Pregel-Like System-1

Since Google's Pregel is not public,open-source Pregel-like systems have been developed recently.Key feature:adopting a message passing model for programming and communication VS a shared memory abstraction.

Apache Giraph:written in Java,built on top of Hadoop.Mind Facebook's contributions to its good performance and scalability.Support multithreading in order to avhieve fine grain parallelism;Old version maintains vertices,edges and messages as native ;new one serializing the edges and messages into byte arrays to reduce the number of objects;Superstep Splitting technique splits a message-heavy superstep into several steps to make sure the number of message transmitted in each step doesnot exceed the memory size,only effective when a received message can be aggregated to a value.