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