Flink Gelly Path/Trail Usecase - path

Question

our team is new to Gelly api. We are looking to implement a simple use case that will list all paths originating from an initial vertice - for e.g.
input edge csv file is
1,2\n2,3\n3,4\n1,5\n5,6
the required output will be (the full path that starts from 1)
1,2,3,4\n1,5,6
Can someone please help.

Answer 1

You can use one of Gelly's iteration abstractions, e.g. vertex-centric iterations. Starting from the source vertex, you can iteratively extend the paths, one hop per superstep. Upon receiving a path, a vertex appends its ID to the path and propagates it to its outgoing neighbors. If a vertex has no outgoing neighbors, then it prints / stores the path and does not propagate it further. To avoid loops a vertex could also check if its ID exists in the path, before propagating. The compute function could look like this:
public static final class ComputePaths extends ComputeFunction<Integer, Boolean, NullValue, ArrayList<Integer>> {
#Override
public void compute(Vertex<Integer, Boolean> vertex, MessageIterator<ArrayList<Integer>> paths) {
if (getSuperstepNumber() == 1) {
// the source propagates its ID
if (vertex.getId().equals(1)) {
ArrayList<Integer> msg = new ArrayList<>();
msg.add(1);
sendMessageToAllNeighbors(msg);
}
}
else {
// go through received messages
for (ArrayList<Integer> p : paths) {
if (!p.contains(vertex.getId())) {
// if no cycle => append ID and forward to neighbors
p.add(vertex.getId());
if (!vertex.getValue()) {
sendMessageToAllNeighbors(p);
}
else {
// no out-neighbors: print p
System.out.println(p);
}
}
else {
// found a cycle => print the path and don't propagate further
System.out.println(p);
}
}
}
}
}
In this code I have assumed that you have pre-processed vertices to mark the ones that have no out-neighbors with a "true" value. You could e.g. use graph.outDegrees() to find those.
Have in mind that enumerating all paths in a large and dense graph is expensive to compute. The intermediate paths state can explode quite quickly. You could look into using a more compact way for representing paths than an using ArrayList of ints, but beware of the cost if you have a dense graph with large diameter.
If you don't need the paths themselves but you're only interested in reachability or shortest paths, then there exist more efficient algorithms.