The Pregel paper introduces a vertex-centric, large-scale graph computational model. Interestingly, the name Pregel comes from the name of the river which the Seven Bridges of Königsberg spanned.
The system takes as input a directed graph with properties assigned to both vertices and edges. The computation consists of a sequence of iterations, called supersteps. In each superstep, a user-defined function is invoked on each vertex in parallel. This function essentially implements the algorithm by specifying the behaviour of a single vertex V during a single superstep S. The function can read messages sent to the vertex V during the previous superstep (S-1), change the state of the vertex or its out-going edges', mutate the graph topology by adding/removing vertices or edges and by sending messages to other vertices that would be received in the next superstep (S+1). Since all computation during a superstep is performed locally, th
