1. A computer-implemented method for determining, in directed graph data maintained in a distributed computing system and describing a directed graph that includes a plurality of vertices for which processing is performed in parallel in a series of synchronized iterations, shortest distances between a source vertex and other vertices from the plurality of vertices, the directed graph data including one or more reference vertices from the plurality of vertices, the method comprising: receiving, at a particular vertex from the plurality of vertices, an update message indicating a distance between the particular vertex and a particular remote vertex, wherein a remote vertex comprises a source vertex or a reference vertex; in response to receiving the update message, updating a distance value for the particular vertex indicating a distance from the particular vertex to the particular remote vertex; determining that the particular vertex is a reference vertex and that the particular remote vertex is the source vertex; in response to determining that the particular vertex is a reference vertex and that the particular remote vertex is the source vertex, sending a global update message including the updated distance value to at least two or more of the plurality of vertices in the directed graph data.