Declarative Patterns for Imperative Distributed Graph Algorithms

We provide an abstraction for expressing graph algorithms in which the vertices and edges of the graph provide locality and communication structure and graph data are represented by property maps that associate vertices and edges to arbitrary user-defined data. Operations on the graph are expressed as patterns, which allow limited traversal of the graph and modification of property maps for the traversed fragments of the graph. Traversal is implicit, and is automatically computed from the pattern's access of property map values. Patterns are declarative, but they can be used in imperative algorithms by using strategies that run in epochs. Strategies are user defined programs that apply patterns in a certain way (e.g., We provide fixed point, once, and ?-stepping strategies), including chaining patterns in an arbitrary way. Patterns are applied in epochs, which provide synchronization across a distributed system, guaranteeing that all patterns have been applied by the end of an epoch.