Many merge operations can be broken up to work incrementally. For example, if the merge operation is to sum *n* rank updates, then you can set mergeMsg = (a, b) => a + b and this function will be applied to all *n* updates in arbitrary order to yield a final sum. Addition, multiplication, min, max, and mean are operations that work in this manner (they are associative and commutative).
If you absolutely must operate on all *n* messages at once, for example to find the median, then a workaround is to emit Array(m) instead of m in the sendMsg function, and then to set mergeMsg = (a, b) => a ++ b. This will accumulate all inbound messages into an array which you can access in vprog. However, it will be much slower for graphs with high-degree vertices, because the accumulated arrays can grow very large. Ankur <http://www.ankurdave.com/>
