Malak, Michael (1 July 2016). Spark GraphX in Action. Manning. p. 89. ISBN9781617292521. https://books.google.com/books?id=8XcPjwEACAAJ. "Pregel and its little sibling aggregateMessages() are the cornerstones of graph processing in GraphX. ... algorithms that require more flexibility for the terminating condition have to be implemented using aggregateMessages()"