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The Pregel processing model

As an optimization for batch processing graphs, the bulk synchronous parallel (BSP) model of computation [70] has become popular. Among others, it is implemented by Apache Giraph [37], Spark’s GraphX API, and Flink’s Gelly API [71]. It is also known as the Pregel model, as Google’s Pregel paper popularized this approach for processing graphs [72].

Recall that in MapReduce, mappers conceptually “send a message” to a particular call of the reducer because the framework collects together all the mapper outputs with the same key. A similar idea is behind Pregel: one vertex can “send a message” to another vertex, and typically those messages are sent along the edges in a graph.

In each iteration, a function is called for each vertex, passing it all the messages that were sent to it—much like a call to the reducer. The difference from MapReduce is that in the Pregel model, a vertex remembers its state in memory from one iteration to the next, so the function only needs to process new incoming messages. If no messages are being sent in some part of the graph, no work needs to be done.

It’s a bit similar to the actor model (see “Distributed actor frameworks” on page 138), if you think of each vertex as an actor, except that vertex state and messages between vertices are fault-tolerant and durable, and communication proceeds in fixed rounds: at every iteration, the framework delivers all messages sent in the previous iteration. Actors normally have no such timing guarantee.

 
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