Saturday, April 18, 2015

GraphX: Graph processing in a distributed dataflow framework

This paper appeared in OSDI'14, and is authored by Joseph E. Gonzalez, University of California, Berkeley; Reynold S. Xin, University of California, Berkeley, and Databricks; Ankur Dave, Daniel Crankshaw, and Michael J. Franklin, University of California, Berkeley; Ion Stoica, University of California, Berkeley, and Databricks. This link includes video and slides which are useful to understand the paper.

This paper comes from the AMP lab at UC Berkeley. (Nice name! AMP stands for Algorithms, Machines, and People.) This lab brought to us Spark and GraphLab. And this paper is a logical successor. This paper is about marrying Spark (dataflow systems) with GraphLab (graph-processing systems).


Here is the motivation for this merger. In large-scale computation, we need both dataflow processing and graph processing systems. Graph-processing systems outperform dataflow processing systems by an order of magnitude for iterative computations on graphs (e.g., connected-component analysis, PageRank analysis). Unfortunately, it is very cumbersome to use two different tools and convert data back and forth between the two. The pipeline becomes very inefficient.

The paper sees an opportunity to unify the two tools (using a narrow-waist data/graph representation in the form of mrTriplets) and provide a single system to address the entire analytics pipeline.

GraphX is actually a thin abstraction layer on top of Spark that provides a conversion from graph computation to dataflow operations (Join, Map, GroupBy). During this reduction from graph computation to dataflow patterns, GraphX applies optimizations based on lessons learned in earlier work on efficient graph-processing (e.g., GraphLab).


GraphX introduces a range of optimizations.

As the programming abstraction GraphX introduces a normalized representation of graphs logically as a pair of vertex and edge property collections. This is called the triplet view.
The GroupBy stage gathers messages destined to the same vertex, an intervening Map operation applies the message sum to update the vertex property, and the Join stage scatters the new vertex property to all adjacent vertices.  This allows GraphX to embed graphs in a distributed dataflow framework. Flexible vertex-cut partitioning is used to encode graphs as horizontally partitioned collections and match the state of the art in distributed graph partitioning.

Here vertex mirroring approach substantially reduces communication for two reasons. First, real-world graphs commonly have orders of magnitude more edges than vertices. Second, a single vertex may have many edges in the same partition, enabling substantial reuse of the vertex property.

As another optimization learned from graph-processing systems, GraphX performs active vertices tracking. In graph algorithms, as algorithm converges, the set of active vertices shrink significantly, and this optimization avoids, wasteful work. GraphX tracks active vertices by restricting the graph using the subgraph operator. The vertex predicate is pushed to the edge partitions, where it can be used to filter the triplets.

GraphX programming

While graph-processing systems, and most famously Pregel, advocated a "think like a vertex" approach to programming, the GraphX programming model is closer to thinking about transformations on data. This may require some getting used to for programmers not familiar with dataflow programming and database operations.


Comparison to Naiad

If you are familiar with the Naiad project, you might be thinking: "Well, Naiad solves the unified general purpose dataflow & graph processing problem and throws in stream-processing and dynamic graphs for good measure". (GraphX does not support dynamic graphs.) So, what are the contributions differences in GraphX over Naiad?

I am new to the dataflow systems domain, and don't know enough to give a more authoritative answer. The contributions in GraphX may be mostly in the idea and academic contributions form. I think the idea of representing graph computations back to dataflow systems is nice. Unfortunately the GraphX paper does not compare with Naiad in terms of performance. And, after the OSDI presentation, there were couple questions/complaints about this point.

GitHub page of the GraphX project

GraphX is available as opensource on GitHub.

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