diff --git a/lightflow/models/dag.py b/lightflow/models/dag.py
index 26220d4..defed38 100644
--- a/lightflow/models/dag.py
+++ b/lightflow/models/dag.py
@@ -36,11 +36,13 @@ class Dag:
         schema (dict): A dictionary with the definition of the task graph.
     """
     def __init__(self, name, *, autostart=True, queue=DefaultJobQueueName.Dag,
-                 schema=None):
+                 schema=None,
+                 graph=None):
         self._name = name
         self._autostart = autostart
         self._queue = queue
         self._schema = schema
+        self._graph = graph
 
         self._copy_counter = 0
         self._workflow_name = None
@@ -114,7 +116,10 @@ def run(self, config, workflow_id, signal, *, data=None):
             DirectedAcyclicGraphInvalid: If the graph is not a dag (e.g. contains loops).
             ConfigNotDefinedError: If the configuration for the dag is empty.
         """
-        graph = self.make_graph(self._schema)
+        if not self._graph:
+            graph = self.make_graph(self._schema)
+        else:
+            graph = self._graph
 
         # pre-checks
         self.validate(graph)
@@ -252,8 +257,7 @@ def validate(self, graph):
         if not nx.is_directed_acyclic_graph(graph):
             raise DirectedAcyclicGraphInvalid(graph_name=self._name)
 
-    @staticmethod
-    def make_graph(schema):
+    def make_graph(self, schema):
         """ Construct the task graph (dag) from a given schema.
 
         Parses the graph schema definition and creates the task graph. Tasks are the
@@ -315,7 +319,8 @@ def make_graph(schema):
                 else:
                     graph.add_node(parent)
 
-        return graph
+        self._graph = graph
+        return self._graph
 
     def __deepcopy__(self, memo):
         """ Create a copy of the dag object.
diff --git a/requirements-dev.txt b/requirements-dev.txt
index f604537..f88f319 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -1,3 +1,3 @@
-pytest
+pytest>=3.6
 pytest-cov
 flake8