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<chapter id="chap-overview"
         xmlns="http://www.scons.org/dbxsd/v1.0"
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         xsi:schemaLocation="http://www.scons.org/dbxsd/v1.0 http://www.scons.org/dbxsd/v1.0/scons.xsd">
<title>Overview</title>

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<section id="sect-architecture">
 <title>Architecture</title>

 <para>

   The heart of &SCons; is its <emphasis>Build Engine</emphasis>.
   The &SCons; Build Engine is a Python module
   that manages dependencies between
   external objects
   such as files or database records.
   The Build Engine is designed to
   be interface-neutral
   and easily embeddable in any
   software system that needs dependency
   analysis between updatable objects.

 </para>

 <para>

   The key parts of the Build Engine architecture
   are captured in the following quasi-UML diagram:

 </para>

   <figure>
    <title>&SCons; Architecture</title>
    <mediaobject>
     <imageobject>
      <imagedata fileref="engine.svg" align="center" scale="50"/>
     </imageobject>
     <imageobject>
      <imagedata fileref="engine.jpg" align="center"/>
     </imageobject>
    </mediaobject>
   </figure>

 <para>

   The point of &SCons; is to manage
   dependencies between arbitrary external objects.
   Consequently, the Build Engine does not restrict or specify
   the nature of the external objects it manages,
   but instead relies on subclass of the &Node;
   class to interact with the external system or systems
   (file systems, database management systems)
   that maintain the objects being examined or updated.

 </para>

 <para>

   The Build Engine presents to the software system in
   which it is embedded
   a Python API for specifying source (input) and target (output) objects,
   rules for building/updating objects,
   rules for scanning objects for dependencies, etc. 
   Above its Python API,
   the Build Engine is completely
   interface-independent,
   and can be encapsulated by any other software
   that supports embedded Python.

 </para>

 <para>

   Software that chooses to use the Build Engine
   for dependency management
   interacts with it
   through <emphasis>Construction Environments</emphasis>.
   A Construction Environment consists
   of a dictionary of environment variables,
   and one or more associated
   &Scanner; objects
   and &Builder; objects.
   The Python API is used to
   form these associations.

 </para>

 <para>

   A &Scanner; object specifies
   how to examine a type of source object
   (C source file, database record)
   for dependency information.
   A &Scanner; object may use
   variables from the associated
   Construction Environment
   to modify how it scans an object:
   specifying a search path for included files,
   which field in a database record to consult,
   etc.

 </para>

 <para>

   A &Builder; object specifies
   how to update a type of target object:
   executable program, object file, database field, etc.
   Like a &Scanner; object,
   a &Builder; object may use
   variables from the associated
   Construction Environment
   to modify how it builds an object:
   specifying flags to a compiler,
   using a different update function,
   etc.

 </para>

 <para>

   &Scanner; and &Builder; objects will return one or more
   &Node; objects that represent external objects.
   &Node; objects are the means by which the
   Build Engine tracks dependencies:
   A &Node; may represent a source (input) object that
   should already exist,
   or a target (output) object which may be built,
   or both.
   The &Node; class is sub-classed to 
   represent external objects of specific type:
   files, directories, database fields or records, etc.
   Because dependency information, however,
   is tracked by the top-level &Node; methods and attributes,
   dependencies can exist
   between nodes representing different external object types.
   For example,
   building a file could be made
   dependent on the value of a given
   field in a database record,
   or a database table could depend
   on the contents of an external file.

 </para>

 <para>

   The Build Engine uses a &Job; class (not displayed)
   to manage the actual work of updating external target objects:
   spawning commands to build files,
   submitting the necessary commands to update a database record,
   etc.
   The &Job; class has sub-classes
   to handle differences between spawning
   jobs in parallel and serially.

 </para>

 <para>

   The Build Engine also uses a
   &Signature; class (not displayed)
   to maintain information about whether
   an external object is up-to-date.
   Target objects with out-of-date signatures
   are updated using the appropriate
   &Builder; object.

 </para>

   <!-- BEGIN HTML -->

   <!--
   Details on the composition, methods,
   and attributes of these classes
   are available in the  A HREF="internals.html" Internals /A  page.
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</section>



<section id="sect-engine">
 <title>Build Engine</title>

 <para>

   More detailed discussion of some of the
   Build Engine's characteristics:

 </para>

 <section>
  <title>Python API</title>

   <para>

   The Build Engine can be embedded in any other software
   that supports embedding Python:
   in a GUI,
   in a wrapper script that
   interprets classic <filename>Makefile</filename> syntax,
   or in any other software that
   can translate its dependency representation
   into the appropriate calls to the Build Engine API.
   <!--<xref linkend="chap-native">--> describes in detail
   the specification for a "Native Python" interface
   that will drive the &SCons; implementation effort.

   </para>

 </section>

 <section>
 <title>Single-image execution</title>

   <para>

   When building/updating the objects,
   the Build Engine operates as a single executable
   with a complete Directed Acyclic Graph (DAG)
   of the dependencies in the entire build tree.
   This is in stark contrast to the
   commonplace recursive use of Make
   to handle hierarchical directory-tree builds.

   </para>

 </section>

 <section>
 <title>Dependency analysis</title>

   <para>

   Dependency analysis is carried out via digital signatures
   (a.k.a. "fingerprints").
   Contents of object are examined and reduced
   to a number that can be stored and compared to
   see if the object has changed.
   Additionally, &SCons; uses the same
   signature technique on the command-lines that
   are executed to update an object.
   If the command-line has changed since the last time,
   then the object must be rebuilt.

   </para>

 </section>

 <section>
 <title>Customized output</title>

   <para>

   The output of Build Engine is customizable
   through user-defined functions.
   This could be used to print additional desired
   information about what &SCons; is doing,
   or tailor output to a specific build analyzer,
   GUI, or IDE.

   </para>

 </section>

 <section>
 <title>Build failures</title>

   <para>

   &SCons; detects build failures via the exit status from the tools
   used to build the target files.  By default, a failed exit status
   (non-zero on UNIX systems) terminates the build with an appropriate
   error message.  An appropriate class from the Python library will
   interpret build-tool failures via an OS-independent API.

   </para>

   <para>

   If multiple tasks are executing in a parallel build, and one tool
   returns failure, &SCons; will not initiate any further build tasks,
   but allow the other build tasks to complete before terminating.

   </para>

   <para>

   A <option>-k</option> command-line option may be used to ignore
   errors and continue building other targets.  In no case will a target
   that depends on a failed build be rebuilt.

   </para>

 </section>

</section>



<section id="sect-interfaces">
 <title>Interfaces</title>

 <para>

   As previously described,
   the &SCons; Build Engine
   is interface-independent above its Python API,
   and can be embedded in any software system
   that can translate its dependency requirements
   into the necessary Python calls.

 </para>

 <para>

   The "main" &SCons; interface
   for implementation purposes,
   uses Python scripts as configuration files.
   Because this exposes the Build Engine's Python API to the user,
   it is current called the "Native Python" interface.

 </para>

 <para>

   This section will also discuss
   how &SCons; will function in the context
   of two other interfaces:
   the &Makefile; interface of the classic &Make; utility,
   and a hypothetical graphical user interface (GUI).

 </para>

 <section>
  <title>Native Python interface</title>

  <para>

   The Native Python interface is intended to be the primary interface
   by which users will know &SCons;--that is,
   it is the interface they will use
   if they actually type &SCons; at a command-line prompt.

  </para>

  <para>

   In the Native Python interface, &SCons; configuration files are simply
   Python scripts that directly invoke methods from the Build Engine's
   Python API to specify target files to be built, rules for building
   the target files, and dependencies.  Additional methods, specific to
   this interface, are added to handle functionality that is specific to
   the Native Python interface:  reading a subsidiary configuration file;
   copying target files to an installation directory; etc.

  </para>

  <para>

   Because configuration files are Python scripts, Python flow control
   can be used to provide very flexible manipulation of objects and
   dependencies.  For example, a function could be used to invoke a common
   set of methods on a file, and called iteratively over an array of
   files.

  </para>

  <para>

   As an additional advantage, syntax errors in &SCons; Native Python
   configuration files will be caught by the Python parser.  Target-building
   does not begin until after all configuration files are read, so a syntax
   error will not cause a build to fail half-way.

  </para>

 </section>

 <section>
  <title>Makefile interface</title>

  <para>

   An alternate &SCons; interface would provide backwards
   compatibility with the classic &Make; utility.
   This would be done by embedding the &SCons; Build Engine
   in a Python script that can translate existing
   &Makefile;s into the underlying calls to the
   Build Engine's Python API
   for building and tracking dependencies.
   Here are approaches to solving some of the issues
   that arise from marrying these two pieces:

  </para>

  <itemizedlist>

   <listitem>
   <para>
   &Makefile; suffix rules can be translated
   into an appropriate &Builder; object
   with suffix maps from the Construction Environment.
   </para>
   </listitem>

   <listitem>
   <para>
   Long lists of static dependences
   appended to a &Makefile; by
   various <command>"make depend"</command> schemes
   can be preserved
   but supplemented by
   the more accurate dependency information
   provided by &Scanner; objects.
   </para>
   </listitem>

   <listitem>
   <para>
   Recursive invocations of &Make;
   can be avoided by reading up
   the subsidiary &Makefile; instead.
   </para>
   </listitem>

  </itemizedlist>

  <para>

   Lest this seem like too outlandish an undertaking,
   there is a working example of this approach:
   Gary Holt's &Makepp; utility
   is a Perl script that provides
   admirably complete parsing of complicated &Makefile;s
   around an internal build engine inspired,
   in part, by the classic <application>Cons</application> utility.

  </para>

 </section>

 <section>
  <title>Graphical interfaces</title>

  <para>

   The &SCons; Build Engine
   is designed from the ground up to be embedded
   into multiple interfaces.
   Consequently, embedding the dependency capabilities
   of &SCons; into graphical interface
   would be a matter of mapping the
   GUI's dependency representation
   (either implicit or explicit)
   into corresponding calls to the Python API
   of the &SCons; Build Engine.

  </para>

  <para>

   Note, however, that this proposal leaves the problem of
   designed a good graphical interface
   for representing software build dependencies
   to people with actual GUI design experience...

  </para>

 </section>

</section>

</chapter>
