AliBaba 2.0-alpha1 - Metadata Server

Metadata Server

The Metadata Server is a general purpose HTTP persistence store. It combines a key/value store with an RDF store to provide general purpose storage and retrieval with searchable metadata.

To start the metadata server run the provided bin/metadata-server.sh (or .bat) file with the main class org.openrdf.server.metadata.Server. The server has optional command line options to assign the repository, data directory, and web directory. For details run the server with the '-h' option.

Once the server is started, it can be accessed from the assigned port (8080 by default). Information can be added to the server via the HTTP PUT method from any supporting HTTP client. Here is an example using the curl client.

 curl -X PUT -H "Content-Type: text/html" --data-binary @welcome.html \
  http://localhost:8080/welcome.html

Once the above command is run successfully, the server will return the a copy of the welcome.html file when accessing the http://localhost:8080/welcome.html URL. The server's copy of a file can be removed using the HTTP DELETE method.

When an RDF file is PUT onto the server, the contents of the file are indexed in the RDF store using the target URL as the named graph. If a client asks for the contents of the graph in a different RDF format, the server will redirect the client and return the indexed graph in the requested format, as shown here.

 curl -X PUT -H "Content-Type: application/x-turtle" --data-binary @my-graph.ttl \
  http://localhost:8080/my-graph
 curl -X GET -L -H "Accept: application/rdf+xml" \
  http://localhost:8080/my-graph

If the client requests a URL as RDF and no RDF graph has been uploaded, the server will search the RDF store and redirect the client to DESCRIBE the RDF resource in the format requested. This allows the metadata server to participate as a linked data node, by returning something useful for any resource request.

The metadata server is easily extended to support other operations or methods. Server extensions can be passed to the server from the command line as an OWL ontology or as a Java JAR file.

By starting the server with the file in Figure 1, any HTML resource with the dc:creator property can be followed using the suffix ?creator.

Figure 1. creator-operation.ttl

@prefix dc:<http://purl.org/dc/elements/1.1/>.
@prefix meta:<http://www.openrdf.org/rdf/2009/metadata#>.
@prefix owl:<http://www.w3.org/2002/07/owl#>.
@prefix rdfs:<http://www.w3.org/2000/01/rdf-schema#>.
@prefix rdf:<http://www.w3.org/1999/02/22-rdf-syntax-ns#>.

dc:creator a rdf:Property;
  meta:operation "creator";
  rdfs:domain <urn:mimetype:text/html>.

For example, if the graph in Figure 2 is uploaded to the server, the request for http://localhost:8080/welcome.html?creator will be redirected to http://localhost:8080/somebody.

Figure 2. creator-graph.ttl

@prefix dc:<http://purl.org/dc/elements/1.1/>.

<http://localhost:8080/welcome.html> dc:creator <http://localhost:8080/somebody>.

The dc:creator property can also be changed by issuing the following PUT request.

 curl -X PUT -H "Content-Location: me" \
  http://localhost:8080/welcome.html?creator

Furthermore if the server is started with the file in Figure 3, the result of the query can be accessed using any HTTP client as shown below using curl.

Figure 3. text-html.ttl

@prefix rdfs:<http://www.w3.org/2000/01/rdf-schema#>.
@prefix owl:<http://www.w3.org/2002/07/owl#>.
@prefix obj:<http://www.openrdf.org/rdf/2009/object#>.
@prefix meta:<http://www.openrdf.org/rdf/2009/metadata#>.

<#text-html> rdfs:subClassOf obj:Message;
  meta:method "GET";
  rdfs:subClassOf [owl:onProperty obj:target;
    owl:allValuesFrom [owl:oneOf (<http://localhost:8080/text-html>)]];
  rdfs:subClassOf [owl:onProperty obj:objectResponse; owl:cardinality 1];
  rdfs:subClassOf [owl:onProperty obj:objectResponse;
    owl:allValuesFrom obj:TupleResult];
  obj:sparql """
SELECT ?html WHERE { ?html a <urn:mimetype:text/html> }
""".

 curl -X GET -H "Accept: application/sparql-results+json" \
  http://localhost:8080/text-html

The server can also be extended through Java classes. If the files in Figure 4 are compiled and included in a JAR or directory on the command line, the server will execute the method for all POST operations on HTML files.

Figure 4. ChangeCreator

// Document.java
import org.openrdf.repository.object.annotations.rdf;
import org.openrdf.server.metadata.annotations.operation;

@rdf("urn:mimetype:text/html")
public interface Document {
  @operation("creator")
  @rdf("http://purl.org/dc/elements/1.1/creator")
  Object getCreator();
  void setCreator(Object creator);
}

// ChangeCreator.java
import org.openrdf.repository.object.*;
import org.openrdf.server.metadata.annotations.method;
import java.util.Map;

public abstract class ChangeCreator implements Document, RDFObject {
  @method("POST")
  public Document changeCreator(Map<String, String[]> param) throws Exception {
    String creator = param.get("creator")[0];
    ObjectConnection con = getObjectConnection();
    setCreator(con.getObject(creator));
    return this;
  }
}

# empty file META-INF/org.openrdf.concepts

# empty file META-INF/org.openrdf.behaviours

As shown below using curl.

 curl -X POST -d "creator=http://localhost:8080/myself" \
  http://localhost:8080/welcome.html

The metadata server is immediately useful as an RDF web server by exposing the contents of an RDF repository over HTTP requests via linked data. Because the metadata server is easily extended, it can scale from a simple static file server to an application controller that facilitates structuring applications around the resources they serve.

Site contents

Metadata Server