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Quoting from Pregel paper:
aggregators are a mechanism for global communica- tioncommunication, 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.
which allow global communication by receiving messages from multiple vertices, combining them and sending the result back to the vertices. They You can think aggregators as a tree, that the leafs (which are the graph vertices) are sending messages to the root (the master aggregator), and then the root is combining all these messages to a value. In the end, this combined values is distributed back to the leafs. Aggregators are useful for statistics (think of an histogram of vertex degrees) or for global controlling.