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From Avery Ching <ach...@apache.org>
Subject Re: Question about TextInputFormat pattern for parsing e.g. RDF
Date Mon, 12 Mar 2012 20:04:58 GMT
Sorry for the delayed response.  Responses inline.

Avery

On 3/8/12 7:14 AM, Benjamin Heitmann wrote:
> 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 ?

This is not possible unfortunately.  It's similar to the Hadoop 
InputFormat.  Vertices (analogous to key-value pairs) are read one at a 
time.  They are not saved for later access (just like Hadoop).

> 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.
>

My suggestion would be the following:

Run a MR job to join all your RDFs on the vertex key and you can either 
convert them to an easy format to parse with a custom VertexInputFormat 
of your choice.  If these are one way relationships, you need not create 
the target vertex.  If they are undirect relationships, when you are 
processing your RDFs in the MR job, add a directed relationship to both 
vertices.

> cheers, Benjamin.
>


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