RDF in Solr: HowTo
The Solr built-in UpdateRequestHandler supports several formats of input data. It delegates the actual data loading to a specific ContentStreamLoader, depending on the content type of the incoming request (i.e. the Content-type header of the HTTP request). Currently, these are the available content types declared in the UpdateRequestHandler class:
- application/xml or text/xml
- application/json or text/json
- application/csv or text/csv
- application/javabin
So, a client has several options to send its data to Solr; all what it needs is to prepare those data in a specific format and call the UpdateRequestHandler (usually located at /update endpoint) specifying the corresponding content type
curl http://localhost:8080/solr/update
-H "Content-Type: text/json" \
--data-binary @/home/agazzarini/data.json
The UpdateRequestHandler can be extended, customized, and replaced; so we can write our own UpdateRequestHandler that accepts a custom format, adding a new content type or overriding the default set of supported content types.
In this brief post, I will describe how to use Jena to load RDF data in Solr, in any format supported by Jena IO API.
This is a quick and easy task mainly because:
- the UpdateRequestHandler already has the logic to index data
- the UpdateRequestHandler can be easily extended
- Jena already provides all the parsers we need
So doing that, is just a matter of subclassing UpdateRequestHandler in order to override the content type registry:
public class RdfDataUpdateRequestHandler extends UpdateRequestHandler {
protected Map createDefaultLoaders(NamedList parameters) {
final Map<String, ContentStreamLoader> registry =
new HashMap<String, ContentStreamLoader>();
final ContentStreamLoader loader = new RdfDataLoader();
for (final Lang language : RDFLanguages.getRegisteredLanguages()) {
registry.put(language.getContentType().toHeaderString(), loader);
}
return registry;
}
- text/turtle
- application/turtle
- application/x-turtle
- application/rdf+xml
- application/rdf+json
- application/ld+json
- text/plain (for n-triple)
- application/n-triples
- (others)
Our RdfDataLoader will be in charge to parse and load the data. Note that the above list is not exhaustive, there a lot of other content types registered in Jena (See the RDFLanguages class).
So, what about the format of the data? Of course, it still depends on the content type of your RDF data, and most important, it has nothing to do with those data we used to send to Solr (i.e. SolrInputDocuments serialised in some format).
The RdfDataLoader is a subclass of ContentStreamLoader
public class RdfDataLoader extends ContentStreamLoader
public void load()
SolrQueryRequest request,
SolrQueryResponse response,
ContentStream stream,
UpdateRequestProcessor processor) throws Exception {
PipedRDFIterator<Triple> iterator =
new PipedRDFIterator<Triple>();
PipedRDFStream<Triple> inputStream =
new PipedTriplesStream(iterator);
// We use an executor for running the parser in a separate thread
ExecutorService executor = Executors.newSingleThreadExecutor();
Runnable parser = new Runnable() {
public void run() {
try {
RDFDataMgr.parse(
inputStream,
stream.getStream(),
RDFLanguages.contentTypeToLang(stream.getContentType()));
} catch (final IOException exception) {
...
}
}
};
executor.submit(parser);
while (iterator.hasNext()) {
Triple triple = iterator.next();
// create and populate the Solr input document
SolrInputDocument document = new SolrInputDocument();
...
// create the update command
AddUpdateCommand command = new AddUpdateCommand(request);
command.solrDoc = document;
processor.processAdd(command);
}
}
That’s all! Once the request handler has been registered in Solr we can send RDF data to Solr using a command like this:
curl http://localhost:8080/solr/store/update \
-H "Content-Type: application/n-triples" \
--data-binary @/home/agazzarini/triples_dogfood.nt