source: downloads/boost_1_33_1/libs/thread/doc/design.xml @ 12

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<section id="threads.design" last-revision="$Date: 2004/07/17 04:33:59 $">
  <title>Design</title>
  <para>With client/server and three-tier architectures becoming common place
  in today's world, it's becoming increasingly important for programs to be
  able to handle parallel processing. Modern day operating systems usually
  provide some support for this through native thread APIs. Unfortunately,
  writing portable code that makes use of parallel processing in C++ is made
  very difficult by a lack of a standard interface for these native APIs.
  Further, these APIs are almost universally C APIs and fail to take
  advantage of C++'s strengths, or to address concepts unique to C++, such as
  exceptions.</para>
  <para>The &Boost.Threads; library is an attempt to define a portable interface
  for writing parallel processes in C++.</para>
  <section id="threads.design.goals">
    <title>Goals</title>
    <para>The &Boost.Threads; library has several goals that should help to set
        it apart from other solutions. These goals are listed in order of precedence
        with full descriptions below.
    <variablelist>
          <varlistentry>
            <term>Portability</term>
                <listitem>
          <para>&Boost.Threads; was designed to be highly portable. The goal is
                  for the interface to be easily implemented on any platform that
                  supports threads, and possibly even on platforms without native thread
                  support.</para>
                </listitem>
          </varlistentry>
          <varlistentry>
        <term>Safety</term>
        <listitem>
          <para>&Boost.Threads; was designed to be as safe as possible. Writing
                  <link linkend="threads.glossary.thread-safe">thread-safe</link>
                  code is very difficult and successful libraries must strive to
                  insulate the programmer from dangerous constructs as much as
                  possible. This is accomplished in several ways:
          <itemizedlist>
            <listitem>
              <para>C++ language features are used to make correct usage easy
                          (if possible) and error-prone usage impossible or at least more
                          difficult. For example, see the <link
                          linkend="threads.concepts.Mutex">Mutex</link> and <link
                          linkend="threads.concepts.Lock">Lock</link> designs, and note
                          how they interact.</para>
            </listitem>
            <listitem>
              <para>Certain traditional concurrent programming features are
                          considered so error-prone that they are not provided at all. For
                          example, see <xref linkend="threads.rationale.events" />.</para>
            </listitem>
            <listitem>
              <para>Dangerous features, or features which may be misused, are
              identified as such in the documentation to make users aware of
              potential pitfalls.</para>
            </listitem>
          </itemizedlist></para>
                </listitem>
          </varlistentry>
      <varlistentry>
            <term>Flexibility</term>
                <listitem>
                  <para>&Boost.Threads; was designed to be flexible. This goal is often
                  at odds with <emphasis>safety</emphasis>. When functionality might be
                  compromised by the desire to keep the interface safe, &Boost.Threads;
                  has been designed to provide the functionality, but to make it's use
                  prohibitive for general use. In other words, the interfaces have been
                  designed such that it's usually obvious when something is unsafe, and
                  the documentation is written to explain why.</para>
        </listitem>
          </varlistentry>
      <varlistentry>
        <term>Efficiency</term>
                <listitem>
          <para>&Boost.Threads; was designed to be as efficient as
                  possible. When building a library on top of another library there is
                  always a danger that the result will be so much slower than the
                  "native" API that programmers are inclined to ignore the higher level
                  API. &Boost.Threads; was designed to minimize the chances of this
                  occurring. The interfaces have been crafted to allow an implementation
                  the greatest chance of being as efficient as possible. This goal is
                  often at odds with the goal for <emphasis>safety</emphasis>. Every
                  effort was made to ensure efficient implementations, but when in
                  conflict <emphasis>safety</emphasis> has always taken
                  precedence.</para>
        </listitem>
          </varlistentry>
    </variablelist></para>
  </section>
  <section>
    <title>Iterative Phases</title>
    <para>Another goal of &Boost.Threads; was to take a dynamic, iterative
        approach in its development. The computing industry is still exploring the
        concepts of parallel programming. Most thread libraries supply only simple
        primitive concepts for thread synchronization. These concepts are very
        simple, but it is very difficult to use them safely or to provide formal
        proofs for constructs built on top of them. There has been a lot of research
        into other concepts, such as in "Communicating Sequential Processes."
        &Boost.Threads; was designed in iterative steps, with each step providing 
        the building blocks necessary for the next step and giving the researcher 
        the tools necessary to explore new concepts in a portable manner.</para>
    <para>Given the goal of following a dynamic, iterative approach
        &Boost.Threads; shall go through several growth cycles. Each phase in its
        development shall be roughly documented here.</para>
  </section>
  <section>
    <title>Phase 1, Synchronization Primitives</title>
    <para>Boost is all about providing high quality libraries with
        implementations for many platforms. Unfortunately, there's a big problem
        faced by developers wishing to supply such high quality libraries, namely
        thread-safety. The C++ standard doesn't address threads at all, but real
        world programs often make use of native threading support. A portable
        library that doesn't address the issue of thread-safety is therefore not
        much help to a programmer who wants to use the library in his multithreaded
        application. So there's a very great need for portable primitives that will
        allow the library developer to create <link
        linkend="threads.glossary.thread-safe">thread-safe</link>
        implementations. This need far out weighs the need for portable methods to
        create and manage threads.</para>
    <para>Because of this need, the first phase of &Boost.Threads; focuses
        solely on providing portable primitive concepts for thread
        synchronization. Types provided in this phase include the
        <classname>boost::mutex</classname>, 
        <classname>boost::try_mutex</classname>,
        <classname>boost::timed_mutex</classname>, 
        <classname>boost::recursive_mutex</classname>,
        <classname>boost::recursive_try_mutex</classname>,
        <classname>boost::recursive_timed_mutex</classname>, and
        <classname>boost::lock_error</classname>. These are considered the "core"
        synchronization primitives, though there are others that will be added in
        later phases.</para>
  </section>
  <section id="threads.design.phase2">
    <title>Phase 2, Thread Management and Thread Specific Storage</title>
    <para>This phase addresses the creation and management of threads and
    provides a mechanism for thread specific storage (data associated with a
    thread instance). Thread management is a tricky issue in C++, so this
    phase addresses only the basic needs of multithreaded program. Later
    phases are likely to add additional functionality in this area. This
    phase of &Boost.Threads; adds the <classname>boost::thread</classname> and
        <classname>boost::thread_specific_ptr</classname> types. With these
        additions the &Boost.Threads; library can be considered minimal but
        complete.</para>
  </section>
  <section>
    <title>The Next Phase</title>
    <para>The next phase will address more advanced synchronization concepts,
    such as read/write mutexes and barriers.</para>
  </section>
</section>