Roman K commented on GIRAPH-141:

I think the best thing to do is to implement multigraph separately from simple 
graph, make some benchmarks to see the performance and memory usage impacts on 
processing (simple graph as multigraph) vs (simple graph) . If the impact could 
be neglected, refactor the code and leave the multigraph implementation only.

Another, more specific question, is what should be done with methods like
   * Request to remove a vertex from the graph
   * (applied just prior to the next superstep).
   * @param vertexId Id of the vertex to be removed.
public void removeEdgeRequest(I sourceVertexId, I destVertexId)
For multigraphs, behavior for this method is quiet undefined.
May be 
public void removeEdgeRequest(I sourceVertexId, I destVertexId, E edgeValue)
should be introduced?
> mulitgraph support in giraph
> ----------------------------
>                 Key: GIRAPH-141
>                 URL: https://issues.apache.org/jira/browse/GIRAPH-141
>             Project: Giraph
>          Issue Type: Improvement
>          Components: graph
>            Reporter: André Kelpe
> The current vertex API only supports simple graphs, meaning that there can 
> only ever be one edge between two vertices. Many graphs like the road network 
> are in fact multigraphs, where many edges can connect two vertices at the 
> same time.
> Support for this could be added by introducing an Iterator<EdgeWritable> 
> getEdgeValue() or a similar construct. Maybe introducing a slim object like a 
> Connector between the edge and the vertex is also a good idea, so that you 
> could do something like:
> {code} 
> for (final Connector<EdgeWritable, VertexWritable> conn: getEdgeValues(){
>      final EdgeWritable edge = conn.getEdge();
>      final VertexWritable otherVertex = conn.getOther();
>      doInterestingStuff(otherVertex);
>      doMoreInterestingStuff(edge);
> }
> {code} 

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