com.bigdata.rdf.graph

Interface IGraphAccessor

All Known Implementing Classes:

BigdataGASEngine.BigdataGraphAccessor, RAMGASEngine.RAMGraphAccessor, SAILGASEngine.SAILGraphAccessor

public interface IGraphAccessor

Interface abstracts access to a backend graph implementation.

Author:

Bryan Thompson

Method Summary

Methods

Modifier and Type	Method and Description
void	advanceView() Hook to advance the view of the graph.
VertexDistribution	getDistribution(Random r) Obtain a weighted distribution of the vertices in the graph from which samples may then be taken.
long	getEdgeCount(IGASContext<?,?,?> ctx, org.openrdf.model.Value u, EdgesEnum edges) Return the #of edges of the specified type for the given vertex.
Iterator<org.openrdf.model.Statement>	getEdges(IGASContext<?,?,?> ctx, org.openrdf.model.Value u, EdgesEnum edges) Return the edges for the given vertex.

Method Detail

getEdgeCount

long getEdgeCount(IGASContext<?,?,?> ctx,
                org.openrdf.model.Value u,
                EdgesEnum edges)

Return the #of edges of the specified type for the given vertex.

Note: This is not always a flyweight operation due to the need to filter for only the observable edge types. If this information is required, it may be best to cache it on the vertex state object for a given IGASProgram.

Parameters:

ctx - The IGASContext.

u - The vertex.

edges - Typesafe enumeration indicating which edges should be visited.

Returns:

An iterator that will visit the edges for that vertex.

getEdges

Iterator<org.openrdf.model.Statement> getEdges(IGASContext<?,?,?> ctx,
                                             org.openrdf.model.Value u,
                                             EdgesEnum edges)

Return the edges for the given vertex.

Parameters:

ctx - The IGASContext.

u - The vertex.

edges - Typesafe enumeration indicating which edges should be visited.

Returns:

An iterator that will visit the edges for that vertex.

advanceView

void advanceView()

Hook to advance the view of the graph. This is invoked at the end of each GAS computation round for a given IGASProgram.

getDistribution

VertexDistribution getDistribution(Random r)

Obtain a weighted distribution of the vertices in the graph from which samples may then be taken. The weight is the #of in-edges and out-edges in which a given vertex appears. Statements that model property values and statement that model link attributes SHOULD NOT be counted in this sample.

Parameters:

r - A random number generator that (a) MAY be used to select the sample distribution (the distribution does not need to be exhaustive so long as it obeys the sampling requirements); and (b) WILL be used to select vertices from that distribution.

Returns:

The distribution. TODO SAMPLING: While the same distribution is used for all SAIL implementations, and while all implementations accept and use a random seed, there is NOT a guarantee the different implementations will sample the same vertices from the same graph. This is because the order in which the vertex sample is collected and the natural order of the vertices currently depends on the implementation objects.

This COULD be fixed if we required the implementation to put the sampled vertices into an order based on their external representation as RDF Values, but still reporting their internal since that is what is used to actually specify a starting vertex of the GAS program. However, even then, implementation specific characteristics might not provide us with the same vertices (e.g., bigdata imposes a canonical mapping on xsd:dateTime).