[to_fork_or_not] / to_fork_or_not_to_fork.xml

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<article id="paper-11194">
  <articleinfo>
    <title>To Fork or Not To Fork</title>
    <subtitle>Lessons From Ubuntu and Debian</subtitle>
    <author>
      <firstname>Benjamin</firstname>
      <othername>Mako</othername>
      <surname>Hill</surname>
      <affiliation>
        <orgname>Canonical Limited</orgname>
      </affiliation>
      <affiliation>
        <orgname>The Debian GNU/Linux Project</orgname>
      </affiliation>
      <affiliation>
        <orgname>Software in the Public Interest, Inc.</orgname>
      </affiliation>

      <authorblurb>
        <para>Benjamin Mako Hill is an intellectual property
          researcher and activist and a professional Free/Open Source
          Software (FOSS) advocate and developer. He is active
          participant in the Debian Project in both technical and
          non-technical roles. He is the author of the Free Software
          Project Management HOWTO and many published works on Free
          and Open Source Software. He currently is working full time
          for Canonical Ltd. on Ubuntu, a new Debian-based
          distribution.</para>
      </authorblurb>

    </author>

    <copyright>
      <year>2005</year>
      <holder>Benjamin Mako Hill</holder>
    </copyright>


    <legalnotice>
      <para>This material is licensed under the <ulink
          url="http://creativecommons.org/licenses/by-sa/2.0/">Creative 
          Commons Attribution-Sharealike 2.0 License</ulink>.</para>

      <para>The canonical location for the most recent version of this
        document is <ulink url="http://mako.cc/">at the author's
          website</ulink>.</para>

    </legalnotice>

    <revhistory>
      <revision>
        <revnumber>0.2</revnumber>
        <date>August 7, 2005</date>
        <revremark>Correction and improvements.</revremark>
      </revision>
      <revision>
        <revnumber>0.1</revnumber>
        <date>May 15, 2005</date>

        <revdescription>
          <para>The first version of this paper was written to an
            accepted talk given at Linuxtag 2005 given in Karlsruhe,
            Germany.</para>
        </revdescription>

      </revision>
    </revhistory>


  </articleinfo>

  <section>
    <title>Introduction</title>
  
    <para>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 &mdash; like GNOME
      and KDE &mdash; and distributions like Debian, RedHat, and
      Gentoo.</para>

    <para>These projects are leveraging the work of thousands of
      programmers &mdash; both volunteer and paid &mdash; 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:</para>

    <itemizedlist>
      <listitem>

        <para>The communities that free software projects &mdash; and
          in particular large projects &mdash; 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.</para>

      </listitem>
      <listitem>

        <para>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.</para>

      </listitem>
    </itemizedlist>

    <para>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.</para>

    <para>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.</para>

    <para>Software makers must pursue the idea of an
      <emphasis>ecosystem</emphasis> 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:</para>

    <itemizedlist>
      <listitem>
        <para>Break down the process of derivation into a set of
          different types of customization and derivation and
          prioritize methods of derivation.</para>
      </listitem>
      <listitem>
        <para>Create and foster social solutions to the social aspects
          of the derivation problem.</para>
      </listitem>
      <listitem>
        <para>Build and use new tools specifically designed to
          coordinate development of software in the context of an
          ecosystem of projects.</para>
      </listitem>
      <listitem>
        <para>Distribute and utilize distributed version control tools
          with an emphasis on maintaining differences over
          time.</para>
      </listitem>
    </itemizedlist>

    <para>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.</para>
  </section>

  <section>
    <title>"Fork" Is A Four Letter Word</title>

    <para>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.</para>

    <para>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 &mdash; EGCS and PGCC &mdash; 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.</para>

    <para>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.</para>

    <para>A fork occurs on the level of code but a fork is not merely
      &mdash; or even primarily &mdash; 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
      <emphasis>single</emphasis> developer or community of developers
      &mdash; even if it does boil down to distinct subgroups within a
      community &mdash; whereas forks are separate projects.</para>

    <para>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 <ulink
        url="http://mako.cc/projects/howto/">Free Software Project
        Management HOWTO</ulink> 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:</para>

    <blockquote>
      <para>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.</para>
    </blockquote>

    <para>In the <emphasis>best</emphasis> situations, a fork means
      that two groups of people need to go on developing features and
      doing work they would ordinarily do <emphasis>in addition
        to</emphasis> 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 &mdash; differently and
      incompatibly.</para>

    <para>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.</para>

    <para>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.</para>

  </section>

  <section>
    <title>Case Study</title>

    <para>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.</para>


    <section>
      <title>The Debian Project</title>

      <para>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.</para>

      <para>At the time that this paper was written, Distrowatch lists
        129 distributions based on Debian<footnote>
          <para>Information is listed on the distrowatch homepage
            here: <ulink
              url="http://distrowatch.com/dwres.php?resource=independence">http://distrowatch.com/dwres.php?resource=independence</ulink></para>

        </footnote> &mdash; most of them
        are currently active to varying degrees. Each distribution
        represents at least one person &mdash; and in most cases a
        community of people &mdash; who disagreed with Debian's vision
        or direction strongly enough to want to create a new
        distribution <emphasis>and</emphasis> who had the technical
        capacity to follow through with this goal. Despite Debian's
        long-standing slogan &mdash; "the universal operating system"
        &mdash; 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 <emphasis>can not</emphasis> fit all.<footnote>
        <para>Netcraft posts yearly updates on the speed at which
        Linux distributions are growing. The one in question can be
        found at: <ulink
        url="http://news.netcraft.com/archives/2004/01/28/debian_fastest_growing_linux_distribution.html">http://news.netcraft.com/archives/2004/01/28/debian_fastest_growing_linux_distribution.html</ulink></para>
        </footnote>
      </para>


      <para>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.<footnote>
          <para>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.</para>
        </footnote> These projects build on the core Debian distribution and
        the canonical archive from <emphasis>within</emphasis> 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 <emphasis>ability</emphasis> to alter the core
        Debian code base through established and policy compliant
        procedures.</para>

<!-- http://linktocddinformation -->

      <para>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.</para>

    </section>


    <section>
      <title>Ubuntu</title>

      <para>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.<footnote>
          <para>Information Ubuntu can be found on the <ulink
              url="http://www.ubuntu.com">Ubuntu homepage.</ulink>
            Information Canonical Limited can be found at <ulink
              url="http://www.canonical.com">Canonical's
              homepage</ulink>.</para>
        </footnote> 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:</para>

      <itemizedlist>
        <listitem>
          <para>Regular and predictable releases &mdash; every six months
            with support for eighteen months.</para>
        </listitem>
        <listitem>
          <para>An emphasis on free software that will maintain the
            derivability of the distribution.</para>
        </listitem>
        <listitem>
          <para>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."</para>
        </listitem>

      </itemizedlist>

      <para>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.<footnote>
          <para>Scott James Remnant maintains a list of these patches
            online here: <ulink
              url="http://people.ubuntu.com/~scott/patches/">http://people.ubuntu.com/~scott/patches/</ulink></para>
        </footnote> The changes that Ubuntu makes are primarily of the
        most intrusive kind &mdash; changes to the code itself.</para>

      <para>That said, the Ubuntu project is explicit about the fact
        that it could not exist without the work done by the Debian
        project.<footnote>
          <para>You can see that explicit statement on Ubuntu's
            website here: <ulink
              url="http://www.ubuntulinux.org/ubuntu/relationship/">http://www.ubuntulinux.org/ubuntu/relationship/</ulink></para>
        </footnote> 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.</para>

      <para>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:<footnote> <para>The
        entire post can be read here: <ulink
        url="http://www.netsplit.com/blog/work/canonical/ubuntu_and_debian.html">http://www.netsplit.com/blog/work/canonical/ubuntu_and_debian.html</ulink></para>
        </footnote>
      </para>

      <blockquote>

        <para>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.</para>

        <para>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.</para>


        <para>
          <inlinemediaobject>
            <imageobject>
              <imagedata fileref="tfontf-picture-01.png" format="PNG"/>
            </imageobject>
          </inlinemediaobject>
        </para>

        <para>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.</para>

        <para>That's why from the very first day we started fixing
          bugs we began sending <ulink
            url="http://www.no-name-yet.com/patches/">the
            patches</ulink> 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.</para>

      </blockquote>

      <para>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
        <emphasis>what</emphasis> 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.</para>

      <para>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.</para>

      <para>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 &mdash; 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.</para>

    </section>

    <section>
      <title>Applicability</title>

      <para>Ubuntu and Debian are distributions and &mdash; as such
        &mdash; 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.</para>

      <para>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.</para>

      <para>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.</para>

      <para>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.</para>
 
      <para>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.</para>

    </section>

  </section>

  <section>
    <title>Balancing Forking With Collaboration</title>

    <section>
      <title>Derivation and Problem Analysis</title>

      <para>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.</para>

      <para>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):</para>

      <orderedlist>
        <listitem>
          <para>Selection of individual pieces of software;</para>
        </listitem>
        <listitem>
          <para>Changes to the way that packages are installed or run
            (e.g., in a Live CD type environment or using a different
            installer);</para>
        </listitem>
        <listitem>
          <para>Configuration of different pieces of software;</para>
        </listitem>
        <listitem>
          <para>Changes made to the actual software package (made on
            the level of changes to the packages code);</para>
        </listitem>
      </orderedlist>

      <para>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.</para>

      <para>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
        <command>main</command> component in Ubuntu. Other packages in
        Debian were included in a separate section of the Ubuntu
        archive called <command>universe</command> 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.</para>

      <para>The most simple derived distributions &mdash; often
        working within the Debian project as CDDs but also including
        projects like Userlinux &mdash; 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
        <emphasis>metapackages</emphasis>: empty packages with long
        lists of "dependencies."</para>

      <para>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 <emphasis>ability</emphasis> 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.<footnote> <para>More information on
            Debconf can be
            found online at: <ulink
              url="http://www.kitenet.net/programs/debconf/">http://www.kitenet.net/programs/debconf/</ulink></para>
        </footnote> 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.</para>

      <para>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.</para>

      <para>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 &mdash; and in the case of some derived
        distributions, <emphasis>most</emphasis> desirable changes
        &mdash; require more invasive techniques. The final and most
        invasive type of change &mdash; changes to code &mdash; 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.</para>

    </section>

    <section>
      <title>Distributed Source Control</title>

      <para>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.</para>

      <para>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 <emphasis>not</emphasis> geared toward a system
        where an arbitrary delta, or sets of deltas, are maintained
        over time.</para>

      <para>Distributed version control aims to solve a number of
        problems introduced by CVS and alluded to above by:</para>

      <itemizedlist>
        <listitem>
          <para>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.</para>
        </listitem>
        <listitem>
          <para>Allowing deltas to be maintained over time.</para>
        </listitem>
      </itemizedlist>

      <para>Ultimately, this requires tools that are better at merging
        changes and in <emphasis>not</emphasis> merging certain
        changes when that is the desired behavior. It also leads to tools capable
        of history-sensitive merging.</para>

      <para>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:</para>

      <blockquote>
        <para>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.<footnote> <para>The full message can be read online
              at: <ulink
                url="http://kerneltrap.org/mailarchive/1/message/48393/thread">http://kerneltrap.org/mailarchive/1/message/48393/thread</ulink></para>
          </footnote>
        </para>
      </blockquote>

      <para>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.</para>

      <para>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.</para>

      <para>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 &mdash; the Bazaar and Bazaar-NG projects.</para>

      <para>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.</para>

      <para>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.</para>

    </section>

    <section>
      <title>Problem Specific Tools</title>

      <para>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.</para>

      <para>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.</para>

      <para>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.</para>

    </section>


    <section>
      <title>Social Solutions</title>

      <para>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 <emphasis>and</emphasis> social problem and
        adequately addressing the larger problems requires approaches that
        take into consideration both types of solution.</para>

      <para>Scott James Remnant compares the relationship between
        distributions and derived distributions as similar to the
        relationship between distributions and upstream
        maintainers:</para>
      <blockquote>

        <para>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.</para>

        <para>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.</para>
      </blockquote>

      <para>Scott alludes the fact that, at least in the world of
        distributions, parallel development is already one way to view
        the <emphasis>modus operandi</emphasis> 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.</para>

      <para>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.</para>

      <para>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.</para>

      <para>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.</para>

      <para>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.</para>

      <para>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.</para>

      <para>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 &mdash; when they begin to employ
        different social models of developer interaction.</para>

      <para>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.</para>

    </section>

  </section>

  <section>
    <title>Conclusions</title>

    <para>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.</para>

  </section>

</article>


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