The problem is that if setNewVertexValue is called more than once, it sends each new value to the out Collector, and these all end up in the workset, but then the coGroups in the two descendants of MessagingUdfWithEdgeValues use only the first value in the state Iterable. I see three ways to resolve this:
1. Add it to the documentation that setNewVertexValue should only be called once, and optionally add a check for this.
2. In setNewVertexValue, do not send the newValue to the out Collector at once, but only record it in outVal, and send the last recorded value after updateVertex returns.
3. Iterate over the entire Iterable in MessagingUdfWithEVsSimpleVV.coGroup and MessagingUdfWithEVsVVWithDegrees.coGroup. (This would probably still need some documentation addition.)
I like 2. the best. What are your opinions?