Benjamin Heitmann commented on GIRAPH-170:

Regarding GIRAPH-141, 
I don't think that true multigraph support is required for Giraph in order to 
use RDF data. 

If I have "subject1 predicate1 object1" and "subject1 predicate1 object2", then 
there will be a total of three vertices with 2 edges, without any conflict. If 
I have the same triple "subject1 predicate1 object1" two or more times, then 
the RDF semantics document states that all of these triples refer to the same 
two vertices and the edge between them in the RDF graph. So there is no need 
for a multigraph again. 

If we introduce literals into the mix, then we have the same thing as above, if 
each literal will be presented by its own Giraph vertex. 

I am not sure if I missed anything, but multigraphs dont seem to be the issue 
here, neither in theory, nor for my already working code. 

An issue which would be more important, is the capability to retrieve and 
modify an already created node from inside the TextVertexInputFormat class (as 
explained above). 
> Workflow for loading RDF graph data into Giraph
> -----------------------------------------------
>                 Key: GIRAPH-170
>                 URL: https://issues.apache.org/jira/browse/GIRAPH-170
>             Project: Giraph
>          Issue Type: New Feature
>            Reporter: Dan Brickley
>            Priority: Minor
> W3C RDF provides a family of Web standards for exchanging graph-based data. 
> RDF uses sets of simple binary relationships, labeling nodes and links with 
> Web identifiers (URIs). Many public datasets are available as RDF, including 
> the "Linked Data" cloud (see http://richard.cyganiak.de/2007/10/lod/ ). Many 
> such datasets are listed at http://thedatahub.org/
> RDF has several standard exchange syntaxes. The oldest is RDF/XML. A simple 
> line-oriented format is N-Triples. A format aligned with RDF's SPARQL query 
> language is Turtle. Apache Jena and Any23 provide software to handle all 
> these; http://incubator.apache.org/jena/ http://incubator.apache.org/any23/
> This JIRA leaves open the strategy for loading RDF data into Giraph. There 
> are various possibilites, including exploitation of intermediate 
> Hadoop-friendly stores, or pre-processing with e.g. Pig-based tools into a 
> more Giraph-friendly form, or writing custom loaders. Even a HOWTO document 
> or implementor notes here would be an advance on the current state of the 
> art. The BluePrints Graph API (Gremlin etc.) has also been aligned with 
> various RDF datasources.
> Related topics: multigraphs https://issues.apache.org/jira/browse/GIRAPH-141 
> touches on the issue (since we can't currently easily represent fully general 
> RDF graphs since two nodes might be connected by more than one typed edge). 
> Even without multigraphs it ought to be possible to bring RDF-sourced data
> into Giraph, e.g. perhaps some app is only interested in say the Movies + 
> People subset of a big RDF collection.
> From Avery in email: "a helper VertexInputFormat (and maybe 
> VertexOutputFormat) would certainly [despite GIRAPH-141] still help"

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