Roman K commented on GIRAPH-141:

On one hand, the problem with two separate constructions for two graph types 
(like 2 separate vertex implementations ) is that it would lead to some 
code-duplications, though not too many but there will be.
On the other hand, if you chose to implement the simple graph as the private 
case of multigraph, it would have some memory overhead (i.e. edge value for 
simple graph per vertex is 4 bytes vs 20 bytes, which turns into additional 2MB 
in heap for 1M vertices) and some slight performance impact (which, in my 
opinion, can be neglected) and the simplicity of the API could be somewhat 
In my opinion, creation of two separate APIs is less preferable because it 
would complicate things for people who want, for some reason, migrate their 
simple graph to multigraph.
> 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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