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From Benjamin Heitmann <benjamin.heitm...@deri.org>
Subject Question about TextInputFormat pattern for parsing e.g. RDF
Date Thu, 08 Mar 2012 15:14:41 GMT
Hello again, 

I am wondering if it would be possible to parse RDF input files from a TextInputFormat class.


The most suitable text format for RDF is called "NTriples", and it has this very simple format:


subject1 predicate1 object1 .\n
subject1 predicate2 object2 .\n
...

So each line contains the subject, which is a vertex, a predicate, which is a typed edge,
and the object, which is another vertex. 
Then the line is terminated by a dot and a new-line. 

In Giraph terms, the result of parsing the first line would be the creation of a vertex for
subject1 with an edge of type predicate1, 
and then the creation of a second vertex for object1. So two vertices need to be created for
that one line. 

Now the second line contains more information about the vertex subject1. 
So in Giraph terms, the vertex which was created for subject1 needs to be retrieved/revisited
and an edge of type predicate2, 
which points to the new vertex object2 needs to be created. And vertex object2 needs to be
created. 

Just to point it out, such RDF NTriples files are unsorted, so information about the same
vertex might appear e.g. at the first and at the last line 
of a multiple GB big file. 

Which interface can be used in a TextInputFormat/VertexReader in order to find an already
created vertex ? 

Are there any other issues when VertexReader.getCurrentVertex() creates two vertices at the
same time ? 


A second related question: 
If I have multiple formats for my input files, how would I implement that ? 
Just by adding a switch to the logic in getCurrentVertex() ? Or is there a better way to switch
the input logic based on the file type ? 
All my input files would result in the same kind of Vertex being created. 


My motivation for doing this, in short: 
I have a large amount of RDF NTriples data which is provided by DBPedia. It amounts to somewhere
between 5 GB and 20 GB, 
depending on which subset is used. Expressing this RDF data, so that each vertex is completely
described in one text line, 
would require me to load it into an RDF store first, and then reprocess the data. In terms
of RDF stores, that is already a non-trivial amount of data
requiring quite a bit of hardware and tweaking. That is the reason why it would be valuable
to directly load the RDF data into Giraph. 


cheers, Benjamin. 


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