GraphFrames detect exclusive outbound relations - apache-spark

Question

In my graph I need to detect vertices that do not have inbound relations. Using the example below, "a" is the only node that is not being related by the anyone.
a --> b
b --> c
c --> d
c --> b
I would really appreciate any examples to detect "a" type nodes in my graph.
Thanks

Answer 1

unfortunately the approach is not as simple because the graph.degress, graph.inDegrees, graph.outDegrees functions are not returning vertices with 0 edges.
(see documentation for Scala which holds true for Python too https://graphframes.github.io/graphframes/docs/_site/api/scala/index.html#org.graphframes.GraphFrame)
so the following code will always return a empty dataframe
g=Graph(vertices,edges)
# check for start points
g.inDegrees.filter("inDegree==0").show()
+---+--------+
| id|inDegree|
+---+--------+
+---+--------+
# or check for end points
g.outDegrees.filter("outDegree==0").show()
+---+---------+
| id|outDegree|
+---+---------+
+---+---------+
# or check for any vertices that are alone without edge
g.degrees.filter("degree==0").show()
+---+------+
| id|degree|
+---+------+
+---+------+
what works is a left, right or full join of the inDegree and outDegree result and filter on the NULL values of the respective column
the join will provide you a merged columns with NULL values on the start and end positions
g.inDegrees.join(g.outDegrees,on="id",how="full").show()
+---+--------+---------+
| id|inDegree|outDegree|
+---+--------+---------+
| b6| 1| null|
| a3| 1| 1|
| a4| 1| null|
| c7| 1| 1|
| b2| 1| 2|
| c9| 3| 1|
| c5| 1| 1|
| c1| null| 1|
| c6| 1| 1|
| a2| 1| 1|
| b3| 1| 1|
| b1| null| 1|
| c8| 3| null|
| a1| null| 1|
| c4| 1| 4|
| c3| 1| 1|
| b4| 1| 1|
| c2| 1| 3|
|c10| 1| null|
| b5| 2| 1|
+---+--------+---------+
now you can filter on what search
my_in_Degrees=g.inDegrees
my_out_Degrees=g.outDegrees
# get starting vertices (no more childs)
my_in_Degrees.join(my_out_Degrees,on="id",how="full").filter(my_in_Degrees.inDegree.isNull()).show()
+---+--------+---------+
| id|inDegree|outDegree|
+---+--------+---------+
| c1| null| 1|
| b1| null| 1|
| a1| null| 1|
+---+--------+---------+
# get ending vertices (no more parents)
my_in_Degrees.join(my_out_Degrees,on="id",how="full").filter(my_out_Degrees.outDegree.isNull()).show()
+---+--------+---------+
| id|inDegree|outDegree|
+---+--------+---------+
| b6| 1| null|
| a4| 1| null|
|c10| 1| null|
+---+--------+---------+