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<!DOCTYPE html>

<head>

<title>Cookbook » Composable Tasking | Taskflow QuickStart</title>

</head>

<body>

<a href="https://taskflow.github.io"><img src="taskflow_logo.png" alt="" />Taskflow</a> | <a href="index.html" class="m-thin">QuickStart</a>

</svg></a>

</div>

<li><a href="pages.html">Handbook</a></li>

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<h1>

<a href="Cookbook.html">Cookbook</a> »

Composable Tasking

</h1>

<h3>Contents</h3>

<ul>

<li><a href="#ComposeATaskflow">Compose a Taskflow</a></li>

<li><a href="#CreateAModuleTaskFromATaskflow">Create a Module Task from a Taskflow</a></li>

<li><a href="#CreateACustomComposableGraph">Create a Custom Composable Graph</a></li>

</ul>

</nav>

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.<section id="ComposeATaskflow"><h2><a href="#ComposeATaskflow">Compose a Taskflow</a></h2>A powerful feature of Taskflow is its composable 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 modularity of writing a parallel task program.<pre class="m-code">// f1 has three independent tasks

tf::Taskflow f1;

f1.name("F1");

tf::Task f1A = f1.emplace([&](){ std::cout << "F1 TaskA\n"; });

tf::Task f1B = f1.emplace([&](){ std::cout << "F1 TaskB\n"; });

tf::Task f1C = f1.emplace([&](){ std::cout << "F1 TaskC\n"; });

f1A.name("f1A");

f1B.name("f1B");

f1C.name("f1C");

f1A.precede(f1C);

f1B.precede(f1C);

// f2A ---

// |----> f2C ----> f1_module_task ----> f2D

// f2B ---

tf::Taskflow f2;

f2.name("F2");

tf::Task f2A = f2.emplace([&](){ std::cout << " F2 TaskA\n"; });

tf::Task f2B = f2.emplace([&](){ std::cout << " F2 TaskB\n"; });

tf::Task f2C = f2.emplace([&](){ std::cout << " F2 TaskC\n"; });

tf::Task f2D = f2.emplace([&](){ std::cout << " F2 TaskD\n"; });

f2A.name("f2A");

f2B.name("f2B");

f2C.name("f2C");

f2D.name("f2D");

f2A.precede(f2C);

f2B.precede(f2C);

tf::Task f1_module_task = f2.composed_of(f1).name("module");

f2C.precede(f1_module_task);

f1_module_task.precede(f2D);

f2.dump(std::cout);</pre><div class="m-graph"><svg style="width: 31.600rem; height: 31.100rem;" viewBox="0.00 0.00 316.00 311.00">

<title>Taskflow</title>

<title>cluster_p0x7ffee9223970</title>

<text text-anchor="middle" x="79" y="-283" font-family="Helvetica,sans-Serif" font-size="10.00">Taskflow: F2</text>

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<text text-anchor="middle" x="229" y="-283" font-family="Helvetica,sans-Serif" font-size="10.00">Taskflow: F1</text>

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<text text-anchor="middle" x="79" y="-103.5" font-family="Helvetica,sans-Serif" font-size="10.00">module [Taskflow: F1]</text>

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</div>The above example first constructs a taskflow consisting of three tasks, <code>f1A</code>, <code>f1B</code>, and <code>f1C</code>, where <code>f1A</code> and <code>f1B</code> execute before <code>f1C</code>. It then creates a second taskflow with four tasks, <code>f2A</code>, <code>f2B</code>, <code>f2C</code>, and <code>f2D</code>. The first taskflow is encapsulated as a module task using <a href="classtf_1_1FlowBuilder.html#ac6f22228d4c2ea2e643c4b0d42c0e92a" class="m-doc">Taskflow::composed_of</a>, allowing it to be embedded within the second taskflow. Dependencies are then established so that <code>f2C</code> must complete before the module task begins, and the module task must finish before <code>f2D</code> executes, thereby integrating the two taskflows into a single execution graph with well-defined ordering constraints.</section><section id="CreateAModuleTaskFromATaskflow"><h2><a href="#CreateAModuleTaskFromATaskflow">Create a Module Task from a Taskflow</a></h2>The task created from <a href="classtf_1_1FlowBuilder.html#ac6f22228d4c2ea2e643c4b0d42c0e92a" class="m-doc">Taskflow::composed_of</a> is a module 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 <code>module1</code> and <code>module2</code> refer to the same taskflow <code>F1</code>, the dependency link prevents <code>F1</code> from multiple executions at the same time.<div class="m-graph"><svg style="width: 31.600rem; height: 38.300rem;" viewBox="0.00 0.00 316.00 383.00">

<title>Taskflow</title>

<title>cluster_p0x7ffee9223970</title>

<text text-anchor="middle" x="79" y="-355" font-family="Helvetica,sans-Serif" font-size="10.00">Taskflow: F2</text>

</g>

<title>cluster_p0x7ffee92238d0</title>

<text text-anchor="middle" x="229" y="-355" font-family="Helvetica,sans-Serif" font-size="10.00">Taskflow: F1</text>

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<text text-anchor="middle" x="79" y="-175.5" font-family="Helvetica,sans-Serif" font-size="10.00">module [Taskflow: F1]</text>

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<text text-anchor="middle" x="79" y="-103.5" font-family="Helvetica,sans-Serif" font-size="10.00">module [Taskflow: F1]</text>

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</div>However, the following composition is invalid. Both module tasks refer to the same taskflow. They can not run at the same time because they are associated with the same graph.<div class="m-graph"><svg style="width: 45.500rem; height: 31.100rem;" viewBox="0.00 0.00 455.00 311.00">

<title>Taskflow</title>

<title>cluster_p0x7ffee9223970</title>

<text text-anchor="middle" x="148.5" y="-283" font-family="Helvetica,sans-Serif" font-size="10.00">Taskflow: F2</text>

</g>

<title>cluster_p0x7ffee92238d0</title>

<text text-anchor="middle" x="368" y="-283" font-family="Helvetica,sans-Serif" font-size="10.00">Taskflow: F1</text>

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<text text-anchor="middle" x="78" y="-103.5" font-family="Helvetica,sans-Serif" font-size="10.00">module [Taskflow: F1]</text>

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<text text-anchor="middle" x="219" y="-103.5" font-family="Helvetica,sans-Serif" font-size="10.00">module [Taskflow: F1]</text>

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</div></section><section id="CreateACustomComposableGraph"><h2><a href="#CreateACustomComposableGraph">Create a Custom Composable Graph</a></h2>Taskflow allows you to create a custom graph object that can participate in the scheduling using composition. To become a module task, your class <code>T</code> must define the method <code>T::graph()</code> that returns a reference to the <a href="classtf_1_1Graph.html" class="m-doc">tf::Graph</a> object managed by <code>T</code>. The following example defines a custom graph object that can be assembled in a taskflow through composition:<pre class="m-code"> 1: struct CustomGraph {

2: tf::Graph graph;

3: CustomGraph() {

4: tf::FlowBuilder builder(graph); // inherit all task builders in tf::Taskflow

5: tf::Task task = builder.emplace([](){

6: std::cout << "a task\n"; // static task

7: });

8: }

9: // returns a reference to the graph for taskflow composition

10: Graph& graph() { return graph; }

11: };

12:

13: CustomGraph obj;

14: tf::Task comp = taskflow.composed_of(obj);</pre>The above code defines a custom graph that can participate in taskflow composition. The graph object is represented using <a href="classtf_1_1Graph.html" class="m-doc">tf::Graph</a>, and its constructor builds the internal task graph through <a href="classtf_1_1FlowBuilder.html" class="m-doc">tf::FlowBuilder</a>. To support composition, the graph implements the required interface method (<code><a href="classtf_1_1Graph.html" class="m-doc">Graph</a>& graph()</code>) that exposes its internal structure to the Taskflow runtime. With this interface in place, the custom graph can then be instantiated as a module task within a larger taskflow, enabling it to be seamlessly composed and scheduled alongside other tasks.<aside class="m-note m-warning"><h4>Attention</h4>Users are responsible for ensuring the given target remains valid throughout its execution. The executor does not assume ownership of the target object.</aside></section>

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Taskflow handbook is part of the <a href="https://taskflow.github.io">Taskflow project</a>, copyright © <a href="https://tsung-wei-huang.github.io/">Dr. Tsung-Wei Huang</a>, 2018–2026. Generated by <a href="https://doxygen.org/">Doxygen</a> 1.9.1 and <a href="https://mcss.mosra.cz/">m.css</a>.

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