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95 lines
2.5 KiB
Markdown
95 lines
2.5 KiB
Markdown
# Direct Acyclic Graph (DAG)：graph_task
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# Sample code
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[tutorial-11-graph_task.cc](/tutorial/tutorial-11-graph_task.cc)
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# About graph_task
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The graph_task example demonstrates how to implement more complex inter-task dependencies by building a DAG.
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# Create tasks in the DAG
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In this tutorial, we create a timer task, two http fetching task, and a 'go' task. Timer task executes a delay of 1 second before
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fetching, http tasks fetch the home page of 'sogou' and 'baidu' in parallel, and after all of that, go task will print the fetching result.
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The dependencies of the tasks are:
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~~~
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+-------+
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+---->| Http1 |-----+
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| +-------+ |
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+-------+ +-v--+
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| Timer | | Go |
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+-------+ +-^--+
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| +-------+ |
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+---->| Http2 |-----+
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+-------+
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~~~
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# Create the graph task
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Graph is a kind of task as well. We can create a graph task by this function：
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~~~cpp
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class WFTaskFactory
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{
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public:
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static WFGraphTask *create_graph_task(graph_callback_t callback);
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...
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};
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~~~
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The graph is a empty graph after it's creation. Of course you may run an empty graph and will get to it callback immediately.
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# Create graph nodes
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We'v got 4 orindary tasks, which can not been added to the graph directly but need to be turned into graph nodes:
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~~~cpp
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{
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/* Create graph nodes */
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WFGraphNode& a = graph->create_graph_node(timer);
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WFGraphNode& b = graph->create_graph_node(http_task1);
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WFGraphNode& c = graph->create_graph_node(http_task2);
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WFGraphNode& d = graph->create_graph_node(go_task);
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}
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~~~
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The ``create_graph_node`` interface of WFGraphTask creates a graph node that refers to a task. And we can use the references of
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graph nodes to specify the dependencies of them. Otherwise, they are all standalone nodes, and will run in parallel when the
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graph task is started.
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# Build the graph
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By using the '-->' or '<--' operators, we can specify the dependencies:
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~~~cpp
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{
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/* Build the graph */
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a-->b;
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a-->c;
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b-->d;
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c-->d;
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}
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~~~
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And now we'v built the graph that we described. And we can use it like an orindary task.
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Also, any of the following codes is legal and equivalent:
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~~~cpp
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{
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a-->b-->d;
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a-->c-->d;
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}
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~~~
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~~~cpp
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{
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d<--b<--a;
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d<--c<--a;
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}
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~~~
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~~~cpp
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{
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d<--b<--a-->c-->d;
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}
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~~~
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# Data passing
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Because the tasks in a graph don't share a same series, there is no general method for passing data between graph nodes.
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# Acknowledgement
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Some designs are inspired by [taskflow](https://github.com/taskflow/taskflow).
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