Translated into French (Français) by Avice Robitaille and into Serbo-Croatian (Srpskohrvatski) by Anja Skrba.
The first version of this paper was written to an accepted talk given at Linuxtag 2005 given in Karlsruhe, Germany.
The explosive growth of free and open source software over the last decade has been mirrored by an equally explosive growth in the ambitiousness of free software projects in choosing and tackling problems. The free software movement approaches these large problems with more code and with more expansive communities than was thinkable a decade ago. Example of these massive projects include desktop environments — like GNOME and KDE — and distributions like Debian, RedHat, and Gentoo.
These projects are leveraging the work of thousands of programmers — both volunteer and paid — and are producing millions of lines of code. Their software is being used by millions of users with diverse sets of needs. This paper focuses on two major effects of this situation:
The communities that free software projects — and in particular large projects — serve are increasingly diverse. It is becoming increasingly difficult for a single large project to release any single product that can cater to all of its potential users.
It's becoming increasingly difficult to reproduce these large projects. While reproducing entire project is impossible for small groups of hackers, it is often not even possible for small groups to even track and maintain a fork of a large project over time.
Taken together, these facts imply an increasingly realized free software community in which programmers frequently derive but where traditional forking is often untenable. "Forks," as they are traditionally defined, must be improved upon. Communities around large free software projects must be smarter about the process of derivation than they have been in the past.
We are already seeing this with GNU/Linux distributions. New distributions are rarely built from scratch today. Instead, they adapted from and built on top of the work of existing projects. As projects and user-bases grow, these derived distributions are increasingly common. Most of what I describe in this essay are tools and experiences of derived distributions.
Software makers must pursue the idea of an ecosystem of free software projects and products that have forked but that maintain a close relationship as they develop parallelly and symbiotically. To do this, developers should:
Break down the process of derivation into a set of different types of customization and derivation and prioritize methods of derivation.
Create and foster social solutions to the social aspects of the derivation problem.
Build and use new tools specifically designed to coordinate development of software in the context of an ecosystem of projects.
Distribute and utilize distributed version control tools with an emphasis on maintaining differences over time.
This paper is an early analysis of this set of problems. As such, it is highly focused on the experience of the Ubuntu project and its existence as a derived Debian distribution. It also pulls from my experience with Debian-NP and the Custom Debian Distribution (CDD) community. Since I participate in both the Ubuntu and CDD projects, these are areas that I can discuss with some degree of knowledge and experience.
The act of taking the code for a free software project and bifurcating it to create a new project is called "forking." There have been a number of famous forks in free software history. One of the most famous was the schism that led to the parallel development of two versions of the Emacs text editor: GNU Emacs and XEmacs. This schism persists to this day.
Some forks, like Emacs and XEmacs, are permanent. Others are relatively short lived. An example of this is the GCC project which saw two forks — EGCS and PGCC — that both eventually merged back into GCC. Forking can happen for any number of reasons. Often developers on a project develop political or personal differences that keep them from continuing to work together. In some cases, maintainers become unresponsive and other developers fork to keep the software alive.
Ultimately though, most forks occur because people do not agree on the features, the mechanisms, or the technology at the core of a project. People have different goals, different problems, and want different tools. Often, these goals, problems and tools are similar up until a certain point before the need to part ways becomes essential.
A fork occurs on the level of code but a fork is not merely — or even primarily — technical. Many projects create "branches." Branches are alternative versions of a piece of software used to experiment with intrusive or unstable features and fixes. Forks are distinguished from branches both in that they are often more significant departures from a technical perspective (i.e., more lines of code have been changed and/or the changes are more invasive or represent a more fundamental rethinking of the problem) and in that they are bifurcations defined in social and political terms. Branches involve a single developer or community of developers — even if it does boil down to distinct subgroups within a community — whereas forks are separate projects.
Forking has historically been viewed as a bad thing in free software communities: they are seen to stem from people's inability to work together and have ended in reproduction of work. When I published the first version of the Free Software Project Management HOWTO more than four years ago, I included a small subsection on forking which described the concept to future free software project leaders with this text:
The short version of the fork section is, don't do them. Forks force developers to choose one project to work with, cause nasty political divisions, and redundancy of work.
In the best situations, a fork means that two groups of people need to go on developing features and doing work they would ordinarily do in addition to tracking the forked project and having to hand-select and apply features and fixes to their own code-base. This level of monitoring and constant comparison can be extremely difficult and time-consuming. The situation is not helped substantially by traditional source control tools like diff, patch, CVS and Subversion which are not optimized for this task. The worse (and much more common) situation occurs when two groups go about their work ignorant or partially ignorant of the code being cut on the other side of the fork. Important features and fixes are implemented twice — differently and incompatibly.
The most substantial bright side to these drawbacks is that the problems associated with forking are so severe and notorious that, in most cases, the threat of a fork is enough to force maintainers to work out solutions that keep the fork from happening in the first place.
Finally, it is worth pointing out that fork is something of a contested term. Because definitions of forks involve, to one degree or another, statements about the political, organization, and technical distinctions between projects, bifurcations that many people call branches or parallel trees are described by others as forks. Recently, fueled by the advent of distributed version control systems, the definition of what is and is not a fork has become increasingly unclear. In part due to the same systems, the benefits and drawbacks of what is increasingly problematically called forking is equally debatable.
In my introduction, I described how the growing scope of free software projects and the rapidly increasingly size and diversity of user communities is spearheading the need for new type of derivation that avoids, as best as possible, the drawbacks of forking. Nowhere is this more evident than in the largest projects with the broadest scope: a small group of projects that includes operating system distributions.
The Debian project is by many counts the largest free software distribution in terms of code. It is the also, arguably, the largest free software project in terms of the number of volunteers. Debian includes more than 15,000 packages and the work of well over 1,000 official volunteers and many more contributors without official membership. Projects without Debian's massive volunteer base cannot replicate what Debian has accomplished; they can rarely hope to even maintain what Debian has produced.
At the time that this paper was written, Distrowatch lists 129 distributions based on Debian — most of them are currently active to varying degrees. Each distribution represents at least one person — and in most cases a community of people — who disagreed with Debian's vision or direction strongly enough to want to create a new distribution and who had the technical capacity to follow through with this goal. Despite Debian's long-standing slogan — "the universal operating system" — the fact that the Debian project has become the fastest growing operating system while spawning so many derivatives is testament to the fact that, as far as software is concerned, one size can not fit all.
Organizationally, Debian derivers are located both inside and outside of the Debian project. A group of derivers working within the Debian project has labeled themselves "Custom Debian Distributions" and has created nearly a dozen projects customizing and deriving from Debian for specific groups of users including non-profit organization, the medical community, lawyers, children and many others. These projects build on the core Debian distribution and the canonical archive from within the organizational and political limits of the Debian project and constantly seek to minimize the delta by focusing on less invasive changes and by advancing creative ways of building the ability to alter the core Debian code base through established and policy compliant procedures.
A second group of Debian customizers includes those working outside of the Debian project organizationally. Notable among this list are (in alphabetical order) Knoppix, Libranet, Linspire (formerly Lindows), Progeny, MEPIS, Ubuntu, Userlinux, and Xandros. With its strong technological base, excellent package management, wide selection of packages to choose from, and strong commitment to software freedom which ensures derivability, Debian provides an ideal point from which to create a GNU/Linux distribution.
The Ubuntu project was started by Mark Shuttleworth in April 2004 and the first version was built almost entirely by a small group of a Debian developers employed by Shuttleworth's company Canonical Limited. It was released to the world in late 2004. The second version was released six months later in April 2005. The goals of Ubuntu are to provide a distribution based on a subset of Debian with:
Regular and predictable releases — every six months with support for eighteen months.
An emphasis on free software that will maintain the derivability of the distribution.
An emphasis on usability and a consistent desktop vision. As an example, this has translated into less questions in the installer and a default selection and configuration of packages that is usable for most desktop users "out of the box."
The Ubuntu project provides an interesting example of a project that aims to derive from Debian to an extensive degree. Ubuntu made code-level changes to nearly 1300 packages in Debian at the time that this paper was written and the speed of changes will not decelerate with time; the total number of changes and the total size of the delta will grow. The changes that Ubuntu makes are primarily of the most intrusive kind — changes to the code itself.
That said, the Ubuntu project is explicit about the fact that it could not exist without the work done by the Debian project. More importantly, Ubuntu explains that it cannot continue to provide the complete set of packages that its users depend on without the ongoing work by the Debian project. Even though Ubuntu has made changes to the nearly 1300 packages, this is less than ten percent of the total packages shipped in Ubuntu and pulled from Debian.
Scott James Remnant, a prominent Debian developer and a hacker on Ubuntu who works for Canonical Ltd., described the situation this way on his web log to introduce the Ubuntu development methodology in the week after the first public announcement of Canonical and Ubuntu:
I don't think Ubuntu is a "fork" of Debian, at least not in the traditional sense. A fork suggests that at some point we go our separate way from Debian and then occasionally merge in changes as we carry on down our own path.
Our model is quite different; every six months we take a snapshot of Debian's unstable distribution, apply any outstanding patches from our last release to it and spend a couple of months testing and bug-fixing it.
One thing that should be obvious from this is that our job is a lot easier if Debian takes all of our changes. The model actually encourages us to give back to Debian.
That's why from the very first day we started fixing bugs we began sending the patches back to Debian through the BTS. Not only will it make our job so much easier when we come to freeze for "hoary", our next release, but it's exactly what every derivative should do in the first place.
There is some debate on the degree to which Ubuntu developers have succeeded in accomplishing the goals laid out by Remnant. Ubuntu has filed hundreds of patches in the bug tracking system but it has also run into problems in deciding what constitutes something that should be fed back to Debian. Many changes are simply not relevant to Debian developers. For example, they may include changes to a package in response to another change made in another package in Ubuntu that will not or has not been taken by Debian. In many other cases, the best action in regards to a particular change, a particular package, and a particular upstream Debian developer is simply unclear.
The Ubuntu project's track record in working constructively with Debian is, at the moment, a mixed one. While an increasingly large number of Debian developers are maintaining their packages actively within both projects, many in both Debian and Ubuntu feel that Ubuntu has work left to do in living up to its own goal of a completely smooth productive relationship with Debian.
That said, the importance of the goals described by Remnant in the context of of the Ubuntu development model cannot be overstated. Every line of delta between Debian and Ubuntu has a cost for Ubuntu developers. Technology, social practices, and wise choices may reduce that cost but it cannot eliminate it. The resources that Ubuntu can bring to bear upon the problem of building a distribution are limited — far more limited than Debian's. As a result, there is a limit to how far Ubuntu can diverge; it is always in Ubuntu's advantage to minimize the delta where possible.
Ubuntu and Debian are distributions and — as such — operate on a different scale than the vast majority of free software projects. They include more code and more people. As a result, there are questions as to whether the experiences and lessons learned from these projects are particularly applicable to the experience of smaller free software projects.
Clearly, because of the difficulties associated with forking massive amount of code and the problems associated with duplicating the work of large volunteer bases, distributions are forced into finding a way to balance the benefits and drawbacks of forking. However, while the need is stronger and more immediate in larger projects, the benefits of their solutions will often be fully transferable.
Clearly, modifiability of free software to better fit the needs of its users lies at the heart of the free software movement's success. However, while modification usually comes in the form of collaboration on a single code-base, this is a function of limitations in software development methodologies and tools rather than the best response to the needs or desires of users or developers.
I believe that the fundamental advantage of free software in the next decade will be in the growing ability of any single free software project to be multiple things to multiple users simultaneously. This will translate into the fact that, in the next ten years, technology and social processes will evolve, so that forking is increasingly less of a bad thing. Free software development methodology will become less dependent on a single project and begin to emphasize parallel development within an ecosystem of related projects. The result is that free software projects will gain a competitive advantage over propriety software projects through their ability to better serve the increasingly diverse needs of increasingly large and increasingly diverse user-bases. Although it sounds paradoxical today, more projects will derive and less redundant code will be written.
Projects more limited in code and scope may use the tools and methods described in the remainder of this paper in different combinations, in different ways, and to different degrees than the examples around distributions introduced here. Different projects with different needs will find that certain solutions work better than others. Because communities of the size of Debian are difficult to fork in a way that is beneficial to any party, it is in these communities that the technology and development methodologies are first emerging. With time, these strategies and tools will find themselves employed productively in a wide variety of projects with a broad spectrum of sizes, needs, scopes and descriptions.
The easiest step in creating a productive derivative software project is to break down the problems of derivations into a series of different classes of modification. Certain types of modification are more easily done and are intrinsically more maintainable.
In the context of distributions, the problem of derivation can be broken down into the following types of changes (sorted roughly according to the intrusiveness inherent in solving the problem and the severity of the long-term maintainability problems that they introduce):
Selection of individual pieces of software;
Changes to the way that packages are installed or run (e.g., in a Live CD type environment or using a different installer);
Configuration of different pieces of software;
Changes made to the actual software package (made on the level of changes to the packages code);
By breaking down the problem in this way, Debian derivers have been able to approach derivation in ways that focus energy on the less intrusive problems first.
The first area that Ubuntu focused on was selecting a subset of packages that Ubuntu would support. Ubuntu selected and supports approximate 2,000 packages. These became the main component in Ubuntu. Other packages in Debian were included in a separate section of the Ubuntu archive called universe but were not guaranteed to be supported with bug or security fixes. By focusing on a small subset of packages, the Ubuntu team was able to select a maintainable subsection of the Debian archive that they could maintain over time.
The most simple derived distributions — often working within the Debian project as CDDs but also including projects like Userlinux — are merely lists of packages and do nothing outside of package selection. The installation of lists of packages and the maintenance of those lists over time can be aided through the creation of what are called metapackages: empty packages with long lists of "dependencies."
The second item, configuration changes, is also relatively low-impact. Focusing on moving as many changes as possible into the realm of configuration changes is a sustainable strategy that derivers working within the Debian project intent on a single code-base have pursued actively. Their idea is that rather than forking a piece of code due to disagreement in how the program should work, they can leave the code intact but add the ability to work in a different way to the software. This alternate functionality is made toggleable through a configuration change in the same manner that applications are configured through questions asked at install time. Since the Debian project has a unified package configuration framework called Debconf, derivers are able to configure an entire system in a highly centralized manner. This is not unlike RedHat's Kickstart although the emphasis is on maintenance of those configuration changes over the life and evolution of the package; Kickstart is focused merely on installation of the package.
A third type of configuration is limited to changes in the environment through which a system is run or installed. One is example is Progeny's Anaconda-based Debian installer which provides an alternate installer but results in an identical system. Another example is the Knoppix project which is famous for its "Live CD" environments. While, Knoppix makes a wide range of invasive changes that span all items in my list above, other Live CD projects, including Ubuntu's "Casper" project, are much closer to an alternate shell through which the same code is run.
Because these three methods are relatively non-invasive, they are reasonable strategies for small teams and individuals working on creating a derived distribution. However, many desirable changes — and in the case of some derived distributions, most desirable changes — require more invasive techniques. The final and most invasive type of change — changes to code — is the most difficult but also the most promising and powerful if it can be done sustainably. Changes of this type involve bifurcations of the code-base and will be the topic of the remainder of this paper.
One promising method of maintaining deltas in forked or branched projects lies in distributed version control systems (VCS). Traditional VCS systems work in a highly centralized fashion. CVS, the archetypal free software VCS and the basis for many others, is based around the model of a single centralized server. Anyone who wishes to commit to a project must commit to the centralized repository. While CVS allows users to create branches, anyone with commit rights has access to the entire repository. The tools for branching and merging over time are not particularly good.
The branching model is primarily geared toward a system where development is bifurcated and then the branch is merged completely back into the main tree. Normal use of a branch might include creating a development branch, making a series of development releases while maintaining and fixing important bugs in the stable primary branch, and then ultimately replacing the stable release with the development release. The CVS model is not geared toward a system where an arbitrary delta, or sets of deltas, are maintained over time.
Distributed version control aims to solve a number of problems introduced by CVS and alluded to above by:
Allowing people to work disconnected from each other and to sync with each other, in whole or in part, in an arbitrary and ad-hoc fashion.
Allowing deltas to be maintained over time.
Ultimately, this requires tools that are better at merging changes and in not merging certain changes when that is the desired behavior. It also leads to tools capable of history-sensitive merging.
The most famous switch to a distributed VCS model from a centralized VCS model was the move by the Linux kernel development community to the proprietary distributed version control system BitKeeper. In his recent announcement of the decision to part ways with BitKeeper, Linus Torvalds said:
In fact, one impact BK has had is to very fundamentally make us (and me in particular) change how we do things. That ranges from the fine-grained changeset tracking to just how I ended up trusting sub-maintainers with much bigger things, and not having to work on a patch-by-patch basis any more.
At the time of the switch, free distributed version control tools were less advanced than they are today. At the moment, an incomplete list of free software VCS tools includes GNU Arch, Bazaar, Bazaar-NG, Darcs, Monotone, SVK (based on Subversion), GIT (a system developed by Linus Torvalds as a replacement for BitKeeper) and others.
Each of these tools, at least after they reach a certain level of maturity, allow or will allow users to develop software in a distributed fashion and to, over time, compare their software and pull changes from others significantly more easily than they could otherwise. The idea of parallel development lies at the heart of the model. The tools for merging and resolving conflicts over time, and the ability to "cherry pick" certain patches or changes from a parallel developer each make this type of development significantly more useful than it has been in the past.
VCSs work entirely on the level of code. Due to the nature of the types of changes that Ubuntu project is making to Debian's code, Ubuntu has focused primarily on this model and Canonical currently funds two major distributed control products — the Bazaar and Bazaar-NG projects.
In many ways, employing distributed version control effectively is a much easier problem to solve for small, more traditional, free software development projects than it is for GNU/Linux distributions. Because the problems associated with maintaining parallel development of a single piece of software in a set of related distributed repositories is the primary use case for distributed version control systems, distributed VCS alone can be a technical solution for certain types of parallel development. As the tools and social processes for distributed VCS evolve, they will become increasingly important tools in the way that free software is developed.
Because the problems of scale associated with building an entire derivative distribution are more complicated than those associated with working with a single "upstream" project, distributed version control is only now being actively deployed in the Ubuntu project. In doing so, the project is focusing on integrating these into problem specific tools built on top of distributed version control.
Another technique that Canonical Ltd. is experimenting with is the creation of high level tools built on top of distributed version control tools specifically designed for maintaining difference between packages. Because packages are usually distributed as a source file with a collection of one or more patches, this introduces the unique possibility of creating a high-level VCS system based around this fact.
In the case of Ubuntu and Debian, the ideal tool creates one branch per patch or feature and uses heuristics to analyze patch files and create these branches intelligently. The package build system section of the total patch can also be kept as a separate branch. Canonical's tool, called the Hypothetical Changeset Tool (HCT) (although no longer hypothetical), is one experimental way of creating a very simple, very streamlined interface for dealing with a particular type of source that is created and distributed in a particular type of way with a particular type of change.
While HCT promises to be very useful for people making derived distributions based on Debian, its application outside distribution makers will, in all likelihood, be limited. That said, it provides an example of the way that problem and context specific tools may play an essential role in the maintenance of derived code more generally.
It has been said that it is a common folly of a technophile to attempt to employ technical solutions toward solving social problems. The problem of deriving software is both a technical and social problem and adequately addressing the larger problems requires approaches that take into consideration both types of solution.
Scott James Remnant compares the relationship between distributions and derived distributions as similar to the relationship between distributions and upstream maintainers:
I don't think this is much different from how Debian maintainers interact with their upstreams. As Debian maintainers we take and package upstream software and then act as a gateway for bugs and problems. Quite often we fix bugs ourselves and apply the patch to the package and send it upstream. Sometimes the upstream don't incorporate that patch and we have to make sure we don't accidentally drop it each subsequent release, we much prefer it if they take them, but we don't get angry if they don't.
This is how I see the relationship between Ubuntu and Debian, we're no more a fork of Debian than a Debian package is a fork of its upstream.
Scott alludes the fact that, at least in the world of distributions, parallel development is already one way to view the modus operandi of existing GNU/Linux distributions. The relationship between a deriver and derivee on the distribution level mirrors the relationship between the distribution and the "upstream" authors of the packages that make up the distribution. These relationships are rarely based around technological tools but are entirely in the realm of social solutions.
Ubuntu has pursued a number of different initiatives along these lines. The first of these has been to regularly file bugs in the Debian bug tracking system when bugs that exist in Debian are fixed in Ubuntu. While this can be partially automated, the choice to automate this and the manner in which it it is set up is a purely social one.
However, as I alluded to above, Ubuntu is still left with questions in regards to changes that are made to packages that do not necessarily fix bugs or that fix bugs that do not exist in Debian but may in the future. Some Debian developers want to hear about the full extent of changes made to their software in Ubuntu while others do not want to be bothered. Ubuntu should continue to work with Debian to find ways to allow developers to stay in sync.
There are also several initiatives by developers in Debian, Ubuntu, and in other derivations to create a stronger relationship between the Debian project and its ecosystem of derivers and between Ubuntu and Debian in particular. While the form that this will ultimately take is unclear, projects existing within an ecosystem should explore the realm of appropriate social relationships that will ensure that they can work together and be informed of each others' work without resorting to "spamming" each other with irrelevant or unnecessary information.
Another issue that has recently played an important role in the Debian/Ubuntu relationship is the importance of both giving adequate credit to the authors or upstream maintainers of software without implying a closer relationship than is the case. Derivers must walk a file line where they credit others' work on a project without implying that the others work for, support, or are connected to the derivers project to which, for any number of reasons, the "upstream" author might not want to be associated.
In the case of Debian and Ubuntu, this has resulted in an emphasis on keeping or importing changelog entries when changes are imported and in noting the pedigree of changes more generally. It has recently also been discussed in terms of the "maintainer" field in each package in Ubuntu. Ubuntu wants to avoid making changes to every unmodified source package (and introducing an unnecessary delta) but does not want to give the impression that the maintainer of the package is someone unassociated with Ubuntu. While no solution has been decided at the time of writing, one idea involved marking the maintainer of the package explicitly as a Debian maintainer at the time that the binary packages are built on the Ubuntu build machines.
The emphasis on social solutions is also essential when using distributed VCS technology. As Linus Torvalds alluded to in the quote above, the importance of technological changes to distributed VCS technology is only felt when people begin to work in a different way — when they begin to employ different social models of developer interaction.
While Ubuntu's experience can provide a good model for tackling some of these source control issues, it can only serve as a model and not as a fixed answer. Social solutions must be appropriate for a given social relationship. Even in situations where a package is branched because of social disagreements, a certain level of collaboration on a social level will be essential to the long term viability of the derivative.
As the techniques described in this paper evolve, the role that they play in free software development becomes increasingly prominent and increasingly important. Joining them will be other techniques and models that I have not described and cannot predict. Because of the size and usefulness of their code and the size of their development communities, large projects like Debian and Ubuntu have been forced into confronting and attempting to mediate the problems inherent in forking and deriving. However, as these problems are negotiated and tools and processes are advanced toward solutions, free software projects of all sizes will be able to offer users exactly what they want with minimal redundancy and little duplication of work. In doing this, free software will harness a power that proprietary models cannot compete with. They will increase their capacity to produce better products and better processes. Ultimately, it will help free software capture more users, bring in more developers, and produce more free software of a higher quality.
 Information is listed on the distrowatch homepage here: http://distrowatch.com/dwres.php?resource=independence
 Netcraft posts yearly updates on the speed at which Linux distributions are growing. The one in question can be found at: http://news.netcraft.com/archives/2004/01/28/debian_fastest_growing_linux_distribution.html
 I spearheaded and help build a now mostly defunct derivation of Debian called Debian-Nonprofit (Debian-NP) geared for non-profit organizations by working within the Debian project.
 The entire post can be read here: http://www.netsplit.com/blog/work/canonical/ubuntu_and_debian.html
 The full message can be read online at: http://kerneltrap.org/mailarchive/1/message/48393/thread