<chapter id="model">
  <title>Model &mdash; The versioning model used by Subversion</title>

  <simplesect>
    <para>This chapter explains the user's view of Subversion &mdash; what
      &ldquo;objects&rdquo; you interact with, how they behave, and how they
      relate to each other.</para>
  </simplesect>

  <sect1 id="model.wc-and-repos">
    <title>Working Directories and Repositories</title>

    <para>Suppose you are using Subversion to manage a software project.  There
      are two things you will interact with: your working directory, and the
      repository.</para>

    <para>Your <firstterm>working directory</firstterm> is an ordinary
      directory tree, on your local system, containing your project's sources.
      You can edit these files and compile your program from them in the usual
      way.  Your working directory is your own private work area: Subversion
      never changes the files in your working directory, or publishes the
      changes you make there, until you explicitly tell it to do so.</para>

    <para>After you've made some changes to the files in your working
      directory, and verified that they work properly, Subversion provides
      commands to publish your changes to the other people working with you on
      your project.  If they publish their own changes, Subversion provides
      commands to incorporate those changes into your working directory.</para>

    <para>A working directory contains some extra files, created and maintained
      by Subversion, to help it carry out these commands.  In particular, these
      files help Subversion recognize which files contain unpublished changes,
      and which files are out-of-date with respect to others' work.</para>

    <para>While your working directory is for your use alone, the
      <firstterm>repository</firstterm> is the common public record you share
      with everyone else working on the project.  To publish your changes, you
      use Subversion to put them in the repository.  (What this means, exactly,
      we explain below.)  Once your changes are in the repository, others can
      tell Subversion to incorporate your changes into their working
      directories.  In a collaborative environment like this, each user will
      typically have their own working directory (or perhaps more than one),
      and all the working directories will be backed by a single repository,
      shared amongst all the users.</para>

    <para>A Subversion repository holds a single directory tree, and records
      the history of changes to that tree.  The repository retains enough
      information to recreate any prior state of the tree, compute the
      differences between any two prior trees, and report the relations between
      files in the tree &mdash; which files are derived from which other
      files.</para>

    <para>A Subversion repository can hold the source code for several
      projects; usually, each project is a subdirectory in the tree.  In this
      arrangement, a working directory will usually correspond to a particular
      subtree of the repository.</para>

    <para>For example, suppose you have a repository laid out like this:</para>

    <programlisting>
/trunk/paint/Makefile
             canvas.c
             brush.c
       write/Makefile
             document.c
             search.c
</programlisting>

    <para>In other words, the repository's root directory has a single
      subdirectory named <filename>trunk</filename>, which itself contains two
      subdirectories: <filename>paint</filename> and
      <filename>write</filename>.</para>

    <para>To get a working directory, you must <firstterm>check out</firstterm>
      some subtree of the repository.  If you check out
      <filename>/trunk/write</filename>, you will get a working directory like
      this:</para>

    <programlisting>
write/Makefile
      document.c
      search.c
      .svn/
</programlisting>

    <para>This working directory is a copy of the repository's
      <filename>/trunk/write</filename> directory, with one additional entry
      &mdash; <filename>.svn</filename> &mdash; which holds the extra
      information needed by Subversion, as mentioned above.</para>

    <para>Suppose you make changes to <filename>search.c</filename>.  Since the
      <filename>.svn</filename> directory remembers the file's modification
      date and original contents, Subversion can tell that you've changed the
      file.  However, Subversion does not make your changes public until you
      explicitly tell it to.</para>

    <para>To publish your changes, you can use Subversion's
      &lsquo;<literal>commit</literal>&rsquo; command:</para>

    <programlisting>
$ pwd
/home/jimb/write
$ ls -a
.svn/   Makefile   document.c    search.c
$ svn commit search.c
$
</programlisting>

    <para>Now your changes to <filename>search.c</filename> have been committed
      to the repository; if another user checks out a working copy of
      <filename>/trunk/write</filename>, they will see your text.</para>

    <para>Suppose you have a collaborator, Felix, who checked out a working
      directory of <filename>/trunk/write</filename> at the same time you did.
      When you commit your change to <filename>search.c</filename>, Felix's
      working copy is left unchanged; Subversion only modifies working
      directories at the user's request.</para>

    <para>To bring his working directory up to date, Felix can use the
      Subversion &lsquo;<literal>update</literal>&rsquo; command.  This will
      incorporate your changes into his working directory, as well as any
      others that have been committed since he checked it out.</para>

    <programlisting>
$ pwd
/home/felix/write
$ ls -a
.svn/    Makefile    document.c    search.c
$ svn update
U search.c
$
</programlisting>

    <para>The output from the &lsquo;<literal>svn update</literal>&rsquo;
      command indicates that Subversion updated the contents of
      <filename>search.c</filename>.  Note that Felix didn't need to specify
      which files to update; Subversion uses the information in the
      <filename>.svn</filename> directory, and further information in the
      repository, to decide which files need to be brought up to date.</para>

    <para>We explain below what happens when both you and Felix make changes to
      the same file.</para>
  </sect1>

  <sect1 id="model.txns-and-revnums">
    <title>Transactions and Revision Numbers</title>

    <para>A Subversion &lsquo;<literal>commit</literal>&rsquo; operation can
      publish changes to any number of files and directories as a single atomic
      transaction.  In your working directory, you can change files' contents,
      create, delete, rename and copy files and directories, and then commit
      the completed set of changes as a unit.</para>

    <para>In the repository, each commit is treated as an atomic transaction:
      either all the commit's changes take place, or none of them take place.
      Subversion tries to retain this atomicity in the face of program crashes,
      system crashes, network problems, and other users' actions.  We may call
      a commit a <firstterm>transaction</firstterm> when we want to emphasize
      its indivisible nature.</para>

    <para>Each time the repository accepts a transaction, this creates a new
      state of the tree, called a <firstterm>revision</firstterm>.  Each
      revision is assigned a unique natural number, one greater than the number
      of the previous revision.  The initial revision of a freshly created
      repository is numbered zero, and consists of an empty root
      directory.</para>

    <para>Since each transaction creates a new revision, with its own number,
      we can also use these numbers to refer to transactions; transaction
      <replaceable>n</replaceable> is the transaction which created revision
      <replaceable>n</replaceable>.  There is no transaction numbered
      zero.</para>

    <para>Unlike those of many other systems, Subversion's revision numbers
      apply to an entire tree, not individual files.  Each revision number
      selects an entire tree.</para>

    <para>It's important to note that working directories do not always
      correspond to any single revision in the repository; they may contain
      files from several different revisions.  For example, suppose you check
      out a working directory from a repository whose most recent revision is
      4:</para>

    <programlisting>
write/Makefile:4
      document.c:4
      search.c:4
</programlisting>

    <para>At the moment, this working directory corresponds exactly to revision
      4 in the repository.  However, suppose you make a change to
      <filename>search.c</filename>, and commit that change.  Assuming no other
      commits have taken place, your commit will create revision 5 of the
      repository, and your working directory will look like this:</para>

    <programlisting>
write/Makefile:4
      document.c:4
      search.c:5
</programlisting>

    <para>Suppose that, at this point, Felix commits a change to
      <filename>document.c</filename>, creating revision 6.  If you use
      &lsquo;<literal>svn update</literal>&rsquo; to bring your working
      directory up to date, then it will look like this:</para>

    <programlisting>
write/Makefile:6
      document.c:6
      search.c:6
</programlisting>

    <para>Felix's changes to <filename>document.c</filename> will appear in
      your working copy of that file, and your change will still be present in
      <filename>search.c</filename>.  In this example, the text of
      <filename>Makefile</filename> is identical in revisions 4, 5, and 6, but
      Subversion will mark your working copy with revision 6 to indicate that
      it is still current.  So, after you do a clean update at the root of your
      working directory, your working directory will generally correspond
      exactly to some revision in the repository.</para>
  </sect1>

  <sect1 id="model.how-wc">
    <title>How Working Directories Track the Repository</title>

    <para>For each file in a working directory, Subversion records two
      essential pieces of information:</para>

    <itemizedlist mark="bullet">
      <listitem><para>what revision of what repository file your working copy
          is based on (this is called the file's <firstterm>base
            revision</firstterm>), and</para></listitem>
      <listitem><para>a timestamp recording when the local copy was last
          updated.</para></listitem>
    </itemizedlist>

    <para>Given this information, by talking to the repository, Subversion can
      tell which of the following four states a file is in:</para>

    <itemizedlist mark="bullet">
      <listitem><para><emphasis role="bold">Unchanged, and current.</emphasis>
          The file is unchanged in the workingdirectory, and no changes to that
          file have been committed to therepository since its base
          revision.</para></listitem>
      <listitem><para><emphasis role="bold">Locally changed, and
            current</emphasis>.  The file has been changed in theworking
          directory, and no changes to that file have been committed tothe
          repository since its base revision.  There are local changes thathave
          not been committed to the repository.</para></listitem>
      <listitem><para><emphasis role="bold">Unchanged, and
            out-of-date</emphasis>.  The file has not been changed in
          theworking directory, but it has been changed in the repository.  The
          fileshould eventually be updated, to make it current with the
          publicrevision.</para></listitem>
      <listitem><para><emphasis role="bold">Locally changed, and
            out-of-date</emphasis>.  The file has been changed bothin the
          working directory, and in the repository.  The file should beupdated;
          Subversion will attempt to merge the public changes with thelocal
          changes.  If it can't complete the merge in a plausible
          wayautomatically, Subversion leaves it to the user to resolve the
          conflict.</para></listitem>
    </itemizedlist>
  </sect1>

  <sect1 id="model.nolock">
    <title>Subversion Does Not Lock Files</title>

    <para>Subversion does not prevent two users from making changes to the same
      file at the same time.  For example, if both you and Felix have checked
      out working directories of <filename>/trunk/write</filename>, Subversion
      will allow both of you to change <filename>write/search.c</filename> in
      your working directories.  Then, the following sequence of events will
      occur:</para>

    <itemizedlist mark="bullet">
      <listitem><para>Suppose Felix tries to commit his changes to
          <filename>search.c</filename> first.  Hiscommit will succeed, and his
          text will appear in the latest revision inthe
          repository.</para></listitem>
      <listitem><para>When you attempt to commit your changes to
          <filename>search.c</filename>, Subversionwill reject your commit, and
          tell you that you must update<filename>search.c</filename> before you
          can commit it.</para></listitem>
      <listitem><para>When you update <filename>search.c</filename>, Subversion
          will try to merge Felix'schanges from the repository with your local
          changes.  By default,Subversion merges as if it were applying a
          patch: if your local changesdo not overlap textually with Felix's,
          then all is well; otherwise,Subversion leaves it to you to resolve
          the overlappingchanges.  In either case,Subversion carefully
          preserves a copy of the original pre-merge text.</para></listitem>
      <listitem><para>Once you have verified that Felix's changes and your
          changes have beenmerged correctly, you can commit the new revision of
          <filename>search.c</filename>,which now contains everyone's
          changes.</para></listitem>
    </itemizedlist>

    <para>Some version control systems provide &ldquo;locks&rdquo;, which
      prevent others from changing a file once one person has begun working on
      it.  In our experience, merging is preferable to locks, because:</para>

    <itemizedlist mark="bullet">
      <listitem><para>changes usually do not conflict, so Subversion's behavior
          does the rightthing by default, while locking can interfere with
          legitimate work;</para></listitem>
      <listitem><para>locking can prevent conflicts within a file, but not
          conflicts betweenfiles (say, between a C header file and another file
          that includes it),so it doesn't really solve the problem; and
          finally,</para></listitem>
      <listitem><para>people often forget that they are holding locks,
          resulting inunnecessary delays and friction.</para></listitem>
    </itemizedlist>

    <para>Of course, the merge process needs to be under the users' control.
      Patch is not appropriate for files with rigid formats, like images or
      executables.  Subversion allows users to customize its merging behavior
      on a per-file basis.  You can direct Subversion to refuse to merge
      changes to certain files, and simply present you with the two original
      texts to choose from.  (Or, someday, you can direct Subversion to merge
      using a tool which respects the semantics of the file format.)</para>
  </sect1>

  <sect1 id="model.props">
    <title>Properties</title>

    <para>Files generally have interesting attributes beyond their contents:
      mime-types, executable permissions, EOL styles, and so on.  Subversion
      attempts to preserve these attributes, or at least record them, when
      doing so would be meaningful.  However, different operating systems
      support very different sets of file attributes: Windows NT supports
      access control lists, while Linux provides only the simpler traditional
      Unix permission bits.</para>

    <para>In order to interoperate well with clients on many different
      operating systems, Subversion supports <firstterm>property
        lists</firstterm>, a simple, general-purpose mechanism which clients
      can use to store arbitrary out-of-band information about files.</para>

    <para>A property list is a set of name / value pairs.  A property name is
      an arbitrary text string, expressed as a Unicode UTF-8 string,
      canonically decomposed and ordered.  A property value is an arbitrary
      string of bytes.  Property values may be of any size, but Subversion may
      not handle very large property values efficiently.  No two properties in
      a given a property list may have the same name.  Although the word `list'
      usually denotes an ordered sequence, there is no fixed order to the
      properties in a property list; the term `property list' is
      historical.</para>

    <para>Each revision number, file, directory, and directory entry in the
      Subversion repository, has its own property list.  Subversion puts these
      property lists to several uses:</para>

    <itemizedlist mark="bullet">
      <listitem><para>Clients can use properties to store file attributes, as
          described above.</para></listitem>
      <listitem><para>The Subversion server uses properties to hold attributes
          of its own,and allow clients to read and modify them.  For example,
          someday ahypothetical &lsquo;<literal>svn-acl</literal>&rsquo;
          property might hold an access control listwhich the Subversion server
          uses to regulate access to repositoryfiles.</para></listitem>
      <listitem><para>Users can invent properties of their own, to store
          arbitrary informationfor use by scripts, build environments, and so
          on.  Names of userproperties should be URI's, to avoid conflicts
          between organizations.</para></listitem>
    </itemizedlist>

    <para>Property lists are versioned, just like file contents.  You can
      change properties in your working directory, but those changes are not
      visible in the repository until you commit your local changes.  If you do
      commit a change to a property value, other users will see your change
      when they update their working directories.</para>
  </sect1>

  <sect1 id="model.merging-and-ancestry">
    <title>Merging and Ancestry</title>

    <para>[WARNING:  this section was written in May 2000, at the very
      beginning of the Subversion project.  This functionality probably will
      not exist in Subversion 1.0, but it's planned for post-1.0.  The problem
      should be reasonably solvable by recording merge data in
      'properties'.]</para>

    <para>Subversion defines merges the same way CVS does: to merge means to
      take a set of previously committed changes and apply them, as a patch, to
      a working copy.  This change can then be committed, like any other
      change.  (In Subversion's case, the patch may include changes to
      directory trees, not just file contents.)</para>

    <para>As defined thus far, merging is equivalent to hand-editing the
      working copy into the same state as would result from the patch
      application.  In fact, in CVS there <emphasis>is</emphasis> no difference
      &ndash; it is equivalent to just editing the files, and there is no
      record of which ancestors these particular changes came from.
      Unfortunately, this leads to conflicts when users unintentionally merge
      the same changes again.  (Experienced CVS users avoid this problem by
      using branch- and merge-point tags, but that involves a lot of unwieldy
      bookkeeping.)</para>

    <para>In Subversion, merges are remembered by recording <firstterm>ancestry
        sets</firstterm>.  A revision's ancestry set is the set of all changes
      "accounted for" in that revision.  By maintaining ancestry sets, and
      consulting them when doing merges, Subversion can detect when it would
      apply the same patch twice, and spare users much bookkeeping.  Ancestry
      sets are stored as properties.</para>

    <para>In the examples below, bear in mind that revision numbers usually
      refer to changes, rather than the full contents of that revision.  For
      example, "the change A:4" means "the delta that resulted in A:4", not
      "the full contents of A:4".</para>

    <para>The simplest ancestor sets are associated with linear histories.  For
      example, here's the history of a file A:</para>

    <programlisting><![CDATA[

 _____        _____        _____        _____        _____
|     |      |     |      |     |      |     |      |     |
| A:1 |----->| A:2 |----->| A:3 |----->| A:4 |----->| A:5 |
|_____|      |_____|      |_____|      |_____|      |_____|

]]></programlisting>

    <para>The ancestor set of A:5 is:</para>

    <programlisting>

  { A:1, A:2, A:3, A:4, A:5 }

</programlisting>

    <para>That is, it includes the change that brought A from nothing to A:1,
      the change from A:1 to A:2, and so on to A:5.  From now on, ranges like
      this will be represented with a more compact notation:</para>

    <programlisting>

  { A:1-5 }

</programlisting>

    <para>Now assume there's a branch B based, or "rooted", at A:2.  (This
      postulates an entirely different revision history, of course, and the
      global revision numbers in the diagrams will change to reflect it.)
      Here's what the project looks like with the branch:</para>

    <programlisting><![CDATA[

 _____        _____        _____        _____        _____        _____
|     |      |     |      |     |      |     |      |     |      |     |
| A:1 |----->| A:2 |----->| A:4 |----->| A:6 |----->| A:8 |----->| A:9 |
|_____|      |_____|      |_____|      |_____|      |_____|      |_____|
                \
                 \
                  \  _____        _____        _____
                   \|     |      |     |      |     |
                    | B:3 |----->| B:5 |----->| B:7 |
                    |_____|      |_____|      |_____|

]]></programlisting>

    <para>If we produce A:9 by merging the B branch back into the
      trunk</para>

    <programlisting><![CDATA[

 _____        _____        _____        _____        _____        _____
|     |      |     |      |     |      |     |      |     |      |     |
| A:1 |----->| A:2 |----->| A:4 |----->| A:6 |----->| A:8 |---.->| A:9 |
|_____|      |_____|      |_____|      |_____|      |_____|  /   |_____|
                \                                            |
                 \                                           |
                  \  _____        _____        _____        /
                   \|     |      |     |      |     |      /
                    | B:3 |----->| B:5 |----->| B:7 |--->-'
                    |_____|      |_____|      |_____|

]]></programlisting>

    <para>then what will A:9's ancestor set be?</para>

    <programlisting>

  { A:1, A:2, A:4, A:6, A:8, A:9, B:3, B:5, B:7}

</programlisting>

    <para>or more compactly:</para>

    <programlisting>

  { A:1-9, B:3-7 }

</programlisting>

    <para>(It's all right that each file's ranges seem to include non-changes;
      this is just a notational convenience, and you can think of the
      non-changes as either not being included, or being included but being
      null deltas as far as that file is concerned).</para>

    <para>All changes along the B line are accounted for (changes B:3-7), and
      so are all changes along the A line, including both the merge and any
      non-merge-related edits made before the commit.</para>

    <para>Although this merge happened to include all the branch changes, that
      needn't be the case.  For example, the next time we merge the B
      line</para>

    <programlisting><![CDATA[

 _____     _____     _____     _____     _____      _____      _____
|     |   |     |   |     |   |     |   |     |    |     |    |     |
| A:1 |-->| A:2 |-->| A:4 |-->| A:6 |-->| A:8 |-.->| A:9 |-.->|A:11 |
|_____|   |_____|   |_____|   |_____|   |_____| |  |_____| |  |_____|
             \                                  /          |
              \                                /           |
               \  _____     _____     _____   / _____      |
                \|     |   |     |   |     | / |     |    /
                 | B:3 |-->| B:5 |-->| B:7 |-->|B:10 |->-'
                 |_____|   |_____|   |_____|   |_____|

]]></programlisting>

    <para>Subversion will know that A's ancestry set already contains B:3-7, so
      only the difference between B:7 and B:10 will be applied.  A's new
      ancestry will be</para>

    <programlisting>

  { A:1-11, B:3-10 }

</programlisting>

    <para>But why limit ourselves to contiguous ranges?  An ancestry set is
      truly a set &ndash; it can be any subset of the changes available:</para>

    <programlisting><![CDATA[

 _____        _____        _____        _____        _____        _____
|     |      |     |      |     |      |     |      |     |      |     |
| A:1 |----->| A:2 |----->| A:4 |----->| A:6 |----->| A:8 |--.-->|A:10 |
|_____|      |_____|      |_____|      |_____|      |_____| /    |_____|
                |                                          /
                |                ______________________.__/
                |               /                      |
                |              /                       |
                \           __/_                      _|__
                 \         {    }                    {    }
                  \  _____        _____        _____        _____
                   \|     |      |     |      |     |      |     |
                    | B:3 |----->| B:5 |----->| B:7 |----->| B:9 |----->
                    |_____|      |_____|      |_____|      |_____|

]]></programlisting>

    <para>In this diagram, the change from B:3-5 and the change from B:7-9 are
      merged into a working copy whose ancestry set (so far) is
      {&nbsp;A:1-8&nbsp;} plus any local changes.  After committing, A:10's
      ancestry set is</para>

    <programlisting>

  { A:1-10, B:5, B:9 }

</programlisting>

    <para>Clearly, saying "Let's merge branch B into A" is a little ambiguous.
      It usually means "Merge all the changes accounted for in B's tip into A",
      but it <emphasis>might</emphasis> mean "Merge the single change that
      resulted in B's tip into A".</para>

    <para>Any merge, when viewed in detail, is an application of a particular
      set of changes &ndash; not necessarily adjacent ones &ndash; to a working
      copy.  The user-level interface may allow some of these changes to be
      specified implicitly.  For example, many merges involve a single,
      contiguous range of changes, with one or both ends of the range easily
      deducible from context (i.e., branch root to branch tip).  These
      inference rules are not specified here, but it should be clear in most
      contexts how they work.</para>

    <para>Because each node knows its ancestors, Subversion never merges the
      same change twice (unless you force it to).  For example, if after the
      above merge, you tell Subversion to merge all B changes into A,
      Subversion will notice that two of them have already been merged, and so
      merge only the other two changes, resulting in a final ancestry set
      of:</para>

    <programlisting>

  { A:1-10, B:3-9 }

</programlisting>

<!--
  Heh, what about this: 

    B:3 adds line 3, with the text "foo". 
    B:5 deletes line 3. 
    B:7 adds line 3, with the text "foo". 
    B:9 deletes line 3. 

  The user first merges B:5 and B:9 into A.  If A had that line, it goes away
  now, nothing more. 

  Next, user merges B:3 and B:7 into A.  The second merge must conflict.

  I'm not sure we need to care about this, I just thought I'd note how even
  merges that seem like they ought to be easily composable can still suck. :-)
-->

    <para>This description of merging and ancestry applies to both intra- and
      inter-repository merges.  However, inter-repository merging will probably
      not be implemented until a future release of Subversion.</para>
  </sect1>
</chapter>
