Multiple Edges in BPMN and graph

[YJDave]

The BPMN model allows to add multiple edges(edges with same source and target nodes). However, the graph from BPMN do not as it is DiGraph.

It has to be MultiDiGraph. However it does not need to pass the name attribute while adding edge in graph to allow multiple edges.

BPMN allows multiple edges by providing different names. However, the graph from bpmn do not show multiple edges. Even if the name attribute is passed to graph(fix in #202) it won't show multiple edges, because bpmn creates nx.DiGraph()

In [26]: y = BPMN.Flow(a, b, name='y')

In [27]: bpmn = BPMN()

In [28]: bpmn.add_flow(x)

In [29]: bpmn.add_flow(y)

In [30]: bpmn.get_flows()
Out[30]:
{id7daf881a-fcb5-4ec3-9dc5-b86978402c47@a -> id4e566611-6a18-4f49-be78-576674b092d0@b,
 id7daf881a-fcb5-4ec3-9dc5-b86978402c47@a -> id4e566611-6a18-4f49-be78-576674b092d0@b}

In [31]: bpmn.get_graph().edges()
Out[31]: OutEdgeView([(7daf881a-fcb5-4ec3-9dc5-b86978402c47@a, 4e566611-6a18-4f49-be78-576674b092d0@b)])

In nx.DiGraph() it is not possible to have multiple edges. The nx.MultiDiGraph() fixes the issue. However the bpmn add/remove flows/tasks needs to tested for MultiDiGraph() to make sure it works as expected, because MultiDiGraph() adds new edge even if all values(source, target, attribute) are same(shown below).

In [32]: g = nx.DiGraph()

In [34]: g.add_edge("a", "b", name='x')

In [35]: g.add_edge("a", "b", name="y")

In [36]: g.edges()
Out[36]: OutEdgeView([('a', 'b')])

In [37]: g.edges(data=True)
Out[37]: OutEdgeDataView([('a', 'b', {'name': 'y'})])

In [38]: g = nx.MultiDiGraph()

In [40]: g.add_edge("a", "b", name='x')
Out[40]: 0

In [41]: g.add_edge("a", "b", name="y")
Out[41]: 1

In [42]: g.edges()
Out[42]: OutMultiEdgeDataView([('a', 'b'), ('a', 'b')])

In [43]: g.edges(data=True)
Out[43]: OutMultiEdgeDataView([('a', 'b', {'name': 'x'}), ('a', 'b', {'name': 'y'})])

In [44]: g = nx.MultiDiGraph()

In [45]: g.add_edge("a", "b")
Out[45]: 0

In [46]: g.add_edge("a", "b")
Out[46]: 1

In [47]: g.edges()
Out[47]: OutMultiEdgeDataView([('a', 'b'), ('a', 'b')])

In [48]: g.edges(data=True)
Out[48]: OutMultiEdgeDataView([('a', 'b', {}), ('a', 'b', {})])

My question is, what is expected? Is multiple edges in BPMN allowed, if yes then why its corresponding graph is DiGraph?

[fit-alessandro-berti]

Good afternoon.

The BPMN standard allows for multiple edges between the same activities. Hence, we will accept your suggestion to use MultiDiGraph for the underlying graph object, after some testing.

[fit-alessandro-berti]

Dear YJDave,

The issue has been resolved in version 2.2.0 !