Aggregators
Description
which allow global communication by receiving messages from multiple vertices, combining them and sending the result back to the vertices. They are useful for statistics (think of an histogram of vertex degrees) or for global controlling
Quoting from Pregel paper:
aggregators are a mechanism for global communica- tion, monitoring, and data. Each vertex can provide a value to an aggregator in superstep S, the system combines those values using a reduction operator, and the resulting value is made available to all vertices in superstep S + 1.