
system should be, which is collecting and eliciting requirements and maintaining those
requirements throughout the life of the system. Because if you just do it once and then
throw it away and let the system evolve, you’re in trouble.
Sholom Cohen (SC): In the case of our work with product lines and architectures,
customers either have nothing in the way of product line requirements and are looking to
establish some basic set of requirements--they aren’t really looking in detail at how to
elicit, manage, or track requirements--or they come to us with an architecture already in
hand and say, “Now what do we do with it?” Or they come to us with a system that’s in
trouble and say, “How do we reengineer it?” So it doesn’t seem like people are coming to
us saying that they need help with managing requirements. We might say that there’s a
problem there because they can’t track the requirements. And they say, “Well, give me a
tool to do it.”
Nancy Mead (NM): We see a lot of people coming to the CERTÒ Coordination Center
and asking us to analyze their existing systems to see where the security holes are. Right
now we’re really trying to push the idea of having them look at their architectures up
front to see if they’re survivable. There is not as much awareness of these up-front
survivability issues as there should be, but the awareness is growing.
Alan Christie (AC): Often what is required in developing a set of requirements is a
broader systems perspective--to be able to see the whole as more than the sum of the
parts, to be able to assess the behavior of the system as an integrated entity. Complex
feedback effects are very difficult to understand and often counter intuitive. As system
complexity increases, it is more and more difficult to predict, at the requirements phase,
what exactly will happen. As a result there is requirements churning, delaying project
deadlines and pushing up costs.
I believe this situation can be addressed with the appropriate use of simulation
techniques. Simulation has been used for many years within the engineering community
to predict everything from traffic flow to aircraft wing loadings to the likelihood of a
reactor accident. However, the software community has, ironically, been slow in adopting
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this technology. I don’t believe that simulation is a silver bullet, but it is more than a
technology in the sense that its use can raise awareness with respect to the importance of
dynamic effects in complex software systems. By using simulation to support
requirements, a customer can achieve a much-improved sense that a supplier understands
the issues. In addition, if changes are proposed, a simulation of the changes is likely to
wring out potentially unforeseen consequences.
Simulation software is now mature and readily available, but it will probably take a few
dramatic examples of simulation supporting software requirements before the community
PP: A lot of our activities in the COTS-Based Systems Program have been based on a
customer coming in and saying, “I’ve got this legacy system, it’s ancient, we have to
build a new one, how do we do it?” And the first thing is for them to even understand
what it is they have in their existing system. They have documents that are hundreds of
pages long that are supposed to describe their requirements, but they don’t. They’re a
mishmash of operational procedure, of detail, of flowcharts...but the documents don’t
provide any understanding of what their requirements are.
NM: And they’re telling you that they really have no choice; they’re up against a wall.
PP: Some are not quite as up against the wall as others. But they’re all under a federal
mandate to use COTS products, one way or the other. And so they’re trying to figure out
how to do it. We tend to work on the design and the technology, but one of their big
problems is understanding the requirements that they need to build to.
NM: The other problem in the requirements area is that requirements are higher risk than
some of the downstream problems we deal with, and we don’t have nice, neat solutions to
hand to people for problems related to requirements. We can’t say “If you go off and use
this analysis technique or that elicitation technique, everything is going to be fine.”
The human side of requirements engineering: natural language vs. formal
BP: Requirements are more than a technical problem; in large part, they are a people
Mark Paulk (MP): Most of the real requirements engineering value is in the people
stuff. It’s fundamental communications, not writing things down in a mathematical
formula. It’s going out there and talking to people, and eliciting what the requirements
are, then capturing them in some way that an MBA can understand.
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Probably the best book I know of in the area is Gause and Weinberg’sExploring
Requirements[Gause 89]. They have exercises where you look at a sentence like “Mary
had a little lamb,” and you see that the meaning changes depending upon which word you
emphasize. So we have to look at the whole idea of the way that we communicate, which
is really an interpersonal skills issue.
There are folks out there who are doing good work. Barry Boehm, for example, with his
win-win stuff [Boehm 99], is in my opinion one of the folks who is at the forefront of this
kind of activity. All of us have to do requirements elicitation and analysis for our
particular projects. But in terms of looking at requirements engineering as a discipline, I
think you really need somebody or a group of people who focus more on the elicitation
side and the capturing of requirements, which is a communications issue more than it is a
PP: I’d like to take issue with one of your points. I agree that requirements are a people
issue, but you jump to the conclusion that writing things down mathematically is not the
right thing to do; I think this is a bit of a leap. I was involved in an exercise with the IEEE
POSIX.21 standard for realtime distributed systems communication. We communicated
in English to the people who were developing the requirements about the flaws that they
had in their existing requirements set, but that communication was based on a
mathematical analysis. I agree with you wholeheartedly that I would not take 20 pages of
mathematics--or even one page--to the average system acquisitions individual and say,
“Here, now you know what the system is.” But you can make the interpretation back into
the system’s own language or domain of discourse because you can see from the
mathematical logic the consequences of your decisions; I will tell you what those
consequences are not in a stream of symbols, but in a stream of words that have meaning
to you, and you can tell me if I’m right or wrong. And if I’m wrong, I can adjust my
model, and if I’m right, you can adjust your requirements.
MP: I agree, you’re absolutely right. Those kinds of analytical techniques are very useful.
The point I was making is that a lot of the work I see that is reported under the
requirements engineering label is really dealing with a requirements specification
language, a formal methods kind of approach: “We’re going to have a complete,
consistent, unambiguous set of requirements.” And then you go out and you try and
explain that, because when you try to translate from the formal specification back to the
English, you inject ambiguity back in. That’s the reason you went to the formal
specification in the first place.
You can get fairly detailed with a rigorous analysis, but when you try to abstract that
back up to the CEO or the MBA or whomever your customer is, things you have captured
absolutely precisely are just not what they really meant. Or as we’ve said several times,
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they don’t know what they really mean, but they’ll figure it out as we iterate with them
on what the system should be.
David Gluch (DG): One of the things we’re talking about in model-based verification is
to use formalism and pragmatic, focused models as communication/understanding
vehicles. As people analyze requirements specifications, they create an abstracted model,
and there is communication. They begin to understand the engineer who is building the
system, and they also make users, clients, and other stakeholders aware of the issues that
emerge. So we see that using formal language effectively as a communication as well as
an analysis vehicle is a very effective way to identify errors in requirements and make
them explicit. People may consciously decide about a certain requirement, for example,
“I’m not sure about this, I’d like to defer it.”
MP: Is this coupled with tools?
DG: It can be, in terms of the actual checking part of the model, but it’s mostly just the
process of building the model--of abstracting out the essential elements--that helps focus
discussion about a system and also identifies ambiguities.
PP: That’s a communication from the people who have the requirements in their heads to
you the engineer. It doesn’t address the communication from you the engineer back to the
people so that you can fix what’s in their heads because they’ve got it wrong.
MP: Let’s broaden what we said earlier, which is, if you know who your audience is,
then you can communicate in a way that they will understand. If you’re talking to a
software programmer, and you say, “Now here is the language specification for C++,”
you might use the formal BNF specification to help the programmer understand the
syntax for the language. But if I were trying to teach some manager how to do
Programming 101 so he could write a little sort program, and I put one of those
specifications down, I can predict that there would be some resistance to that form of
PP: A course at the University of Manchester actually did that. They gave first-year
graduate students a formal specification, plus a natural-language discussion, for their first
exercise. In fact, for all of their programming careers, students used formal specifications
in their exercises. And the university found that successful--that’s why they did it.
MP: I made that mistake too when I was starting out teaching. It didn’t work out as well
for me. I guess that natural-language discussion is critical: the better you are at that, the
more success you’re going to have.
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PP: Yes, you need the two together.
NM: I think that’s a problem that we see a lot. We do need both, and people don’t want to
invest in both. They want to pick one or the other, and I’m not sure that one or the other
can do the job that needs to be done.
AC: I don't think that there is any real conflict between natural language and formal
requirements. Usually formal requirements are applied to narrowly focused elements of
the problem while natural-language requirements complement this in the broader scope.
For high-reliability and safety-critical systems, formal requirements make a lot of sense
for precisely describing the system's interactions, but that's not necessarily best for
providing an understanding of overall capability.
PP: The most important part of a formal specification is the natural language that
surrounds it. Any mathematical model can state very precisely what the requirements are,
but there’s no intuition. And you can provide all the intuition about that precision in
natural language, so that it’s not just 20 pages of mathematics. It’s mathematics plus text
that says what the mathematics mean.
Requirements engineering in other disciplines: Is software unique?
BP: Is requirements engineering particularly difficult with software?
DG: We often seem to function in isolation, but formal representations are routinely used
in structural engineering. They go to mathematics very quickly, they go to formal
representations, and they’re integrated--mathematics are part of how they define
requirements, part of how they accomplish this communication and translation to the
engineering staff and to the other people involved. I don’t think there is anything
fundamentally different about building software than building anything else, but there’s a
larger focus on intellectual intangibles up front that are more difficult to describe.
PP: The issue is one of complexity. We are creating software with thousands of
functions, many times more complicated than any other manufactured artifacts. Even
well-understood programs have incredible complexity. In a previous roundtable
discussion inSEI Interactive
David Carney likened COTS software to bridges as opposed to screws; however, I think
that he’s doing software an injustice--it’s more like building a country’s road system
when the pieces we have to interconnect are the towns! This complexity takes software
out of the realm of the other engineering disciplines.
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A second significant difference is that software is, by its nature, of extreme generality. A
toaster makes toast and could, if you bought an expensive enough toaster, be coupled to
all sorts of other household artifacts so that your morning toast was available, say, 10
minutes after your morning shower--whenever that occurred. That said, it’s still a toaster,
and if I wished I could create the mathematical language to describe and engineer
The mathematics of bridges are well understood and relatively simple. However, for
software, since it has no defined purpose, the best we can do is create a mathematics
capable of modeling any aspect of software. Unfortunately, this leads us to all
computable functions (a technical term, not to be confused with computation)--this is a
lot of mathematics to cover and means that we are without a domain-specific language to
use for our engineering (unlike bridges that deal in tensile strength as well as forces and
Another point to bear in mind is that software development is really new; we’ve only
been developing what we consider to be software for a tiny fraction of the time during
which we’ve been building bridges.
AC: One distinction between most other engineering disciplines and software is the fact
that, in traditional engineering, the results of what you produce are mostly tangible.
Buildings, bridges, and aircraft all have a physical presence that software does not. I
think this physical presence allows one to more easily visualize the issues and to think in
metaphors. For example if I understand the reason why bridges are structurally sound, I
can probably use that knowledge in designing safe buildings--or even aircraft. I'm not
sure that this intellectual reuse is as portable with software. At least it is not being used
that way today.
DG: My comment relating to specifications for other disciplines was intended to be a
very general observation that other engineering disciplines rely heavily on mathematics
as a basis for describing and understanding their systems. I did not mean to select
structural engineering specifically. I believe this is true across the board in engineering
My point is simply that other engineering disciplines rely on mathematics (formalism)
and so, if software engineering is to be an engineering discipline, it too should include
mathematical formalism as integral to doing “good” engineering for software systems.
The issue is not whether formalism should be included but rather how it should become
part of the discipline.
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Also, I was speculating on what differentiates software from the other engineering
disciplines. While it may seem that software engineering does not have much in common
with mechanical engineering, I think it does. But consider another example--electrical
engineers (digital) designing microprocessors or custom application-specific integrated
circuits. I believe the issues and problems here closely align with those of software
implementation. It can be argued, though, that the similarity is at the
design/implementation levels rather than at requirements. But in looking at requirements,
one may ask how software requirements engineering differs from systems requirements
Pat, relating to the complexity premise, I am not sure about this perspective. Consider the
Boeing 777. I believe that the complexity of the entire aircraft is greater than the
complexity of the software that comprises only part of it, and that the requirements
specification for the software can be considered as resulting from doing the system
design for the aircraft.
MP: I agree with Alan that it is easier to have a set of shared paradigms when you have
something tangible to look at. So when you’re saying “I want to build a bridge,” if I’m
the mayor of Pittsburgh, I can say “I want a bridge to go from here to over there and I
want to be able to run 50,000 cars every day over it.” And so you can specify from the
Dave is absolutely right, when the engineers started doing that, they started getting into
the mathematics from Day 1, but those mathematics are communicated to the city
engineering office, they’re not communicated to the mayor. There are different audiences
for the different ways that you capture things, and the problem that we run into in the
software world is not that we don’t have different audiences, it is that we try to
communicate the same way to all of them.
NM: In one of the projects that I was on, we had a series of design reviews. And one of
the later design reviews was a review of the user interface and displays. At that review,
we found out that these folks who had sat through all the previous reviews had no
understanding of what was going on, because they were looking for data on the displays
that wasn’t being computed in the system; and if they had understood everything that had
come before then, they would have realized that not only was the data not being
displayed, it wasn’t there, it didn’t exist. Because they didn’t understand the methods that
we were using to present our designs, they were unable to internalize that until they
actually saw the user interface; that was the only way they could comprehend what it was
that we were trying to do.
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MP: That’s why you see such a big emphasis on use cases these days. Operational
scenarios are popular because it’s in the actual use that people internalize what it is that
they’re getting. Rapid prototyping is another technique--all of these techniques are ways
of getting the user wrapped up into the system.
DG: Achieving this balance is a key problem--keeping it in the environment. Any
descriptions of requirements that evolve should be characterized in terms of the
environment so that they become more real. The entities that are identified need not be
tangible, but they must be real in terms of the user, and I think it’s important to keep a
balance between what’s real on the one hand and the need to consider COTS products on
the other. Because COTS products will constrain the designer and ultimately perhaps the
performance of the system. This is not explicitly communication, but it’s really wrestling
with ideas that have traditionally been discussed in terms of the “what” and the “why.”
Use cases, scenarios, and other similar strategies enable people to describe the system
and how it works within the environment.
SC: Use cases came up for us recently. We had someone come in to talk about them.
This person’s organization had developed, I think, 156 use cases to describe its system.
The use cases were stated in terms of processes that the system had to go through. We
were sitting there with the person who had developed all the requirements, and also
across the table were all of the program managers. He said “What I would like to do is hit
on one of these functions and see what use cases apply.” And the guy’s jaw kind of
dropped, because there was no mapping between the use cases, which were process
oriented--how to do tracking, analysis, engagement planning--and the functions that were
actually built into the system.
AC: From an organizational perspective, it's increasingly common for a customer for
software and the supplier of that software to be geographically distributed. This usually
means that they have to get together on a periodic basis to “sync up.” During the
formative stages of a project, when requirements are being developed, this interaction can
be particularly critical, and interactions should be most frequent. However, the
impediment of distance means that such interactions do not take place as frequently as
they should, with the result that communications quality can be degraded. Phone and
email communications just do not measure up when high bandwidth interactions are
required. Clearly the quality of the requirements lays a foundation for the quality of the
resulting software system, so there should be a great motivation to really work hard at
effective means to communicate easily early during the early stages of such projects.
This is why I believe that collaboration technologies should be used to help groups
develop requirements when these groups are geographically separated. These
technologies are rapidly maturing and coming down in cost, so it is becoming
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increasingly appealing to use them. They allow for a wide variety of types of interactions
among individuals, and the automatic capture of session interactions provides a historical
record of activities.
What are the “real” requirements for the system?
BP: It seems that requirements can exist at different levels of granularity. How can
software engineers manage these different levels and determine the “real” requirements
of their systems?
PP: Let’s take the FAA system as an example. The FAA has a CD-ROM full of
functionality, such as “The user types these two letters and these two digits, and this is
what will happen.” So in essence, they are the requirements for the system, and that
document is maintained and is kept up to date. And, given the life-threatening nature of
their systems, people do not make changes without good reason. But is that document
really a “requirements document?” My guess is that it is not, at least at the level of “What
are the requirements of the system?”
The requirements of the system are that it be able to maintain separation of aircraft, to
maintain the safety of people as they’re flying en route and as they’re landing; it’s not
that the controllers shall be able to type these two letters and these two digits and achieve
this effect. I think that people lose sight of the real requirements, and they don’t see the
value of investing the effort in maintaining currency between whatever the requirements
document is and the code. Because while the design documentation and the architecture
documentation may never change, they know they’re going to change the code. So there
is a tendency to view everything else as overhead. That’s why you get the situation that
we’ve all seen with systems that evolve, where the initial requirements document
becomes irrelevant. We do not recognize that the requirements are recorded in a living
document that must be maintained and that must survive and evolve throughout the life of
the system. We just don’t do that, because it’s expensive and there’s no return on it; it
doesn’t get another byte of code written.
NM: One interesting example is the use of the Patriot systems during the Gulf War. By
keeping the system in operation much longer than was intended, accuracy was lost, and
hence the anti-missiles didn’t go in the right direction. The change was a procedural one--
just take it down and re-initialize it periodically.
PP: Early versions of IBM’s AIX operating system had a memory fragmentation
problem. After a couple of weeks of continual operation, they’d run out of virtual
memory. Procedurally, you needed to reboot the system. But if you’re running on a 24-
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hour-a-day, 7-day-a-week system, you do not want to reboot that system. Yes, you can
get around these things, but I see confusion in the fact that people have these operational
procedures documents but don’t realize that there are also requirements on the overall
system. We think of requirements that are system requirements or software requirements,
but we don’t consider that the man in the loop and other environmental factors really
form this thing that is the system.
MP: Let me ask a question that builds on what you and Nancy have been saying. Should
the user manual or the operator’s manual be considered the real requirements
specification for the system? For example, in the case of the FAA and the need for
separation of aircraft, a requirement might be that there should be an alarm that goes off
or some kind of blinking light that says these craft are getting too close together. Or the
procedure for operating the system should say that when these two aircraft get too close
together you should tell them to go in different directions or something. I know that there
are non-technical requirements in there, but one of the things that I’ve encouraged folks
to do is to make sure that their user manuals and what the code actually does are
consistent with one another.
PP: But this still misses the point. Doing that gets you into the world of “We have 2000
requirements, and X number of ‘shalls’ in our document,” and this is what you have to
address. At some level, yes, you have to have that, that forms a contract. But is that
something that’s manageable or maintainable, or even something that I can elicit from
MP: Let me rephrase the question. Should anything be considered a real requirement that
is, in essence, invisible to the user? A lot of times when we get into functional
requirements, they’re actually design statements, they’re not really requirements. It’s the
requirements about what the system will do, what the user can expect to achieve, which I
would hope would be captured in some kind of user manual or something like that.
PP: But there are cases where it is important to require things that may be invisible to the
user, that may impinge upon the design. For example, if the FAA writes requirements
only in terms of things that are visible to the user, they will have many different types of
maintenance to do. If they can require that everybody runs on this operating system or
uses that hardware, which is completely invisible to the user, they can save themselves a
whole bunch of maintenance.
MP: I would characterize that as being part of the maintenance manual. Depending on
what environment you’re from, a lot of times there’s a maintenance document that is part
of the deliverable set, which includes the user manual, the operations manual, the
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PP: Yes, but in the world of COTS, you don’t have a maintenance manual. You don’t
maintain the operating system. Periodically you will have to do operating system
upgrades, but you don’t maintain the operating system. I think that you would rather have
one operating system and one set of procedures for how to reinstall that operating system
than to have 20.
NM: I think that the place where this breaks down is that, again to use the FAA example,
all of the operational procedures tell you what to do with aircraft with the assumption that
you’re tracking them correctly. But there’s really a requirement of the system that you
track all aircraft, and that might not appear in the user manual. And all of the software
that’s underneath it isn’t captured in the statement that you’re going to display.
PP: The user manual is all about how and what you can do with the information that you
have available. But there are other requirements for completeness and currency of that
information that probably don’t appear.
Documenting requirements and maintaining architectural integrity
DG: If I could put a little different spin on this, one of the key areas I see is getting the
thing right. Many if not most of the errors that occur downstream, even in code, were
requirements errors. So there are two elements of getting requirements right. One is
something we’ve been talking about--eliciting them and capturing the real needs. Once
that’s accomplished and we have in fact described real needs, are they consistent and
correct in and of themselves? That’s the kind of analysis that we’re focusing on. Are they
complete, consistent, do they make sense, do they really accomplish the objectives that
the stakeholders wanted? These questions must be addressed at the requirements level,
especially given the data that shows that the cost of correcting an error that occurs
downstream that was originally injected in the requirements is quite significant.
MP: How do you ensure that you keep a consistent record of what the requirements are?
We’ve all observed that, over time, the code changes, the design changes, and the
requirements change. So the point that Dave is making is absolutely valid, but the
problem is that if we had a stable set of requirements that was unchanging, then the more
rigorous techniques would be much easier to use. But when you have a dynamically
changing environment, how do you deal with the consistency and make sure that
everything is kept up to date? It’s a human behavior issue as much as anything else.
DG: Right, and that’s true regardless of the language that’s used, whether it’s formal
language or natural language. But a potential solution is to capture as many of the
requirements as appropriate and can be effectively accomplished in a formal language
and then allow automation to handle a lot of the application-independent things that are
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problematic and that require a lot of time but not a lot of intellectual activity on the part
of humans. There are solutions that are tending in that direction, so that you have a line
from requirements all the way through to code, and the ability to then do checking and
SC: At conferences that I have attended in the past couple of years, the emphasis has not
been on the requirements phase. Even when they talk about analysis, they’re talking
about something that is much closer to design than it is to requirements analysis. The
approach starts with the assumption that the requirements already exist, and elicitation is
not the emphasis.
In the product line area, we tend to apply object-oriented analysis to the up-front
elicitation; but again, it’s in a rather narrowly focused area within a bigger problem. And
once we give our results back to the client, there’s a tendency to look at architecture
development activities, and then never to return back to the requirements as they exist in
that up-front analysis effort. The requirements will be evolved through the architectures,
through change scenarios, or some other approach, which lets clients explore what
they’ve got in hand, what kinds of changes the system is likely to undergo in the future,
and to refine with those things in mind. They’re not going to go back and elicit further
requirements through a formal elicitation or management process. And I think that’s
likely to cause a problem downstream. At the end of implementation or even design, the
clients don’t really know what requirements they have.
This is similar to what Pat said earlier--a lot of people come in with these legacy systems
and ask “What do we have?” So one of the things we can take back to other customers is
to say, our experience has shown that, without formally tracking and managing
requirements effectively, you’re going to run into some of the same problems
downstream as those you’ve encountered in the past. They may hit later than they do
now, but they’re still going to hit, and they’re going to probably be as severe if not more
so, because systems are bigger and more complex, and they affect more people than the
old standalone systems used to.
The situation we see commonly is that people do some up-front analysis, and they have
some elicitation and some requirements, but as they get into the architectural design, they
never go back and update the requirements. So within months after the first set of
requirements is complete, the requirements have already been overcome by the
architecture, and they are never re-examined. To these people, requirements elicitation
and management aren’t even issues. The issues now are management and evolution of the
architecture. There may have been some requirements notions up front, but now they’re
embodied in the implementation.
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NM: I think that what happens is that, over time, people lose intellectual control, and they
lose the sense of integrity of the architecture, and going back further, the set of
requirements, and they start changing things in ways that they don’t reflect on. They
don’t go back and say “Does this match the original vision for this?” Sometimes the
vision isn’t documented or expressed really well, and so you don’t really know what the
impact is. But many times, people just don’t think about it; they think about the
immediate problem at hand and not the impact that it has on the original vision of the
system, which is why you see systems that become a mishmash even if they had a pretty
decent set of requirements or a decent architecture to begin with.
Improving requirements engineering
BP: We’ve had a very rich discussion about the problems associated with requirements
engineering. In the time remaining, I’d like us to focus on some of the solutions in the
various domains that you represent. Nancy, you mentioned an analysis method in the
NM: In the area of survivable systems, we are developing an analysis method that we call
the Survivable Network Analysis (SNA) method for software architectures. The SNA
method is causing us to think about requirements for survivable systems. We find that
most clients have not given a lot of thought to how to get the essential functions of their
systems to survive an attack or a break-in and to continue to operate--how to recognize
such an attack and how to recover from it. We call this the “three Rs”: resistance,
recognition, and recovery. For readers who are interested, we wrote up one of our
survivable network analysis case studies
We’re finding that in doing architectural analysis, we are stepping back and making
recommendations for requirements changes as well as recommendations for survivability.
We think there is a lot of work that needs to be done here. Many users, from defense to
finance, just haven’t given that much thought to the vulnerabilities that will occur when
they take their systems and distribute them over a network.
One of the things we’re trying to do this year in our research is to think about intrusion
scenarios that we develop and work with a client on. We’d like to have a canonical or
relatively complete set of possible intrusions at the architecture level. In our initial work
with this, we basically used our own intellect to figure out what the typical types of
intrusions might be, but we’d really like to reduce that to something that could be used in
a more analytical way, so the customers could feel that they had taken into account most
of the bad things that could occur. So our focus this year is in part on that and in part on
SEI Interactive, 03/99 page 15
taking into account risk analysis, because intrusions are not your normal occurrence,
they’re at the far end of the bell curve of activities relative to the systems. And in
conjunction with that, we’re looking at formalisms that support the work we’re doing.
BP: In the sense that Pat and Dave have spoken of?
NM: There’s certainly some relationship, yes.
AC: I mentioned this before but I think it's worth mentioning again. I don't believe
behavioral properties of software systems are being addressed early enough. Too many
systems have been built that don't address performance issues at an early stage, and
consequently they get wiped out during operational tests. Likewise, complex system may
not respond as intended because of unforeseen dependencies between loosely coupled
components. I think simulation can help here, and in fact, a simulation model can become
an integral part of the system's specification. It is almost certain that, after the
requirements development phase is completed, requirements will change. Such
simulation models can thus be used to investigate and address new issues as they arise--
before they become significant problems.
SC: We’ve found that a lot of the people we’re working with are taking the scenario
approach or use-case approach. Many people are adopting this approach to move toward
object-oriented development. But we still come back to the fact that they get to a certain
point with the use cases and then kind of abandon further evolution and maintenance of
the requirements. Instead, the class hierarchy or class structure, whatever the next stage
is, is maintained because it’s closer to the code or the implementation. It seems like there
are instances though, where requirements are placed under very tight control, which
would work if the requirements were true requirements. Often they degenerate into
functional descriptions, and then you’re really just writing in text what’s happening in the
design--the example mentioned earlier of hitting these two keys and having this result.
Those are not really requirements, they’re a description of what’s going on the code.
PP: I don’t think we have solutions in the COTS domain. We have, at best, advice, which
is perhaps little more than a collection of things that you should know anyway. One of
the more innovative approaches that we’ve seen is to get recognized consultants from the
industry--from the marketplace--to help set the requirements for a particular product for a
new system to be developed. That is, instead of getting only experts in the system to be
developed, you also get marketplace leaders who can say, “If you go down this route, you
will never be able to acquire anything that’s COTS based.” We are beginning to collect
these sorts of successful approaches together. But are they repeatable? Who knows?
BP: Maybe these are sort of “best practices.”
SEI Interactive, 03/99 page 16
PP: Or suggested practices. Things like setting the levels of stretch of the requirements is
a real problem. We see problems where you go into so much detail that you don’t admit
COTS solutions. That may be what you need to do. On the other hand, we’ve seen cases
where to ensure the use of COTS-based systems, people set the requirements so
abstractly that they get completely unsuitable systems offered in bids, and they have no
way to reject them because they meet the stated requirements. There was no way to say
that this one, which was the lowest cost alternative, cannot be employed, because its user
interface, for example--which we didn’t mention in our requirements--is completely
unsuitable for our users, and they will reject it, and it will never get installed. So we’re at
that sort of level--we have advice, but not solutions.
With regard to engineering business processes, sometimes it’s much easier to change the
way you do business than it is to acquire a system that does the business that you’re in.
We saw one case where the DoD installation said, “It’s much easier to adopt Oracle
Financial and get that approved within the Air Force as a reporting mechanism for
financial information than it is to try and acquire a system that reports on the existing
reports that we’re producing. But again, this is mostly anecdotal; there’s no real
We talked earlier about the POSIX work. One of the things that we did with POSIX.21
was that, when the requirements were still in the formative stage, we did an analysis of
the requirements and formalized them, and then reported the problems that arose from the
formalization back to the requirements group and changed the requirements because of
the problems that arose from the formalization. So this was a real requirements
engineering elicitation. The entire working group believed then and still believes now
that this was a valuable exercise and a really great way to home in on the requirements
and get higher quality requirements faster than they could have gotten without them.
Again, this is another piece of anecdotal experience.
SC: We did a case study about three years ago of CelsiusTech, a Swedish firm. They
have an elaborate requirements database that they’ve maintained. It has allowed them to
work with new customers to say, “Given this set of requirements that we can support in
our architecture, how do you envision your systems?” So the new system requirements
are developed in light of this requirements database that’s been proven over 10 years or
so in implemented systems. That’s one instance of a managed, tracked, effective database
BP: Is the requirements aspect specifically discussed in the CelsiusTech technical report
SEI Interactive, 03/99 page 17
SC: Yes, but the report is a high-level overview, so the information about the database is
probably only a paragraph or two. The counter example to that are things that come up in
our architecture analyses for applying the Architecture Tradeoff Analysis Method
(ATAM) or Software Architecture Analysis Method (SAAM). You get stakeholders
together sometimes who haven’t worked together, and we’re taking an existing
architecture and eliciting scenarios about how the system will change, how it will be
used, and how it’s envisioned. And new requirements pop up, or requirements that exist
that were unknown to other people in the room. So that’s an example of requirements that
weren’t developed, tracked, or fully understood, and now that the system has been built,
things are suddenly coming to light. Again, there’s no recommendation for how this
could have been done more effectively, but at least bringing the stakeholders together and
working through these kinds of analyses are helpful for bringing these things out early in
the development--anything you can do to avoid sudden recognition when the system is
being tested, as in Nancy’s case of the people who didn’t understand the system until they
looked for data on the screen. So we hope that the architecture analysis brings the
stakeholders together to recognize problems with requirements up front when the system
is being designed.
So we encourage people to examine and consider requirements up front, and at least if
there’s a core requirement that’s being used to define the evolution of the system,
maintain that core even if the other details aren’t maintained. If analysis turns up new
requirements, we have to pull those back into this core. But I don’t think we’ll ever get to
the stage where in the majority of cases, requirements are fully defined and explored as
the system evolves.
DG: I think that what we really have been talking about here is making requirements
engineering part of our systems engineering activities. That’s really the essence of all of
this. A lot of the problems we’ve identified are simply a failure to be engineering-like.
We need a disciplined tracking practice as well as a technically sound approach to
capturing and analyzing requirements.
PP: I agree. Engineering discipline takes a lot of time to develop—I just read Petroski’s
To Engineer is Human[Petroski 85] to understand the millennia spent “hacking” bridges,
and everything else for that matter. So, it’s not a surprise that software isn’t engineered in
the same way as the products of other disciplines. But that doesn’t mean we shouldn’t try
and put it on a sounder footing.
[Boehm 99] Boehm, Barry & Port, Dan. “Escaping the Software Tar Pit: Model Clashes
and How to Avoid Them.”Association for Computing Machinery (ACM) Software
Engineering Notes 23, 1 (January 1999): 36-48.
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[Brownsword 96] Brownsword, Lisa & Clements, Paul.A Case Study in Successful
Product Line Management(CMU/SEI-96-TR-016, ADA 315802). Pittsburgh, Pa.:
Software Engineering Institute, Carnegie Mellon University, 1996. Available at
[Gause 89] Gause, Donald C. & Weinberg, Gerald M.Exploring Requirements: Quality
Before Design.New York, NY: Dorset House Pub., 1989.
[Petroski 85] Petroski, Henry.To Engineer is Human: The Role of Failure in Successful
Design.New York, NY: St. Martin’s Press, 1985.
SEI Interactive, 03/99 page 19
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