# Cookbook » Composable Tasking

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.

## Compose a Taskflow

A powerful feature of tf::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.

     1: // f1 has three independent tasks
     2: tf::Taskflow f1;
     3: f1.name("F1");
     4: tf::Task f1A = f1.emplace([&](){ std::cout << "F1 TaskA\n"; });
     5: tf::Task f1B = f1.emplace([&](){ std::cout << "F1 TaskB\n"; });
     6: tf::Task f1C = f1.emplace([&](){ std::cout << "F1 TaskC\n"; });
     7:
     8: f1A.name("f1A");
     9: f1B.name("f1B");
    10: f1C.name("f1C");
    11: f1A.precede(f1C);
    12: f1B.precede(f1C);
    13:
    14: // f2A ---
    15: //        |----> f2C ----> f1_module_task ----> f2D
    16: // f2B ---
    17: tf::Taskflow f2;
    18: f2.name("F2");
    19: tf::Task f2A = f2.emplace([&](){ std::cout << "  F2 TaskA\n"; });
    20: tf::Task f2B = f2.emplace([&](){ std::cout << "  F2 TaskB\n"; });
    21: tf::Task f2C = f2.emplace([&](){ std::cout << "  F2 TaskC\n"; });
    22: tf::Task f2D = f2.emplace([&](){ std::cout << "  F2 TaskD\n"; });
    23:
    24: f2A.name("f2A");
    25: f2B.name("f2B");
    26: f2C.name("f2C");
    27: f2D.name("f2D");
    28:
    29: f2A.precede(f2C);
    30: f2B.precede(f2C);
    31:
    32: tf::Task f1_module_task = f2.composed_of(f1).name("module");
    33: f2C.precede(f1_module_task);
    34: f1_module_task.precede(f2D);
    35:
    36: f2.dump(std::cout);

Debrief:

- Lines 1-12 create a taskflow of three tasks f1A, f1B, and f1C with f1A and f1B preceding f1C
- Lines 17-30 create a taskflow of four tasks f2A, f2B, f2C, and f2D
- Line 32 creates a module task from taskflow f1 through the method Taskflow::composed_of
- Line 33 enforces task f2C to run before the module task
- Line 34 enforces the module task to run before task f2D

## Create a Module Task

The task created from Taskflow::composed_of 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 `module1` and `module2` refer to the same taskflow `F1`, the dependency link prevents `F1` from multiple executions at the same time.

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.

## Create a Custom Composable Graph

Taskflow allows you to create a custom graph object that can participate in the scheduling using composition. To become a module task, your class `T` must define a method `T::graph()` that returns a reference to a tf::Graph object. The following example defines a custom graph object that can be assembled in a taskflow throw composition:

     1: struct CustomGraph {
     2:   tf::Graph graph;
     3:   CustomGraph() {
     4:     tf::FlowBuilder builder(graph);
     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);

Debrief:

- Lines 1-11 define a custom graph interface to participate in taskflow composition
- Line 2 defines the graph object using tf::Graph
- Lines 3-8 defines the constructor that constructs the task graph using tf::FlowBuilder
- Line 10 defines the required method for taskflow composition
- Lines 13-14 creates a module task for the declared graph object in the taskflow

The composition method tf::Taskflow::composed_of requires the target to define the `graph()` method that returns a reference to a tf::Graph 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 tf::Pipeline.

#### Note

While Taskflow gives you the flexibility to create a composable graph object, you should consider using tf::Graph 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.

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