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The open source community may now have more than ideology on their
side, with researchers showing mathematically that their "release
early, release often" software development model is the quickest way
to bug-free code.
The bazaar versus...
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Software is usually created in one of two ways.
Eric Raymond coined names for the two approaches:
the cathedral
approach, where a team of programmers work as an isolated
group creating the software, only releasing it to their users every so
often after substantial changes; and the bazaar approach,
where new versions of the software are released as often
as possible, perhaps as a result of work done by a community of
user-programmers.
Most proprietary software, such as Microsoft Windows, falls into the cathedral
category, while the Linux operating system and Apache, the most popular web server, are examples of the bazaar approach.
In a paper
currently being refereed,
Damien
Challet and Yann
Le Du from the University of Oxford characterised open source
software as that developed in the bazaar style, and closed
source as that created with the cathedral approach. They
developed a mathematical model of how bugs, or errors in the software, are
detected and resolved for the two types of software development.
...the cathedral
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The model assumes that each software program consists of a number of
independent parts, and that users
will spot bugs present in a
part of the program with a certain probability. Once a bug is
reported, the chance that some programmer will be able
to fix it is
given by another probability.
The model of open source software assumes that at each step, the users
are using a new version of the software containing all the changes
made by the
programmers in the previous step. In contrast, for the closed source model,
new
versions of the software is released to users less frequently.
The study shows that under the same conditions
- the same number of users with the same chance
of spotting bugs, the same number of
equally able programmers, and the same initial level of bugs -
open source software will always become bug-free quicker than closed
source software. For closed source software to become
completely reliable in the same amount of time, the programmers must
be more able, or there must be more of them, than for open
source software.
The researchers explain that this is a result of the release cycles
for closed source software. When a new version is released, after an
initial rush of bug finds by users, the programmers are left to find
and correct errors with little feedback until the next release. In contrast,
the frequent releases of open source software allow for continuous
testing by users, and feedback to programmers.
The researchers were also able to use their model to investigate the
dynamics of real life projects, such as the development of the
open source project Linux, by using records of the number of users and
programmers, and the size of the program measured in
lines of code. Since its birth in 1991, the growth of the Linux code has been
approximately quadratic - much faster than the linear growth of other
software projects such as Mozilla - and yet it has a reputation for
being very reliable. According to the researchers, "this leaves the
question of how Linux could grow at such a pace without compromising
its quality".
The answer appears to lie in the quality of Linux programmers. The
researchers had to put a lower bound on their ability -
the probability that they would successfully
fix a bug - to make the model emulate the
development of Linux. The researchers concluded that "rapid software
growth can indeed lead to high quality software, even in adverse
conditions, provided that the programmers' quality is high enough."
The researchers are aware that there are limitations to their current
model, including the assumption that a program is made up of independent
parts. The next step is to apply the model to a scale free
network, which Ricard
Solé from the Complex Systems Lab,
Barcelona, and his colleagues from the Santa Fe Institute, New Mexico, have shown to be a
more accurate description of the structure of software.
A scale free network is a network of points, called nodes,
joined together in such a way that most of the nodes are linked
directly to only a few others, while an important few nodes are linked
to many others. These highly linked nodes act like hubs, and a
consequence is that the average distance between any two nodes
in the network (the number of links you have to follow to get from one
to the other) is small. Many real-world networks
have recently been shown to be scale free, including
the World Wide Web, with sites like
yahoo acting as hubs, and the internet, the physical network of routers and
wires behind the WWW - even some metabolic networks and food webs
appear to be scale free.
So as research continues, many in the programming community will be
waiting to see if it provides more ammunition for the open source
debate. Meanwhile, software users will be hoping that it will lead to more
reliable software, as we head towards an ever more computer-dependent future.
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