path: root/doc/user/caching.in

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  <para>

  On multi-developer software projects,
  you can sometimes speed up every developer's builds a lot by
  allowing them to share the derived files that they build.
  &SCons; makes this easy, as well as reliable.

  </para>

  <section>
  <title>Specifying the Shared Cache Directory</title>

    <para>

    To enable sharing of derived files,
    use the &CacheDir; function
    in any &SConscript; file:

    </para>

    <scons_example name="ex1">
       <file name="SConstruct">
       env = Environment()
       env.Program('hello.c')
       CacheDir('cache')
       </file>
       <file name="hello.c">
       hello.c
       </file>
       <directory name="cache">
       </directory>
       <file name="not_used" printme="1">
       CacheDir('/usr/local/build_cache')
       </file>
    </scons_example>

    <para>

    Note that the directory you specify must already exist
    and be readable and writable by all developers
    who will be sharing derived files.
    It should also be in some central location
    that all builds will be able to access.
    In environments where developers are using separate systems
    (like individual workstations) for builds,
    this directory would typically be
    on a shared or NFS-mounted file system.

    </para>

    <para>

    Here's what happens:
    When a build has a &CacheDir; specified,
    every time a file is built,
    it is stored in the shared cache directory
    along with its MD5 build signature.
    On subsequent builds,
    before an action is invoked to build a file,
    &SCons; will check the shared cache directory
    to see if a file with the exact same build
    signature already exists.
    If so, the derived file will not be built locally,
    but will be copied into the local build directory
    from the shared cache directory,
    like so:

    </para>

    <scons_output example="ex1">
      <command>scons -Q</command>
      <command>scons -Q -c</command>
      <command>scons -Q</command>
    </scons_output>

  </section>

  <section>
  <title>Keeping Build Output Consistent</title>

    <para>

    One potential drawback to using a shared cache
    is that your build output can be inconsistent
    from invocation to invocation,
    because any given file may be rebuilt one time
    and retrieved from the shared cache the next time.
    This can make analyzing build output more difficult,
    especially for automated scripts that
    expect consistent output each time.

    </para>

    <para>

    If, however, you use the <literal>--cache-show</literal> option,
    &SCons; will print the command line that it
    <emphasis>would</emphasis> have executed
    to build the file,
    even when it is retrieving the file from the shared cache.
    This makes the build output consistent
    every time the build is run:

    </para>

    <scons_output example="ex1">
      <command>scons -Q</command>
      <command>scons -Q -c</command>
      <command>scons -Q --cache-show</command>
    </scons_output>

    <para>

    The trade-off, of course, is that you no longer
    know whether or not &SCons;
    has retrieved a derived file from cache
    or has rebuilt it locally.

    </para>

  </section>

  <section>
  <title>Not Retrieving Files From a Shared Cache</title>

    <para>

    Retrieving an already-built file
    from the shared cache
    is usually a significant time-savings
    over rebuilding the file,
    but how much of a savings
    (or even whether it saves time at all)
    can depend a great deal on your
    system or network configuration.
    For example, retrieving cached files
    from a busy server over a busy network
    might end up being slower than
    rebuilding the files locally.

    </para>

    <para>

    In these cases, you can specify
    the <literal>--cache-disable</literal>
    command-line option to tell &SCons;
    to not retrieve already-built files from the
    shared cache directory:

    </para>

    <scons_output example="ex1">
      <command>scons -Q</command>
      <command>scons -Q -c</command>
      <command>scons -Q</command>
      <command>scons -Q -c</command>
      <command>scons -Q --cache-disable</command>
    </scons_output>

  </section>

  <section>
  <title>Populating a Shared Cache With Already-Built Files</title>

    <para>

    Sometimes, you may have one or more derived files
    already built in your local build tree
    that you wish to make available to other people doing builds.
    For example, you may find it more effective to perform
    integration builds with the cache disabled
    (per the previous section)
    and only populate the shared cache directory
    with the built files after the integration build
    has completed successfully.
    This way, the cache will only get filled up
    with derived files that are part of a complete, successful build
    not with files that might be later overwritten
    while you debug integration problems.

    </para>

    <para>

    In this case, you can use the
    the <literal>--cache-force</literal> option
    to tell &SCons; to put all derived files in the cache,
    even if the files had already been built
    by a previous invocation:

    </para>

    <scons_output example="ex1">
      <command>scons -Q --cache-disable</command>
      <command>scons -Q -c</command>
      <command>scons -Q --cache-disable</command>
      <command>scons -Q --cache-force</command>
      <command>scons -Q -c</command>
      <command>scons -Q</command>
    </scons_output>

    <para>

    Notice how the above sample run
    demonstrates that the <literal>--cache-disable</literal>
    option avoids putting the built
    <filename>hello.o</filename>
    and 
    <filename>hello</filename> files in the cache,
    but after using the <literal>--cache-force</literal> option,
    the files have been put in the cache
    for the next invocation to retrieve.

    </para>

  </section>