Windows Tech Journal, November 1997
"How long do you think version 3.0
will take?" Kim asked the assembled project team
members. She'd been assigned to lead a major upgrade to
Square-Calc, Square-Techs signature spreadsheet
program. Square-Calc 3.0 would use Square-Calc 2.5s
code base as the jumping-off point for a series of major
"Ive reviewed each of the
features carefully," Josť said. "It looks to me like it would take about
6 months for our 4-person development team to implement this
functionality from scratch. I wasnt involved in version
2.5, and I havent had a chance to look over the code
base, but the from a user point of view, the product looks
pretty good, so I think 6 months is a safe estimate."
"I wouldnt be too sure about
that," George said. George was a senior developer who
had worked on several of Square-Techs projects. "I
didnt work on Square-Calc 2.5, but I know that that
team had problems. The contract programmer who was in charge
of the graphics updates delivered his code 2 months late.
Then the team had problems integrating his code with the rest
of the code, and it took another month to stabilize the
software. They released a good product, but I dont know
what will happen once we start changing the code. If it would
take 6 months to implement the version 3.0 features from
scratch, Id allow at least 8 months to add the same
features to the existing code baseespecially since none
of us worked on version 2.5."
"Ill tell my boss that we need
at least 6 months, but that he should be prepared to accept a
2-month overrun," Kim said. "How do you want to
proceed with requirements specification and design?"
"The company has been over this ground
before," Bob, the third developer, said. "I think
we all have a pretty good idea about what the software should
look like. I dont see the point of wasting our time on
a huge, bureaucratic requirements document."
"Thats OK with me, Josť said.
"I used to work on military software projects, and those
government specs are a total waste of time. Lets
just create a checklist of features and implement the
software based on the list."
"I think formal design is about as
useless as formal requirements," Bob said. "We all
know what were doing. We can just noodle some ideas
around on the whiteboard. I think that will be good
"Sounds good to me," Josť said.
"OK," Kim said. "Lets
get to work."
The team members had decided earlier that
they needed to make quick progress to gain upper
managements confidence, so they coded a handful of the
easiest features first. Some features took a little longer
than expected, but they attributed that to learning to work
with an unfamiliar code base. They made their first internal
release 2 months into their 6 month schedule, and then began
work on the meatier features.
Josť turned first to adding two new
graphs. After working for a few weeks, the team held a status
meeting. Josť reported encountering a few problems.
"The graphics code is really unstable," he said.
"Im doing the best I can with it, but its
slow going. I cant seem to add any new graphs without
breaking all the existing graphs. That contractors code
from version 2.5 is terrible. It doesnt follow the
projects coding conventions, and I dont see how
they ever got most of it to work at all." Bob and George
reported that the code they were working on was less
problematic, and they were making good progress.
"Lets just see how your work
goes a little longer," Kim said to Josť. "If you
keep running into problems, Bob or George can give you a
A few weeks later, the team met again.
"Im running into even more problems than
before," Josť reported. "It seems like every time
I fix one bug in that old graphics code, I uncover two or
three more. I think that whole graphics engine needs to be
"Bob, can you spare some time to help
Josť?" Kim asked.
"Im running into problems
myself," Bob said. "Ive been working on
speeding up the spreadsheet recalculation engine. I thought
the approach I had chosen would work, but Ive run into
a flaw in my design, and I have to rewrite a big chunk of
what Ive already coded."
"How about you George?"
"Ive been doing my own design
work as I go along," George said, "so I
havent run into any major problems. But working with
that old code is tough. The code is really brittle, so I have
to make my changes carefully. There are some squirrelly
workarounds that the version 2.5 team used to hack the
integration with that contractors graphics code. When
Josť changes that code, I keep needing to change my code. I
dont think Im making progress fast enough to be
able to help Josť."
"OK, well keep the assignments
as they are," Kim decided.
As the project wore on, the development
team found that the pattern of slow progress didnt
change much. Josť eventually did rewrite the graphics
subsystem, and the redesign of that code caused George to
rewrite many of the interfaces to the code he had been
working on. Bobs work on the recalculation engine kept
pace roughly with Josťs graphics work, and the team
reached its feature complete milestone after 9 months. At
that point, the marketing department reviewed the software
and found several "essential" features that had not
been included. The development team spent 2 more months
adding those features. The developers needed 2 months after
that to stabilize the software, and they finally released
Square-Tech 3.0 after 13 months.
How Fast Can You Swim
Although the Square-Calc case study is
fictional, the general pattern is one that has been repeated
many times. Development teams take shortcuts early in the
project to save time, and in the end those shortcuts come
back to haunt both the current project team and the project
teams on future projects that try to extend those designs and
Youll sometimes hear experienced
software developers talk about the "upstream" and
"downstream" parts of a software project. The word
"upstream" refers to the early parts of a project,
and "downstream" refers to the later parts. I have
found this distinction between "upstream" and
"downstream" to be a fundamentally useful way to
think about software projects. The work developers perform
early in the project flows into the later part of the
project. Good early work sets up the project for a gentle
float through placid waters downstream. Poor quality work
sets up the project for a rough and tumble ride through rocky
Barry Boehm and Philip Papaccio found that
an error created early in the project, for example during
requirements specification or architecture, costs 50 to 200
times as much to correct late in the project as it does to
correct close to the point where it was originally created.
Figure 1 illustrates this effect.
Increase in defect cost as
time between defect creation and defect correction increases.
Effective projects practice "phase
containment"detecting and correcting defects in
the same phase theyre created.
Why are errors so much more costly to
correct downstream? One sentence in a requirements
specification can easily turn into several design diagrams.
Later in the project, those diagrams can turn into hundreds
of lines of source code, dozens of test cases, many pages of
end-user documentation, help screens, instructions for
technical support personnel, and so on.
If the project team has an opportunity to
correct a mistake at requirements time when the only work
that has been done is the creation of a one-sentence
requirements statement, it will be wise to correct that
single sentence rather than waiting until it also has to
correct all the various manifestations of the requirements
problem downstream. This idea is sometimes called "phase
containment," which refers to detecting and correcting
defects in the same phase in which theyre created.
The Square-Calc 3.0 team fell victim to a
major upstream problem: It didnt perform the upstream
activities of requirements development and design, which
meant that it didnt give itself an opportunity to
detect and correct upstream problems cheaply. It could
correct problems only downstream, during coding, and only at
Well run software projects root out
problems early. They create opportunities to correct upstream
problems. They develop and review their requirements
carefully to correct as many requirements problems as they
can "in phase." They develop and review their
designs with just as much care. In the case study, Bob
shouldnt have spent several weeks writing new code for
the recalc engine before discovering a design flaw. That flaw
should have been corrected cheaply during an upstream design
Because no code is being generated while
these upstream activities are being conducted, these
activities might seem as though they are delaying "the
real work" of the project. In the case study, Bob was
eager to begin coding. The temptation to start coding before
nailing down the requirements or thinking through high level
designs is a strong temptation to developers who enjoy coding
(including me). But the upstream activities of requirements
development and design dont delay the project; they lay
the groundwork for the projects success, and they
shouldnt be abbreviated.
You might worry about erring on the side of
spending too much time on upstream work and increasing the
projects overhead. That is a slight risk, but very few
projects err on that side. The more severe risk is erring on
the side of too little upstream work, which allows defects to
slip through that must be corrected at 50 to 200 times the
cost of correcting them earlier. The smart money errs on the
side of too much upstream work rather than too little.
Life Preservers for
Project teams create software under intense
schedule pressure, which leads them to take shortcuts and
produce low quality design and code bases. When the project
team finally stabilizes the software enough to release it,
the team members heave huge sighs of relief and cross their
fingers, hoping that the next project will work out better.
The next project hardly ever works out
better. Even though a project team knows that the old designs
and code are shaky, because of pressure to release timely
upgrades it will continue to build the next version of its
product on that shaky foundation. The project pays a steep
price for using that previous projects low quality
code. Newly added code will expose latent defects in the old
code, which must be corrected. Debugging is time consuming
because new bugs can arise from either new code or low
quality old code. As problems mount, the project team
realizes that the notion of using a poor quality code base to
support quick development is a cruel jokein reality,
doing that severely hobbles the project teams ability
to extend its software and ultimately delays the
softwares delivery. Josť might have been right that
implementing Square-Calc 3.0s functionality from
scratch would take only 6 months, but implementing it within
the context of Square-Calc 2.5s existing low-quality
code base took much longer.
Developers maintaining low quality code
they wrote themselves is bad enough, but Richard Thomas found
that ten generations of maintenance programmers work on an
average program before its rewritten. If the code
isnt designed and written well in the first place,
those 10 generations of programmers will have a devil of a
time modifying it.
Because so many generations of programmers
work on any particular program, it isnt surprising that
each of them have to invest a substantial amount of time
understanding the code theyre revising. Garish Parikh
and Nicholas Zvegintzov report that maintenance programmers
spend 50 to 60 percent of their time simply trying to
understand the code theyre working on. Create high
quality designs and code in the first place, and 10
generations of maintenance programmers will thank you for it!
Watch for the Undertow
The desire to make the maintenance
programmers jobs easier is noble, but a more immediate
reason to develop high quality designs and code is that high
quality designs and code make initial development easier.
Software changes substantially during its initial
development. When requirements change in the middle of a
release, developers find themselves revising their own
designs and code to accommodate the changes. They are
downstream from their own work. In the case study, marketing
had the developers make two months worth of changes after
they were already 9 months into their project. Even on
well-managed projects, Capers Jones reports that requirements
change between 1 to 4 percent per month. If youre
looking for a reason to do high quality work upstream,
realize that "downstream" might be closer than it
If youre interested in learning more
about the downstream consequences of upstream decisions,
several books and articles have more to say about it.
Barry W. Boehm and Philip N. Papaccio were
some of the first people to call attention to the cost of
upstream mistakes in their article "Understanding and
Controlling Software Costs" (IEEE Transactions on
Software Engineering, v. 14, no. 10, October 1988, pp.
Steve McConnells Software Project
Survival Guide maps a course through an entire project
which is designed to minimize the number of upstream defects
without making the project bureaucratic (Expected October
1997, Microsoft Press, ISBN ????).
Capers Jones discusses the dynamic of
requirements changes in Applied Software Measurement:
Assuring Productivity and Quality, 2d Ed. (1997,
Richard A. Thomas reported that 10
generations of maintenance programmers work on a program
before its discarded in his 1984 article, "Using
Comments to Aid Program Maintenance" (Byte, May
1984, pp. 415-422).
Although somewhat dated, Garish Parikh and
Nicholas Zvegintzovs collection of papers titled Tutorial
on Software Maintenance is an excellent source of
fundamental information on software maintenance (1983, IEEE
Computer Society Press).
About the Author
Steve McConnell is the author of Code
Complete (1993); Rapid Development (1996); and
numerous technical articles. Rapid Development received
Windows Tech Journals Star Tech award for
"outstanding innovation, significance, and
usefulness." He is currently Chief Software Engineer at
Construx Software Builders where he divides his time between
leading custom software projects, consulting to companies in
the shrink-wrap industry, and writing books and articles. His
newest book, Software Project Survival Guide, was
published by Microsoft Press in fall 1997. Steve can be
reached at email@example.com.