| t byfield on Sun, 29 Mar 1998 08:50:44 +0200 (MET DST) |
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| <nettime> Eric S. Raymond: "The Cathedral and the Bazaar" |
[<http://earthspace.net/~esr/writings/cathedral-bazaar/index.html>
As the author remarks, "This paper influenced Netscape's decision
to release Communicator 5.0 in source, and there are hopeful signs
that it may be launching a long-overdue revolution in the software
industry." Whether *it* will launch a "revolution"...that it influ-
enced Netscape's decision is a fact; and if Netscape's decision is
successful, then other software companies will follow. And that is
a Good Thing. If you've already read this, please bear with us; if
you haven't, well, as they say, share and enjoy. -T]
The Cathedral and the Bazaar
by Eric S. Raymond
$Date: 1998/03/27 18:52:18 $
I anatomize a successful open-source project, fetchmail, that was run
as a deliberate test of some surprising theories about software engi-
neering suggested by the history of Linux. I discuss these theories
in terms of two fundamentally different development styles, the
``cathedral'' model of most of the commercial world versus the
``bazaar'' model of the Linux world. I show that these models derive
from opposing assumptions about the nature of the software-debugging
task. I then make a sustained argument from the Linux experience for
the proposition that ``Given enough eyeballs, all bugs are shallow'',
suggest productive analogies with other self-correcting systems of
selfish agents, and conclude with some exploration of the implications
of this insight for the future of software.
Table of Contents
1. The Cathedral and the Bazaar
2. The Mail Must Get Through
3. The Importance of Having Users
4. Release Early, Release Often
5. When Is A Rose Not A Rose?
6. Popclient becomes Fetchmail
7. Fetchmail Grows Up
8. A Few More Lessons From Fetchmail
9. Necessary Preconditions for the Bazaar Style
10. The Social Context of Open-Source Software
11. Acknowledgements
12. For Further Reading
13. Epilog: Netscape Embraces the Bazaar!
14. Version and Change History
---
1. The Cathedral and the Bazaar
Linux is subversive. Who would have thought even five years ago that
a world-class operating system could coalesce as if by magic out of
part-time hacking by several thousand developers scattered all over
the planet, connected only by the tenuous strands of the Internet?
Certainly not I. By the time Linux swam onto my radar screen in early
1993, I had already been involved in Unix and open-source development
for ten years. I was one of the first GNU contributors in the
mid-1980s. I had released a good deal of open-source software onto
the net, developing or co-developing several programs (nethack, Emacs
VC and GUD modes, xlife, and others) that are still in wide use today.
I thought I knew how it was done.
Linux overturned much of what I thought I knew. I had been preaching
the Unix gospel of small tools, rapid prototyping and evolutionary
programming for years. But I also believed there was a certain
critical complexity above which a more centralized, a priori approach
was required. I believed that the most important software (operating
systems and really large tools like Emacs) needed to be built like
cathedrals, carefully crafted by individual wizards or small bands of
mages working in splendid isolation, with no beta to be released
before its time.
Linus Torvalds's style of development - release early and often,
delegate everything you can, be open to the point of promiscuity -
came as a surprise. No quiet, reverent cathedral-building here --
rather, the Linux community seemed to resemble a great babbling bazaar
of differing agendas and approaches (aptly symbolized by the Linux
archive sites, who'd take submissions from anyone) out of which a
coherent and stable system could seemingly emerge only by a succession
of miracles.
The fact that this bazaar style seemed to work, and work well, came as
a distinct shock. As I learned my way around, I worked hard not just
at individual projects, but also at trying to understand why the Linux
world not only didn't fly apart in confusion but seemed to go from
strength to strength at a speed barely imaginable to cathedral-
builders.
By mid-1996 I thought I was beginning to understand. Chance handed me
a perfect way to test my theory, in the form of an open-source project
which I could consciously try to run in the bazaar style. So I did --
and it was a significant success.
In the rest of this article, I'll tell the story of that project, and
I'll use it to propose some aphorisms about effective open-source
development. Not all of these are things I first learned in the Linux
world, but we'll see how the Linux world gives them particular point.
If I'm correct, they'll help you understand exactly what it is that
makes the Linux community such a fountain of good software -- and help
you become more productive yourself.
---
2. The Mail Must Get Through
Since 1993 I'd been running the technical side of a small free-access
ISP called Chester County InterLink (CCIL) in West Chester,
Pennsylvania (I co-founded CCIL and wrote our unique multiuser
bulletin-board software -- you can check it out by telnetting to
locke.ccil.org <telnet://locke.ccil.org>. Today it supports almost
three thousand users on nineteen lines.) The job allowed me 24-hour-
a-day access to the net through CCIL's 56K line -- in fact, it
practically demanded it!
Accordingly, I had gotten quite used to instant Internet email. For
complicated reasons, it was hard to get SLIP to work between my home
machine (snark.thyrsus.com) and CCIL. When I finally succeeded, I
found having to periodically telnet over to locke to check my mail
annoying. What I wanted was for my mail to be delivered on snark so
that I would be notified when it arrived and could handle it using all
my local tools.
Simple sendmail forwarding wouldn't work, because my personal machine
isn't always on the net and doesn't have a static IP address. What I
needed was a program that would reach out over my SLIP connection and
pull across my mail to be delivered locally. I knew such things
existed, and that most of them used a simple application protocol
called POP (Post Office Protocol). And sure enough, there was already
a POP3 server included with locke's BSD/OS operating system.
I needed a POP3 client. So I went out on the net and found one.
Actually, I found three or four. I used pop-perl for a while, but it
was missing what seemed an obvious feature, the ability to hack the
addresses on fetched mail so replies would work properly.
The problem was this: suppose someone named `joe' on locke sent me
mail. If I fetched the mail to snark and then tried to reply to it,
my mailer would cheerfully try to ship it to a nonexistent `joe' on
snark. Hand-editing reply addresses to tack on `@ccil.org' quickly
got to be a serious pain.
This was clearly something the computer ought to be doing for me. But
none of the existing POP clients knew how! And this brings us to the
first lesson:
1. Every good work of software starts by scratching a developer's
personal itch.
Perhaps this should have been obvious (it's long been proverbial that
``Necessity is the mother of invention'') but too often software
developers spend their days grinding away for pay at programs they
neither need nor love. But not in the Linux world -- which may
explain why the average quality of software originated in the Linux
community is so high.
So, did I immediately launch into a furious whirl of coding up a
brand-new POP3 client to compete with the existing ones? Not on your
life! I looked carefully at the POP utilities I had in hand, asking
myself ``which one is closest to what I want?''. Because
2. Good programmers know what to write. Great ones know what to
rewrite (and reuse).
While I don't claim to be a great programmer, I try to imitate one.
An important trait of the great ones is constructive laziness. They
know that you get an A not for effort but for results, and that it's
almost always easier to start from a good partial solution than from
nothing at all.
Linus Torvalds <http://www.earthspace.net/~esr/faqs/linus>, for
example, didn't actually try to write Linux from scratch. Instead, he
started by reusing code and ideas from Minix, a tiny Unix-like OS for
386 machines. Eventually all the Minix code went away or was
completely rewritten -- but while it was there, it provided
scaffolding for the infant that would eventually become Linux.
In the same spirit, I went looking for an existing POP utility that
was reasonably well coded, to use as a development base.
The source-sharing tradition of the Unix world has always been
friendly to code reuse (this is why the GNU project chose Unix as a
base OS, in spite of serious reservations about the OS itself). The
Linux world has taken this tradition nearly to its technological
limit; it has terabytes of open sources generally available. So
spending time looking for some else's almost-good-enough is more
likely to give you good results in the Linux world than anywhere else.
And it did for me. With those I'd found earlier, my second search
made up a total of nine candidates -- fetchpop, PopTart, get-mail,
gwpop, pimp, pop-perl, popc, popmail and upop. The one I first
settled on was `fetchpop' by Seung-Hong Oh. I put my header-rewrite
feature in it, and made various other improvements which the author
accepted into his 1.9 release.
A few weeks later, though, I stumbled across the code for `popclient'
by Carl Harris, and found I had a problem. Though fetchpop had some
good original ideas in it (such as its daemon mode), it could only
handle POP3 and was rather amateurishly coded (Seung-Hong was a bright
but inexperienced programmer, and both traits showed). Carl's code
was better, quite professional and solid, but his program lacked
several important and rather tricky-to-implement fetchpop features
(including those I'd coded myself).
Stay or switch? If I switched, I'd be throwing away the coding I'd
already done in exchange for a better development base.
A practical motive to switch was the presence of multiple-protocol
support. POP3 is the most commonly used of the post-office server
protocols, but not the only one. Fetchpop and the other competition
didn't do POP2, RPOP, or APOP, and I was already having vague thoughts
of perhaps adding IMAP <http://www.imap.org> (Internet Message Access
Protocol, the most recently designed and most powerful post-office
protocol) just for fun.
But I had a more theoretical reason to think switching might be as
good an idea as well, something I learned long before Linux.
3. ``Plan to throw one away; you will, anyhow.'' (Fred Brooks, ``The
Mythical Man-Month'', Chapter 11)
Or, to put it another way, you often don't really understand the
problem until after the first time you implement a solution. The
second time, maybe you know enough to do it right. So if you want to
get it right, be ready to start over at least once.
Well (I told myself) the changes to fetchpop had been my first try.
So I switched.
After I sent my first set of popclient patches to Carl Harris on 25
June 1996, I found out that he had basically lost interest in
popclient some time before. The code was a bit dusty, with minor bugs
hanging out. I had many changes to make, and we quickly agreed that
the logical thing for me to do was take over the program.
Without my actually noticing, the project had escalated. No longer
was I just contemplating minor patches to an existing POP client. I
took on maintaining an entire one, and there were ideas bubbling in my
head that I knew would probably lead to major changes.
In a software culture that encourages code-sharing, this is a natural
way for a project to evolve. I was acting out this:
4. If you have the right attitude, interesting problems will find you.
But Carl Harris's attitude was even more important. He understood
that
5. When you lose interest in a program, your last duty to it is to
hand it off to a competent successor.
Without ever having to discuss it, Carl and I knew we had a common
goal of having the best solution out there. The only question for
either of us was whether I could establish that I was a safe pair of
hands. Once I did that, he acted with grace and dispatch. I hope I
will act as well when it comes my turn.
---
3. The Importance of Having Users
And so I inherited popclient. Just as importantly, I inherited
popclient's user base. Users are wonderful things to have, and not
just because they demonstrate that you're serving a need, that you've
done something right. Properly cultivated, they can become co-
developers.
Another strength of the Unix tradition, one that Linux pushes to a
happy extreme, is that a lot of users are hackers too. Because source
code is available, they can be effective hackers. This can be
tremendously useful for shortening debugging time. Given a bit of
encouragement, your users will diagnose problems, suggest fixes, and
help improve the code far more quickly than you could unaided.
6. Treating your users as co-developers is your least-hassle route to
rapid code improvement and effective debugging.
The power of this effect is easy to underestimate. In fact, pretty
well all of us in the open-source world drastically underestimated how
well it would scale up with number of users and against system
complexity, until Linus Torvalds showed us differently.
In fact, I think Linus's cleverest and most consequential hack was not
the construction of the Linux kernel itself, but rather his invention
of the Linux development model. When I expressed this opinion in his
presence once, he smiled and quietly repeated something he has often
said: ``I'm basically a very lazy person who likes to get credit for
things other people actually do.'' Lazy like a fox. Or, as Robert
Heinlein might have said, too lazy to fail.
In retrospect, one precedent for the methods and success of Linux can
be seen in the development of the GNU Emacs Lisp library and Lisp code
archives. In contrast to the cathedral-building style of the Emacs C
core and most other FSF tools, the evolution of the Lisp code pool was
fluid and very user-driven. Ideas and prototype modes were often
rewritten three or four times before reaching a stable final form.
And loosely-coupled collaborations enabled by the Internet, a la
Linux, were frequent.
Indeed, my own most successful single hack previous to fetchmail was
probably Emacs VC mode, a Linux-like collaboration by email with three
other people, only one of whom (Richard Stallman, the author of Emacs
and founder of the FSF <http://www.fsf.org>) I have met to this day.
It was a front-end for SCCS, RCS and later CVS from within Emacs that
offered ``one-touch'' version control operations. It evolved from a
tiny, crude sccs.el mode somebody else had written. And the
development of VC succeeded because, unlike Emacs itself, Emacs Lisp
code could go through release/test/improve generations very quickly.
One unexpected side-effect of FSF's policy of trying to legally bind
code into the GPL is that it becomes procedurally harder for FSF to
use the bazaar mode, since they believe they must get a copyright
assignment for every individual contribution of more than twenty lines
in order to immunize GPLed code from challenge under copyright law.
People who copyright using the BSD and MIT X Consortium licenses don't
have this problem; they're not trying to reserve rights that anyone
might have an incentive to challenge.
---
4. Release Early, Release Often
Early and frequent releases are a critical part of the Linux
development model. Most developers (including me) used to believe
this was bad policy for larger than trivial projects, because early
versions are almost by definition buggy versions and you don't want to
wear out the patience of your users.
This belief reinforced the general commitment to a cathedral-building
style of development. If the overriding objective was for users to
see as few bugs as possible, why then you'd only release one every six
months (or less often), and work like a dog on debugging between
releases. The Emacs C core was developed this way. The Lisp library,
in effect, was not -- because there were active Lisp archives outside
the FSF's control, where you could go to find new and development code
versions independently of Emacs's release cycle.
The most important of these, the Ohio State elisp archive, anticipated
the spirit and many of the features of today's big Linux archives.
But few of us really thought very hard about what we were doing, or
about what the very existence of that archive suggested about problems
in FSF's cathedral-building development model. I made one serious
attempt around 1992 to get a lot of the Ohio code formally merged into
the official Emacs Lisp library. I ran into political trouble and was
largely unsuccessful.
But by a year later, as Linux became widely visible, it was clear that
something different and much healthier was going on there. Linus's
open development policy was the very opposite of cathedral-building.
The sunsite and tsx-11 archives were burgeoning, multiple
distributions were being floated. And all of this was driven by an
unheard-of frequency of core system releases.
Linus was treating his users as co-developers in the most effective
possible way:
7. Release early. Release often. And listen to your customers.
Linus's innovation wasn't so much in doing this (something like it had
been Unix-world tradition for a long time), but in scaling it up to a
level of intensity that matched the complexity of what he was
developing. In those early times (around 1991) it wasn't unknown for
him to release a new kernel more than once a day! Because he
cultivated his base of co-developers and leveraged the Internet for
collaboration harder than anyone else, this worked.
But how did it work? And was it something I could duplicate, or did
it rely on some unique genius of Linus Torvalds?
I didn't think so. Granted, Linus is a damn fine hacker (how many of
us could engineer an entire production-quality operating system
kernel?). But Linux didn't represent any awesome conceptual leap
forward. Linus is not (or at least, not yet) an innovative genius of
design in the way that, say, Richard Stallman or James Gosling (of
NeWS and Java) are. Rather, Linus seems to me to be a genius of
engineering, with a sixth sense for avoiding bugs and development
dead-ends and a true knack for finding the minimum-effort path from
point A to point B. Indeed, the whole design of Linux breathes this
quality and mirrors Linus's essentially conservative and simplifying
design approach.
So, if rapid releases and leveraging the Internet medium to the hilt
were not accidents but integral parts of Linus's engineering-genius
insight into the minimum-effort path, what was he maximizing? What
was he cranking out of the machinery?
Put that way, the question answers itself. Linus was keeping his
hacker/users constantly stimulated and rewarded -- stimulated by the
prospect of having an ego-satisfying piece of the action, rewarded by
the sight of constant (even daily) improvement in their work.
Linus was directly aiming to maximize the number of person-hours
thrown at debugging and development, even at the possible cost of
instability in the code and user-base burnout if any serious bug
proved intractable. Linus was behaving as though he believed
something like this:
8. Given a large enough beta-tester and co-developer base, almost
every problem will be characterized quickly and the fix obvious to
someone.
Or, less formally, ``Given enough eyeballs, all bugs are shallow.'' I
dub this: ``Linus's Law''.
My original formulation was that every problem ``will be transparent
to somebody''. Linus demurred that the person who understands and
fixes the problem is not necessarily or even usually the person who
first characterizes it. ``Somebody finds the problem,'' he says,
``and somebody else understands it. And I'll go on record as saying
that finding it is the bigger challenge.'' But the point is that both
things tend to happen quickly.
Here, I think, is the core difference underlying the cathedral-builder
and bazaar styles. In the cathedral-builder view of programming, bugs
and development problems are tricky, insidious, deep phenomena. It
takes months of scrutiny by a dedicated few to develop confidence that
you've winkled them all out. Thus the long release intervals, and the
inevitable disappointment when long-awaited releases are not perfect.
In the bazaar view, on the other hand, you assume that bugs are
generally shallow phenomena -- or, at least, that they turn shallow
pretty quick when exposed to a thousand eager co-developers pounding
on every single new release. Accordingly you release often in order
to get more corrections, and as a beneficial side effect you have less
to lose if an occasional botch gets out the door.
And that's it. That's enough. If ``Linus's Law'' is false, then any
system as complex as the Linux kernel, being hacked over by as many
hands as the Linux kernel, should at some point have collapsed under
the weight of unforseen bad interactions and undiscovered ``deep''
bugs. If it's true, on the other hand, it is sufficient to explain
Linux's relative lack of bugginess.
And maybe it shouldn't have been such a surprise, at that.
Sociologists years ago discovered that the averaged opinion of a mass
of equally expert (or equally ignorant) observers is quite a bit more
reliable a predictor than that of a single randomly-chosen one of the
observers. They called this the ``Delphi effect''. It appears that
what Linus has shown is that this applies even to debugging an
operating system -- that the Delphi effect can tame development
complexity even at the complexity level of an OS kernel.
I am indebted to Jeff Dutky <[email protected]> for pointing out that
Linus's Law can be rephrased as ``Debugging is parallelizable''. Jeff
observes that although debugging requires debuggers to communicate
with some coordinating developer, it doesn't require significant
coordination between debuggers. Thus it doesn't fall prey to the same
quadratic complexity and management costs that make adding developers
problematic.
In practice, the theoretical loss of efficiency due to duplication of
work by debuggers almost never seems to be an issue in the Linux
world. One effect of a ``release early and often policy'' is to
minimize such duplication by propagating fed-back fixes quickly.
Brooks even made an off-hand observation related to Jeff's: ``The
total cost of maintaining a widely used program is typically 40
percent or more of the cost of developing it. Surprisingly this cost
is strongly affected by the number of users. More users find more
bugs.'' (my emphasis).
More users find more bugs because adding more users adds more
different ways of stressing the program. This effect is amplified
when the users are co-developers. Each one approaches the task of bug
characterization with a slightly different perceptual set and
analytical toolkit, a different angle on the problem. The ``Delphi
effect'' seems to work precisely because of this variation. In the
specific context of debugging, the variation also tends to reduce
duplication of effort.
So adding more beta-testers may not reduce the complexity of the
current ``deepest'' bug from the developer's point of view, but it
increases the probability that someone's toolkit will be matched to
the problem in such a way that the bug is shallow to that person.
Linus coppers his bets, too. In case there are serious bugs, Linux
kernel version are numbered in such a way that potential users can
make a choice either to run the last version designated ``stable'' or
to ride the cutting edge and risk bugs in order to get new features.
This tactic is not yet formally imitated by most Linux hackers, but
perhaps it should be; the fact that either choice is available makes
both more attractive.
---
5. When Is a Rose Not a Rose?
Having studied Linus's behavior and formed a theory about why it was
successful, I made a conscious decision to test this theory on my new
(admittedly much less complex and ambitious) project.
But the first thing I did was reorganize and simplify popclient a lot.
Carl Harris's implementation was very sound, but exhibited a kind of
unnecessary complexity common to many C programmers. He treated the
code as central and the data structures as support for the code. As a
result, the code was beautiful but the data structure design ad-hoc
and rather ugly (at least by the high standards of this old LISP
hacker).
I had another purpose for rewriting besides improving the code and the
data structure design, however. That was to evolve it into something
I understood completely. It's no fun to be responsible for fixing
bugs in a program you don't understand.
For the first month or so, then, I was simply following out the
implications of Carl's basic design. The first serious change I made
was to add IMAP support. I did this by reorganizing the protocol
machines into a generic driver and three method tables (for POP2,
POP3, and IMAP). This and the previous changes illustrate a general
principle that's good for programmers to keep in mind, especially in
languages like C that don't naturally do dynamic typing:
9. Smart data structures and dumb code works a lot better than the
other way around.
Brooks, Chapter 9: ``Show me your [code] and conceal your [data
structures], and I shall continue to be mystified. Show me your [data
structures], and I won't usually need your [code]; it'll be obvious.''
Actually, he said ``flowcharts'' and ``tables''. But allowing for
thirty years of terminological/cultural shift, it's almost the same
point.
At this point (early September 1996, about six weeks from zero) I
started thinking that a name change might be in order -- after all, it
wasn't just a POP client any more. But I hesitated, because there was
as yet nothing genuinely new in the design. My version of popclient
had yet to develop an identity of its own.
That changed, radically, when fetchmail learned how to forward fetched
mail to the SMTP port. I'll get to that in a moment. But first: I
said above that I'd decided to use this project to test my theory
about what Linus Torvalds had done right. How (you may well ask) did
I do that? In these ways:
1. I released early and often (almost never less often than every ten
days; during periods of intense development, once a day).
2. I grew my beta list by adding to it everyone who contacted me about
fetchmail.
3. I sent chatty announcements to the beta list whenever I released,
encouraging people to participate.
4. And I listened to my beta testers, polling them about design
decisions and stroking them whenever they sent in patches and
feedback.
The payoff from these simple measures was immediate. From the
beginning of the project, I got bug reports of a quality most
developers would kill for, often with good fixes attached. I got
thoughtful criticism, I got fan mail, I got intelligent feature
suggestions. Which leads to:
10. If you treat your beta-testers as if they're your most valuable
resource, they will respond by becoming your most valuable resource.
One interesting measure of fetchmail's success is the sheer size of
the project beta list, fetchmail-friends. At time of writing it has
249 members and is adding two or three a week.
Actually, as I revise in late May 1997 the list is beginning to lose
members from its high of close to 300 for an interesting reason.
Several people have asked me to unsubscribe them because fetchmail is
working so well for them that they no longer need to see the list
traffic! Perhaps this is part of the normal life-cycle of a mature
bazaar-style project.
---
6. Popclient Becomes Fetchmail
The real turning point in the project was when Harry Hochheiser sent
me his scratch code for forwarding mail to the client machine's SMTP
port. I realized almost immediately that a reliable implementation of
this feature would make all the other delivery modes next to obsolete.
For many weeks I had been tweaking fetchmail rather incrementally
while feeling like the interface design was serviceable but grubby --
inelegant and with too many exiguous options hanging out all over.
The options to dump fetched mail to a mailbox file or standard output
particularly bothered me, but I couldn't figure out why.
What I saw when I thought about SMTP forwarding was that popclient had
been trying to do too many things. It had been designed to be both a
mail transport agent (MTA) and a local delivery agent (MDA). With
SMTP forwarding, it could get out of the MDA business and be a pure
MTA, handing off mail to other programs for local delivery just as
sendmail does.
Why mess with all the complexity of configuring a mail delivery agent
or setting up lock-and-append on a mailbox when port 25 is almost
guaranteed to be there on any platform with TCP/IP support in the
first place? Especially when this means retrieved mail is guaranteed
to look like normal sender-initiated SMTP mail, which is really what
we want anyway.
There are several lessons here. First, this SMTP-forwarding idea was
the biggest single payoff I got from consciously trying to emulate
Linus's methods. A user gave me this terrific idea -- all I had to do
was understand the implications.
11. The next best thing to having good ideas is recognizing good ideas
from your users. Sometimes the latter is better.
Interestingly enough, you will quickly find that if you are completely
and self-deprecatingly truthful about how much you owe other people,
the world at large will treat you like you did every bit of the
invention yourself and are just being becomingly modest about your
innate genius. We can all see how well this worked for Linus!
(When I gave this paper at the Perl conference in August 1997, Larry
Wall was in the front row. As I got to the last line above he called
out, religious-revival style, ``Tell, it, tell it, brother!''. The
whole audience laughed, because they knew it had worked for the
inventor of Perl too.)
After a very few weeks of running the project in the same spirit, I
began to get similar praise not just from my users but from other
people to whom the word leaked out. I stashed away some of that
email; I'll look at it again sometime if I ever start wondering
whether my life has been worthwhile :-).
But there are two more fundamental, non-political lessons here that
are general to all kinds of design.
12. Often, the most striking and innovative solutions come from
realizing that your concept of the problem was wrong.
I had been trying to solve the wrong problem by continuing to develop
popclient as a combined MTA/MDA with all kinds of funky local delivery
modes. Fetchmail's design needed to be rethought from the ground up
as a pure MTA, a part of the normal SMTP-speaking Internet mail path.
When you hit a wall in development -- when you find yourself hard put
to think past the next patch -- it's often time to ask not whether
you've got the right answer, but whether you're asking the right
question. Perhaps the problem needs to be reframed.
Well, I had reframed my problem. Clearly, the right thing to do was
(1) hack SMTP forwarding support into the generic driver, (2) make it
the default mode, and (3) eventually throw out all the other delivery
modes, especially the deliver-to-file and deliver-to-standard-output
options.
I hesitated over step 3 for some time, fearing to upset long-time
popclient users dependent on the alternate delivery mechanisms. In
theory, they could immediately switch to .forward files or their non-
sendmail equivalents to get the same effects. In practice the
transition might have been messy.
But when I did it, the benefits proved huge. The cruftiest parts of
the driver code vanished. Configuration got radically simpler -- no
more grovelling around for the system MDA and user's mailbox, no more
worries about whether the underlying OS supports file locking.
Also, the only way to lose mail vanished. If you specified delivery
to a file and the disk got full, your mail got lost. This can't
happen with SMTP forwarding because your SMTP listener won't return OK
unless the message can be delivered or at least spooled for later
delivery.
Also, performance improved (though not so you'd notice it in a single
run). Another not insignificant benefit of this change was that the
manual page got a lot simpler.
Later, I had to bring delivery via a user-specified local MDA back in
order to allow handling of some obscure situations involving dynamic
SLIP. But I found a much simpler way to do it.
The moral? Don't hesitate to throw away superannuated features when
you can do it without loss of effectiveness. Antoine de Saint-Exupery
(who was an aviator and aircraft designer when he wasn't being the
author of classic children's books) said:
13. ``Perfection (in design) is achieved not when there is nothing
more to add, but rather when there is nothing more to take away.''
When your code is getting both better and simpler, that is when you
know it's right. And in the process, the fetchmail design acquired an
identity of its own, different from the ancestral popclient.
It was time for the name change. The new design looked much more like
a dual of sendmail than the old popclient had; both are MTAs, but
where sendmail pushes then delivers, the new popclient pulls then
delivers. So, two months off the blocks, I renamed it fetchmail.
---
7. Fetchmail Grows Up
There I was with a neat and innovative design, code that I knew worked
well because I used it every day, and a burgeoning beta list. It
gradually dawned on me that I was no longer engaged in a trivial
personal hack that might happen to be useful to few other people. I
had my hands on a program every hacker with a Unix box and a SLIP/PPP
mail connection really needs.
With the SMTP forwarding feature, it pulled far enough in front of the
competition to potentially become a ``category killer'', one of those
classic programs that fills its niche so competently that the
alternatives are not just discarded but almost forgotten.
I think you can't really aim or plan for a result like this. You have
to get pulled into it by design ideas so powerful that afterward the
results just seem inevitable, natural, even foreordained. The only
way to try for ideas like that is by having lots of ideas -- or by
having the engineering judgment to take other peoples' good ideas
beyond where the originators thought they could go.
Andrew Tanenbaum had the original idea to build a simple native Unix
for the 386, for use as a teaching tool. Linus Torvalds pushed the
Minix concept further than Andrew probably thought it could go -- and
it grew into something wonderful. In the same way (though on a
smaller scale), I took some ideas by Carl Harris and Harry Hochheiser
and pushed them hard. Neither of us was `original' in the romantic
way people think is genius. But then, most science and engineering
and software development isn't done by original genius, hacker
mythology to the contrary.
The results were pretty heady stuff all the same -- in fact, just the
kind of success every hacker lives for! And they meant I would have
to set my standards even higher. To make fetchmail as good as I now
saw it could be, I'd have to write not just for my own needs, but also
include and support features necessary to others but outside my orbit.
And do that while keeping the program simple and robust.
The first and overwhelmingly most important feature I wrote after
realizing this was multidrop support -- the ability to fetch mail from
mailboxes that had accumulated all mail for a group of users, and then
route each piece of mail to its individual recipients.
I decided to add the multidrop support partly because some users were
clamoring for it, but mostly because I thought it would shake bugs out
of the single-drop code by forcing me to deal with addressing in full
generality. And so it proved. Getting RFC 822
<http://www.internic.net/rfc/rfc822.txt> parsing right took me a
remarkably long time, not because any individual piece of it is hard
but because it involved a pile of interdependent and fussy details.
But multidrop addressing turned out to be an excellent design decision
as well. Here's how I knew:
14. Any tool should be useful in the expected way, but a truly great
tool lends itself to uses you never expected.
The unexpected use for multi-drop fetchmail is to run mailing lists
with the list kept, and alias expansion done, on the client side of
the SLIP/PPP connection. This means someone running a personal
machine through an ISP account can manage a mailing list without
continuing access to the ISP's alias files.
Another important change demanded by my beta testers was support for
8-bit MIME operation. This was pretty easy to do, because I had been
careful to keep the code 8-bit clean. Not because I anticipated the
demand for this feature, but rather in obedience to another rule:
15. When writing gateway software of any kind, take pains to disturb
the data stream as little as possible -- and *never* throw away
information unless the recipient forces you to!
Had I not obeyed this rule, 8-bit MIME support would have been
difficult and buggy. As it was, all I had to do is read RFC 1652
<http://www.internic.net/rfc/rfc1652.txt> and add a trivial bit of
header-generation logic.
Some European users bugged me into adding an option to limit the
number of messages retrieved per session (so they can control costs
from their expensive phone networks). I resisted this for a long
time, and I'm still not entirely happy about it. But if you're
writing for the world, you have to listen to your customers -- this
doesn't change just because they're not paying you in money.
---
8. A Few More Lessons From Fetchmail
Before we go back to general software-engineering issues, there are a
couple more specific lessons from the fetchmail experience to ponder.
The rc file syntax includes optional `noise' keywords that are
entirely ignored by the parser. The English-like syntax they allow is
considerably more readable than the traditional terse keyword-value
pairs you get when you strip them all out.
These started out as a late-night experiment when I noticed how much
the rc file declarations were beginning to resemble an imperative
minilanguage. (This is also why I changed the original popclient
`server' keyword to `poll').
It seemed to me that trying to make that imperative minilanguage more
like English might make it easier to use. Now, although I'm a
convinced partisan of the ``make it a language'' school of design as
exemplified by Emacs and HTML and many database engines, I am not
normally a big fan of ``English-like'' syntaxes.
Traditionally programmers have tended to favor control syntaxes that
are very precise and compact and have no redundancy at all. This is a
cultural legacy from when computing resources were expensive, so
parsing stages had to be as cheap and simple as possible. English,
with about 50% redundancy, looked like a very inappropriate model
then.
This is not my reason for normally avoiding English-like syntaxes; I
mention it here only to demolish it. With cheap cycles and core,
terseness should not be an end in itself. Nowadays it's more
important for a language to be convenient for humans than to be cheap
for the computer.
There are, however, good reasons to be wary. One is the complexity
cost of the parsing stage -- you don't want to raise that to the point
where it's a significant source of bugs and user confusion in itself.
Another is that trying to make a language syntax English-like often
demands that the ``English'' it speaks be bent seriously out of shape,
so much so that the superficial resemblance to natural language is as
confusing as a traditional syntax would have been. (You see this in a
lot of so-called ``fourth generation'' and commercial database-query
languages.)
The fetchmail control syntax seems to avoid these problems because the
language domain is extremely restricted. It's nowhere near a general-
purpose language; the things it says simply are not very complicated,
so there's little potential for confusion in moving mentally between a
tiny subset of English and the actual control language. I think there
may be a wider lesson here:
16. When your language is nowhere near Turing-complete, syntactic
sugar can be your friend.
Another lesson is about security by obscurity. Some fetchmail users
asked me to change the software to store passwords encrypted in the rc
file, so snoopers wouldn't be able to casually see them.
I didn't do it, because this doesn't actually add protection. Anyone
who's acquired permissions to read your rc file will be able to run
fetchmail as you anyway -- and if it's your password they're after,
they'd be able to rip the necessary decoder out of the fetchmail code
itself to get it.
All .fetchmailrc password encryption would have done is give a false
sense of security to people who don't think very hard. The general
rule here is:
17. A security system is only as secure as its secret. Beware of
pseudo-secrets.
---
9. Necessary Preconditions for the Bazaar Style
Early reviewers and test audiences for this paper consistently raised
questions about the preconditions for successful bazaar-style
development, including both the qualifications of the project leader
and the state of code at the time one goes public and starts to try to
build a co-developer community.
It's fairly clear that one cannot code from the ground up in bazaar
style. One can test, debug and improve in bazaar style, but it would
be very hard to originate a project in bazaar mode. Linus didn't try
it. I didn't either. Your nascent developer community needs to have
something runnable and testable to play with.
When you start community-building, what you need to be able to present
is a plausible promise. Your program doesn't have to work
particularly well. It can be crude, buggy, incomplete, and poorly
documented. What it must not fail to do is convince potential co-
developers that it can be evolved into something really neat in the
foreseeable future.
Linux and fetchmail both went public with strong, attractive basic
designs. Many people thinking about the bazaar model as I have
presented it have correctly considered this critical, then jumped from
it to the conclusion that a high degree of design intuition and
cleverness in the project leader is indispensable.
But Linus got his design from Unix. I got mine initially from the
ancestral popclient (though it would later change a great deal, much
more proportionately speaking than has Linux). So does the
leader/coordinator for a bazaar-style effort really have to have
exceptional design talent, or can he get by on leveraging the design
talent of others?
I think it is not critical that the coordinator be able to originate
designs of exceptional brilliance, but it is absolutely critical that
the coordinator be able to recognize good design ideas from others.
Both the Linux and fetchmail projects show evidence of this. Linus,
while not (as previously discussed) a spectacularly original designer,
has displayed a powerful knack for recognizing good design and
integrating it into the Linux kernel. And I have already described
how the single most powerful design idea in fetchmail (SMTP
forwarding) came from somebody else.
Early audiences of this paper complimented me by suggesting that I am
prone to undervalue design originality in bazaar projects because I
have a lot of it myself, and therefore take it for granted. There may
be some truth to this; design (as opposed to coding or debugging) is
certainly my strongest skill.
But the problem with being clever and original in software design is
that it gets to be a habit -- you start reflexively making things cute
and complicated when you should be keeping them robust and simple. I
have had projects crash on me because I made this mistake, but I
managed not to with fetchmail.
So I believe the fetchmail project succeeded partly because I
restrained my tendency to be clever; this argues (at least) against
design originality being essential for successful bazaar projects.
And consider Linux. Suppose Linus Torvalds had been trying to pull
off fundamental innovations in operating system design during the
development; does it seem at all likely that the resulting kernel
would be as stable and successful as what we have?
A certain base level of design and coding skill is required, of
course, but I expect almost anybody seriously thinking of launching a
bazaar effort will already be above that minimum. The open-source
community's internal market in reputation exerts subtle pressure on
people not to launch development efforts they're not competent to
follow through on. So far this seems to have worked pretty well.
There is another kind of skill not normally associated with software
development which I think is as important as design cleverness to
bazaar projects -- and it may be more important. A bazaar project
coordinator or leader must have good people and communications skills.
This should be obvious. In order to build a development community,
you need to attract people, interest them in what you're doing, and
keep them happy about the amount of work they're doing. Technical
sizzle will go a long way towards accomplishing this, but it's far
from the whole story. The personality you project matters, too.
It is not a coincidence that Linus is a nice guy who makes people like
him and want to help him. It's not a coincidence that I'm an
energetic extrovert who enjoys working a crowd and has some of the
delivery and instincts of a stand-up comic. To make the bazaar model
work, it helps enormously if you have at least a little skill at
charming people.
---
10. The Social Context of Open-Source Software
It is truly written: the best hacks start out as personal solutions to
the author's everyday problems, and spread because the problem turns
out to be typical for a large class of users. This takes us back to
the matter of rule 1, restated in a perhaps more useful way:
18. To solve an interesting problem, start by finding a problem that
is interesting to you.
So it was with Carl Harris and the ancestral popclient, and so with me
and fetchmail. But this has been understood for a long time. The
interesting point, the point that the histories of Linux and fetchmail
seem to demand we focus on, is the next stage -- the evolution of
software in the presence of a large and active community of users and
co-developers.
In ``The Mythical Man-Month'', Fred Brooks observed that programmer
time is not fungible; adding developers to a late software project
makes it later. He argued that the complexity and communication costs
of a project rise with the square of the number of developers, while
work done only rises linearly. This claim has since become known as
``Brooks's Law'' and is widely regarded as a truism. But if Brooks's
Law were the whole picture, Linux would be impossible.
A few years later Gerald Weinberg's classic ``The Psychology Of
Computer Programming'' supplied what, in hindsight, we can see as a
vital correction to Brooks. In his discussion of ``egoless
programming'', Weinberg observed that in shops where developers are
not territorial about their code, and encourage other people to look
for bugs and potential improvements in it, improvement happens
dramatically faster than elsewhere.
Weinberg's choice of terminology has perhaps prevented his analysis
from gaining the acceptance it deserved -- one has to smile at the
thought of describing Internet hackers as ``egoless''. But I think
his argument looks more compelling today than ever.
The history of Unix should have prepared us for what we're learning
from Linux (and what I've verified experimentally on a smaller scale
by deliberately copying Linus's methods). That is, that while coding
remains an essentially solitary activity, the really great hacks come
from harnessing the attention and brainpower of entire communities.
The developer who uses only his or her own brain in a closed project
is going to fall behind the developer who knows how to create an open,
evolutionary context in which bug-spotting and improvements get done
by hundreds of people.
But the traditional Unix world was prevented from pushing this
approach to the ultimate by several factors. One was the legal
contraints of various licenses, trade secrets, and commercial
interests. Another (in hindsight) was that the Internet wasn't yet
good enough.
Before cheap Internet, there were some geographically compact
communities where the culture encouraged Weinberg's ``egoless''
programming, and a developer could easily attract a lot of skilled
kibitzers and co-developers. Bell Labs, the MIT AI Lab, UC Berkeley
-- these became the home of innovations that are legendary and still
potent.
Linux was the first project to make a conscious and successful effort
to use the entire world as its talent pool. I don't think it's a
coincidence that the gestation period of Linux coincided with the
birth of the World Wide Web, and that Linux left its infancy during
the same period in 1993-1994 that saw the takeoff of the ISP industry
and the explosion of mainstream interest in the Internet. Linus was
the first person who learned how to play by the new rules that
pervasive Internet made possible.
While cheap Internet was a necessary condition for the Linux model to
evolve, I think it was not by itself a sufficient condition. Another
vital factor was the development of a leadership style and set of
cooperative customs that could allow developers to attract co-
developers and get maximum leverage out of the medium.
But what is this leadership style and what are these customs? They
cannot be based on power relationships -- and even if they could be,
leadership by coercion would not produce the results we see. Weinberg
quotes the autobiography of the 19th-century Russian anarchist Pyotr
Alexeyvich Kropotkin's ``Memoirs of a Revolutionist'' to good effect
on this subject:
``Having been brought up in a serf-owner's family, I entered active
life, like all young men of my time, with a great deal of confidence
in the necessity of commanding, ordering, scolding, punishing and the
like. But when, at an early stage, I had to manage serious enterprises
and to deal with [free] men, and when each mistake would lead at once
to heavy consequences, I began to appreciate the difference between
acting on the principle of command and discipline and acting on the
principle of common understanding. The former works admirably in a
military parade, but it is worth nothing where real life is concerned,
and the aim can be achieved only through the severe effort of many
converging wills.''
The ``severe effort of many converging wills'' is precisely what a
project like Linux requires -- and the ``principle of command'' is
effectively impossible to apply among volunteers in the anarchist's
paradise we call the Internet. To operate and compete effectively,
hackers who want to lead collaborative projects have to learn how to
recruit and energize effective communities of interest in the mode
vaguely suggested by Kropotkin's ``principle of understanding''. They
must learn to use Linus's Law.
Earlier I referred to the ``Delphi effect'' as a possible explanation
for Linus's Law. But more powerful analogies to adaptive systems in
biology and economics also irresistably suggest themselves. The Linux
world behaves in many respects like a free market or an ecology, a
collection of selfish agents attempting to maximize utility which in
the process produces a self-correcting spontaneous order more
elaborate and efficient than any amount of central planning could have
achieved. Here, then, is the place to seek the ``principle of
understanding''.
The ``utility function'' Linux hackers are maximizing is not
classically economic, but is the intangible of their own ego
satisfaction and reputation among other hackers. (One may call their
motivation ``altruistic'', but this ignores the fact that altruism is
itself a form of ego satisfaction for the altruist). Voluntary
cultures that work this way are not actually uncommon; one other in
which I have long participated is science fiction fandom, which unlike
hackerdom explicitly recognizes ``egoboo'' (the enhancement of one's
reputation among other fans) as the basic drive behind volunteer
activity.
Linus, by successfully positioning himself as the gatekeeper of a
project in which the development is mostly done by others, and
nurturing interest in the project until it became self-sustaining, has
shown an acute grasp of Kropotkin's ``principle of shared
understanding''. This quasi-economic view of the Linux world enables
us to see how that understanding is applied.
We may view Linus's method as a way to create an efficient market in
``egoboo'' -- to connect the selfishness of individual hackers as
firmly as possible to difficult ends that can only be achieved by
sustained cooperation. With the fetchmail project I have shown
(albeit on a smaller scale) that his methods can be duplicated with
good results. Perhaps I have even done it a bit more consciously and
systematically than he.
Many people (especially those who politically distrust free markets)
would expect a culture of self-directed egoists to be fragmented,
territorial, wasteful, secretive, and hostile. But this expectation
is clearly falsified by (to give just one example) the stunning
variety, quality and depth of Linux documentation. It is a hallowed
given that programmers hate documenting; how is it, then, that Linux
hackers generate so much of it? Evidently Linux's free market in
egoboo works better to produce virtuous, other-directed behavior than
the massively-funded documentation shops of commercial software
producers.
Both the fetchmail and Linux kernel projects show that by properly
rewarding the egos of many other hackers, a strong
developer/coordinator can use the Internet to capture the benefits of
having lots of co-developers without having a project collapse into a
chaotic mess. So to Brooks's Law I counter-propose the following:
19: Provided the development coordinator has a medium at least as good
as the Internet, and knows how to lead without coercion, many heads
are inevitably better than one.
I think the future of open-source software will increasingly belong to
people who know how to play Linus's game, people who leave behind the
cathedral and embrace the bazaar. This is not to say that individual
vision and brilliance will no longer matter; rather, I think that the
cutting edge of open-source software will belong to people who start
from individual vision and brilliance, then amplify it through the
effective construction of voluntary communities of interest.
And perhaps not only the future of open-source software. No
commercial developer can match the pool of talent the Linux community
can bring to bear on a problem. Very few could afford even to hire
the more than two hundred people who have contributed to fetchmail!
Perhaps in the end the open-source culture will triumph not because
cooperation is morally right or software ``hoarding'' is morally wrong
(assuming you believe the latter, which neither Linus nor I do), but
simply because the commercial world cannot win an evolutionary arms
race with open-source communities that can put orders of magnitude
more skilled time into a problem.
---
11. Acknowledgements
This paper was improved by conversations with a large number of people
who helped debug it. Particular thanks to Jeff Dutky
<[email protected]>, who suggested the ``debugging is parallelizable''
formulation, and helped develop the analysis that proceeds from it.
Also to Nancy Lebovitz <[email protected]> for her suggestion
that I emulate Weinberg by quoting Kropotkin. Perceptive criticisms
also came from Joan Eslinger <[email protected]> and
Marty Franz <[email protected]> of the General Technics list. Paul
Eggert <[email protected]> noticed the conflict between GPL and the
bazaar model. I'm grateful to the members of PLUG, the Philadelphia
Linux User's group, for providing the first test audience for the
first public version of this paper. Finally, Linus Torvalds's
comments were helpful and his early endorsement very encouraging.
---
12. For Further Reading
I quoted several bits from Frederick P. Brooks's classic The Mythical
Man-Month because, in many respects, his insights have yet to be
improved upon. I heartily recommend the 25th Anniversary edition from
Addison-Wesley (ISBN 0-201-83595-9), which adds his 1986 ``No Silver
Bullet'' paper.
The new edition is wrapped up by an invaluable 20-years-later
retrospective in which Brooks forthrightly admits to the few
judgements in the original text which have not stood the test of time.
I first read the retrospective after this paper was substantially
complete, and was surprised to discover that Brooks attributes bazaar-
like practices to Microsoft!
Gerald M. Weinberg's The Psychology Of Computer Programming (New York,
Van Nostrand Reinhold 1971) introduced the rather unfortunately-
labeled concept of ``egoless programming''. While he was nowhere near
the first person to realize the futility of the ``principle of
command'', he was probably the first to recognize and argue the point
in particular connection with software development.
Richard P. Gabriel, contemplating the Unix culture of the pre-Linux
era, reluctantly argued for the superiority of a primitive bazaar-like
model in his 1989 paper Lisp: Good News, Bad News, and How To Win Big.
Though dated in some respects, this essay is still rightly celebrated
among Lisp fans (including me). A correspondent reminded me that the
section titled ``Worse Is Better'' reads almost as an anticipation of
Linux. The paper is accessible on the World Wide Web at
<http://www.naggum.no/worse-is-better.html>.
De Marco and Lister's Peopleware: Productive Projects and Teams (New
York; Dorset House, 1987; ISBN 0-932633-05-6) is an underappreciated
gem which I was delighted to see Fred Brooks cite in his
retrospective. While little of what the authors have to say is
directly applicable to the Linux or open-source communities, the
authors' insight into the conditions necessary for creative work is
acute and worthwhile for anyone attempting to import some of the
bazaar model's virtues into a more commercial context.
Finally, I must admit that I very nearly called this paper ``The
Cathedral and the Agora'', the latter term being the Greek for an open
market or public meeting place. The seminal ``agoric systems'' papers
by Mark Miller and Eric Drexler, by describing the emergent properties
of market-like computational ecologies, helped prepare me to think
clearly about analogous phenomena in the open-source culture when
Linux rubbed my nose in them five years later. These papers are
available on the Web at <http://www.agorics.com/agorpapers.html>.
---
13. Epilog: Netscape Embraces the Bazaar!!
It's a strange feeling to realize you're helping make history....
On January 22 1998, approximately seven months after I first published
this paper, Netscape Communications, Inc. announced plans to give away
the source for Netscape Communicator <http://www.netscape.com/newsref/
pr/newsrelease558.html>. I had had no clue this was going to happen
before the day of the announcement.
Eric Hahn, Executive Vice President and Chief Technology Officer at
Netscape, emailed me shortly afterwards as follows: ``On behalf of
everyone at Netscape, I want to thank you for helping us get to this
point in the first place. Your thinking and writings were fundamental
inspirations to our decision.''
The following week I flew out to Silicon Valley at Netscape's
invitation for a day-long strategy conference (on Feb 4 1998) with
some of their top executives and technical people. We designed
Netscape's source-release strategy and license together, and laid some
more plans that we hope will eventually have far-reaching and positive
impacts on the open-source community. As I write, it is a bit too
soon to be more specific; but details should be forthcoming within
weeks.
Netscape is about to provide us with a large-scale, real-world test of
the bazaar model in the commercial world. The open-source culture now
faces a danger; if Netscape's execution doesn't work, the open-source
concept may be so discredited that the commercial world won't touch it
again for another decade.
On the other hand, this is also a spectacular opportunity. Initial
reaction to the move on Wall Street and elsewhere has been cautiously
positive. We're being given a chance to prove ourselves, too. If
Netscape regains substantial market share through this move, it just
may set off a long-overdue revolution in the computer industry.
The next year should be a very instructive and interesting time.
14. Version and Change History
$Id: cathedral-bazaar.sgml,v 1.36 1998/03/27 18:52:18 esr Exp $
I gave 1.16 at the Linux Kongress, May 21 1997.
I added the bibliography July 7 1997 in 1.20.
I added the Perl Conference anecdote November 18 1997 in 1.27.
I changed ``free software'' to ``open source'' February 9 1998 in
1.29.
I added ``Epilog: Netscape Embraces the Bazaar!'' on February 10 1998
in 1.31
Other revision levels incorporate minor editorial and markup fixes.
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