<?xml version='1.0' encoding='UTF-8' standalone='no'?>
<doxygen xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="compound.xsd" version="1.9.6" xml:lang="en-US">
  <compounddef id="ComposableTasking" kind="page">
    <compoundname>ComposableTasking</compoundname>
    <title>Composable Tasking</title>
    <tableofcontents>
      <tocsect>
        <name>Compose a Taskflow</name>
        <reference>ComposableTasking_1ComposeATaskflow</reference>
    </tocsect>
      <tocsect>
        <name>Create a Module Task</name>
        <reference>ComposableTasking_1CreateAModuleTask</reference>
    </tocsect>
      <tocsect>
        <name>Create a Custom Composable Graph</name>
        <reference>ComposableTasking_1CreateACustomComposableGraph</reference>
    </tocsect>
    </tableofcontents>
    <briefdescription>
    </briefdescription>
    <detaileddescription>
<para>Composition is a key to improve the programmability of a complex workflow. This chapter describes how to create a large parallel graph through composition of modular and reusable blocks that are easier to optimize.</para>
<sect1 id="ComposableTasking_1ComposeATaskflow">
<title>Compose a Taskflow</title>
<para>A powerful feature of <ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref> is its <emphasis>composable</emphasis> interface. You can break down a large parallel workload into smaller pieces each designed to run a specific task dependency graph. This largely facilitates the <emphasis>modularity</emphasis> of writing a parallel task program.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal"><sp/>1:<sp/></highlight><highlight class="comment">//<sp/>f1<sp/>has<sp/>three<sp/>independent<sp/>tasks</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/>2:<sp/><ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref><sp/>f1;</highlight></codeline>
<codeline><highlight class="normal"><sp/>3:<sp/>f1.<ref refid="classtf_1_1Taskflow_1ad5706e5819aa01a63c4aa2e3485546b9" kindref="member">name</ref>(</highlight><highlight class="stringliteral">&quot;F1&quot;</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal"><sp/>4:<sp/><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>f1A<sp/>=<sp/>f1.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([&amp;](){<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;F1<sp/>TaskA\n&quot;</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
<codeline><highlight class="normal"><sp/>5:<sp/><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>f1B<sp/>=<sp/>f1.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([&amp;](){<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;F1<sp/>TaskB\n&quot;</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
<codeline><highlight class="normal"><sp/>6:<sp/><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>f1C<sp/>=<sp/>f1.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([&amp;](){<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;F1<sp/>TaskC\n&quot;</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
<codeline><highlight class="normal"><sp/>7:<sp/></highlight></codeline>
<codeline><highlight class="normal"><sp/>8:<sp/>f1A.<ref refid="classtf_1_1Task_1a08ada0425b490997b6ff7f310107e5e3" kindref="member">name</ref>(</highlight><highlight class="stringliteral">&quot;f1A&quot;</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal"><sp/>9:<sp/>f1B.<ref refid="classtf_1_1Task_1a08ada0425b490997b6ff7f310107e5e3" kindref="member">name</ref>(</highlight><highlight class="stringliteral">&quot;f1B&quot;</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal">10:<sp/>f1C.<ref refid="classtf_1_1Task_1a08ada0425b490997b6ff7f310107e5e3" kindref="member">name</ref>(</highlight><highlight class="stringliteral">&quot;f1C&quot;</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal">11:<sp/>f1A.<ref refid="classtf_1_1Task_1a8c78c453295a553c1c016e4062da8588" kindref="member">precede</ref>(f1C);</highlight></codeline>
<codeline><highlight class="normal">12:<sp/>f1B.<ref refid="classtf_1_1Task_1a8c78c453295a553c1c016e4062da8588" kindref="member">precede</ref>(f1C);</highlight></codeline>
<codeline><highlight class="normal">13:</highlight></codeline>
<codeline><highlight class="normal">14:<sp/></highlight><highlight class="comment">//<sp/>f2A<sp/>---</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">15:<sp/></highlight><highlight class="comment">//<sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/>|----&gt;<sp/>f2C<sp/>----&gt;<sp/>f1_module_task<sp/>----&gt;<sp/>f2D</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">16:<sp/></highlight><highlight class="comment">//<sp/>f2B<sp/>---<sp/></highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">17:<sp/><ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref><sp/>f2;</highlight></codeline>
<codeline><highlight class="normal">18:<sp/>f2.<ref refid="classtf_1_1Taskflow_1ad5706e5819aa01a63c4aa2e3485546b9" kindref="member">name</ref>(</highlight><highlight class="stringliteral">&quot;F2&quot;</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal">19:<sp/><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>f2A<sp/>=<sp/>f2.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([&amp;](){<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;<sp/><sp/>F2<sp/>TaskA\n&quot;</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
<codeline><highlight class="normal">20:<sp/><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>f2B<sp/>=<sp/>f2.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([&amp;](){<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;<sp/><sp/>F2<sp/>TaskB\n&quot;</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
<codeline><highlight class="normal">21:<sp/><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>f2C<sp/>=<sp/>f2.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([&amp;](){<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;<sp/><sp/>F2<sp/>TaskC\n&quot;</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
<codeline><highlight class="normal">22:<sp/><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>f2D<sp/>=<sp/>f2.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([&amp;](){<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;<sp/><sp/>F2<sp/>TaskD\n&quot;</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
<codeline><highlight class="normal">23:<sp/></highlight></codeline>
<codeline><highlight class="normal">24:<sp/>f2A.<ref refid="classtf_1_1Task_1a08ada0425b490997b6ff7f310107e5e3" kindref="member">name</ref>(</highlight><highlight class="stringliteral">&quot;f2A&quot;</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal">25:<sp/>f2B.<ref refid="classtf_1_1Task_1a08ada0425b490997b6ff7f310107e5e3" kindref="member">name</ref>(</highlight><highlight class="stringliteral">&quot;f2B&quot;</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal">26:<sp/>f2C.<ref refid="classtf_1_1Task_1a08ada0425b490997b6ff7f310107e5e3" kindref="member">name</ref>(</highlight><highlight class="stringliteral">&quot;f2C&quot;</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal">27:<sp/>f2D.<ref refid="classtf_1_1Task_1a08ada0425b490997b6ff7f310107e5e3" kindref="member">name</ref>(</highlight><highlight class="stringliteral">&quot;f2D&quot;</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal">28:</highlight></codeline>
<codeline><highlight class="normal">29:<sp/>f2A.<ref refid="classtf_1_1Task_1a8c78c453295a553c1c016e4062da8588" kindref="member">precede</ref>(f2C);</highlight></codeline>
<codeline><highlight class="normal">30:<sp/>f2B.<ref refid="classtf_1_1Task_1a8c78c453295a553c1c016e4062da8588" kindref="member">precede</ref>(f2C);</highlight></codeline>
<codeline><highlight class="normal">31:</highlight></codeline>
<codeline><highlight class="normal">32:<sp/><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>f1_module_task<sp/>=<sp/>f2.<ref refid="classtf_1_1FlowBuilder_1ac6f22228d4c2ea2e643c4b0d42c0e92a" kindref="member">composed_of</ref>(f1).<ref refid="classtf_1_1Task_1a08ada0425b490997b6ff7f310107e5e3" kindref="member">name</ref>(</highlight><highlight class="stringliteral">&quot;module&quot;</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal">33:<sp/>f2C.<ref refid="classtf_1_1Task_1a8c78c453295a553c1c016e4062da8588" kindref="member">precede</ref>(f1_module_task);</highlight></codeline>
<codeline><highlight class="normal">34:<sp/>f1_module_task.<ref refid="classtf_1_1Task_1a8c78c453295a553c1c016e4062da8588" kindref="member">precede</ref>(f2D);</highlight></codeline>
<codeline><highlight class="normal">35:</highlight></codeline>
<codeline><highlight class="normal">36:<sp/>f2.<ref refid="classtf_1_1Taskflow_1ac433018262e44b12c4cc9f0c4748d758" kindref="member">dump</ref>(<ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref>);</highlight></codeline>
</programlisting></para>
<para><dotfile name="composition_static_1.dot"></dotfile>
</para>
<para>Debrief:</para>
<para><itemizedlist>
<listitem><para>Lines 1-12 create a taskflow of three tasks f1A, f1B, and f1C with f1A and f1B preceding f1C </para>
</listitem>
<listitem><para>Lines 17-30 create a taskflow of four tasks f2A, f2B, f2C, and f2D </para>
</listitem>
<listitem><para>Line 32 creates a module task from taskflow f1 through the method <ref refid="classtf_1_1FlowBuilder_1ac6f22228d4c2ea2e643c4b0d42c0e92a" kindref="member">Taskflow::composed_of</ref> </para>
</listitem>
<listitem><para>Line 33 enforces task f2C to run before the module task </para>
</listitem>
<listitem><para>Line 34 enforces the module task to run before task f2D</para>
</listitem>
</itemizedlist>
</para>
</sect1>
<sect1 id="ComposableTasking_1CreateAModuleTask">
<title>Create a Module Task</title>
<para>The task created from <ref refid="classtf_1_1FlowBuilder_1ac6f22228d4c2ea2e643c4b0d42c0e92a" kindref="member">Taskflow::composed_of</ref> is a <emphasis>module</emphasis> task that runs on a pre-defined taskflow. A module task does not own the taskflow but maintains a soft mapping to the taskflow. You can create multiple module tasks from the same taskflow but only one module task can run at one time. For example, the following composition is valid. Even though the two module tasks <computeroutput>module1</computeroutput> and <computeroutput>module2</computeroutput> refer to the same taskflow <computeroutput>F1</computeroutput>, the dependency link prevents <computeroutput>F1</computeroutput> from multiple executions at the same time.</para>
<para><dotfile name="composition_static_2.dot"></dotfile>
</para>
<para>However, the following composition is <emphasis>invalid</emphasis>. Both module tasks refer to the same taskflow. They can not run at the same time because they are associated with the same graph.</para>
<para><dotfile name="composition_static_invalid.dot"></dotfile>
</para>
</sect1>
<sect1 id="ComposableTasking_1CreateACustomComposableGraph">
<title>Create a Custom Composable Graph</title>
<para>Taskflow allows you to create a custom graph object that can participate in the scheduling using composition. To become a module task, your class <computeroutput>T</computeroutput> must define a method <computeroutput>T::graph()</computeroutput> that returns a reference to a <ref refid="classtf_1_1Graph" kindref="compound">tf::Graph</ref> object. The following example defines a custom graph object that can be assembled in a taskflow throw composition:</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal"><sp/>1:<sp/></highlight><highlight class="keyword">struct<sp/></highlight><highlight class="normal">CustomGraph<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/>2:<sp/><sp/><sp/><ref refid="classtf_1_1Graph" kindref="compound">tf::Graph</ref><sp/>graph;</highlight></codeline>
<codeline><highlight class="normal"><sp/>3:<sp/><sp/><sp/>CustomGraph()<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/>4:<sp/><sp/><sp/><sp/><sp/><ref refid="classtf_1_1FlowBuilder" kindref="compound">tf::FlowBuilder</ref><sp/>builder(graph);</highlight></codeline>
<codeline><highlight class="normal"><sp/>5:<sp/><sp/><sp/><sp/><sp/><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>task<sp/>=<sp/>builder.emplace([](){</highlight></codeline>
<codeline><highlight class="normal"><sp/>6:<sp/><sp/><sp/><sp/><sp/><sp/><sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;a<sp/>task\n&quot;</highlight><highlight class="normal">;<sp/><sp/></highlight><highlight class="comment">//<sp/>static<sp/>task</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/>7:<sp/><sp/><sp/><sp/><sp/>});</highlight></codeline>
<codeline><highlight class="normal"><sp/>8:<sp/><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal"><sp/>9:<sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>returns<sp/>a<sp/>reference<sp/>to<sp/>the<sp/>graph<sp/>for<sp/>taskflow<sp/>composition</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">10:<sp/><sp/><sp/>Graph&amp;<sp/>graph()<sp/>{<sp/></highlight><highlight class="keywordflow">return</highlight><highlight class="normal"><sp/>graph;<sp/>}</highlight></codeline>
<codeline><highlight class="normal">11:<sp/>};</highlight></codeline>
<codeline><highlight class="normal">12:</highlight></codeline>
<codeline><highlight class="normal">13:<sp/>CustomGraph<sp/>obj;</highlight></codeline>
<codeline><highlight class="normal">14:<sp/><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>comp<sp/>=<sp/>taskflow.<ref refid="classtf_1_1Task_1ab38be520fe700cb4ca1f312308a95585" kindref="member">composed_of</ref>(obj);</highlight></codeline>
</programlisting></para>
<para>Debrief:<itemizedlist>
<listitem><para>Lines 1-11 define a custom graph interface to participate in taskflow composition</para>
</listitem><listitem><para>Line 2 defines the graph object using <ref refid="classtf_1_1Graph" kindref="compound">tf::Graph</ref></para>
</listitem><listitem><para>Lines 3-8 defines the constructor that constructs the task graph using <ref refid="classtf_1_1FlowBuilder" kindref="compound">tf::FlowBuilder</ref></para>
</listitem><listitem><para>Line 10 defines the required method for taskflow composition</para>
</listitem><listitem><para>Lines 13-14 creates a module task for the declared graph object in the taskflow</para>
</listitem></itemizedlist>
</para>
<para>The composition method <ref refid="classtf_1_1FlowBuilder_1ac6f22228d4c2ea2e643c4b0d42c0e92a" kindref="member">tf::Taskflow::composed_of</ref> requires the target to define the <computeroutput>graph()</computeroutput> method that returns a reference to a <ref refid="classtf_1_1Graph" kindref="compound">tf::Graph</ref> object defined by the target. At runtime, the executor will run dependent tasks in that graph using the same work-stealing scheduling algorithm as other taskflows. Taskflow leverages this powerful feature to design high-level algorithms, such as <ref refid="classtf_1_1Pipeline" kindref="compound">tf::Pipeline</ref>.</para>
<para><simplesect kind="note"><para>While Taskflow gives you the flexibility to create a composable graph object, you should consider using <ref refid="classtf_1_1Graph" kindref="compound">tf::Graph</ref> as an opaque data structure just to interact with the library. Additionally, as other module tasks, Taskflow does not own the lifetime of a custom composable graph object but keeps a soft mapping to it. You should keep the graph object alive during its execution. </para>
</simplesect>
</para>
</sect1>
    </detaileddescription>
    <location file="cookbook/composable_tasking.dox"/>
  </compounddef>
</doxygen>
