Mulgara

= Connection API =

== Overview ==

Mulgara now supports a new interface called a `Connection`. This will be replacing the current `ItqlInterpreterBean` interface as the full suite of utilities becomes available.

The existing `ItqlInterpreterBean` interface provides a completely automated connection mechanism, based on the URLs of graphs referenced in TQL queries. While convenient, this has several undesirable effects:

 * All queries issued to this interface must contain at least one URL for a graph on the server that will handle the query.

 * Arbitrary graph names are not permitted. Graph names must follow a particular format, and will include the hostname of the server containing the graph.

 * Queries against graphs must be changed when the server hosting the graph changes.

 * Connections to a server cannot be cached.

 * Distributed queries (queries where the requested data comes from more than one server) are sent to an arbitrary server and not necessarily the most desirable one.

 * Query languages which use a default graph name (like [http://www.w3.org/TR/rdf-sparql-query/ SPARQL]) cannot identify a server if no graph names are given.

The `Connection` interface avoids these issues by establishing a link to a specified server. This link can be to a server running in the current process, over inter-process-communication (IPC) to a server running on the same machine, or across a network. Queries and commands can be sent along this link, and all operations will be performed by the server that has been connected to.

The `Connection` interface and its supporting classes are found in the `org.mulgara.connection` package.

== Connection Factory ==

While Connections can be created directly, the `ConnectionFactory` class attempts to manage connections to ensure that the correct type is created, and reduce network overhead. The connections returned by a `ConnectionFactory` are only managed by that factory, and will not interfere with connections from a separate factory instance.

`Connection`s can be established based on the URL of a database (in the form ''protocol''://''host''/''service'') or by wrapping a `Session` instance. The `Session` interface has existed in Mulgara for some time, and it may be more appropriate for existing code to use its internal `Session` instances to create new `Connection`s.

For the moment, the only supported protocol is ''rmi''. Other protocols, including ''http'' are expected shortly.

== Using a Connection ==

=== Commands ===

Once a `Connection` has been created, commands can be sent on it using the `execute` method. Alternatively, commands also have their own `execute` method that can be applied to a connection, with the same effect.

The following is an example of using a Connection to create a graph, and load a file into that graph:

{{{

#!java

    // define the server we want to connect to

    URI serverURI = new URI("rmi://localhost/server1");

    // define the file we want to load

    URI dataFile = new File("data.rdf").toURI();

    // define the name of the graph to load the data into

    URI myData = new URI("test:data");

    // Create a factory, and connect to the server

    ConnectionFactory factory = new ConnectionFactory();

    Connection connection = factory.newConnection(serverUri);

    // execute a CREATE command

    connection.execute(new CreateGraph(myData));

    // execute a LOAD command

    connection.execute(new Load(dataFile, myData, true));

    // cleaning up the connection allows the network resources to be re-used

    connection.close();

}}}

Executing a command returns a String containing a message related to the success of the command. If commands are kept, then it is also possible to retrieve the last message from a command with the `getResultMessage` method.

=== Queries ===

Queries are a type of command that return an `Answer` instead of a message string. Queries can be created using either the TQL or SPARQL parsers:

{{{

#!java

    // define the server we want to connect to

    URI serverURI = new URI("rmi://localhost/server1");

    // Create a factory, and connect to the server

    ConnectionFactory factory = new ConnectionFactory();

    Connection connection = factory.newConnection(serverUri);

    // Use a SPARQL query

    Query query = new SparqlInterpreter().parseQuery(

                  "SELECT * FROM <test:data> WHERE { ?s ?p ?o }");

    Answer answer = connection.execute(query);

    // display the result

    answer.beforeFirst();

    while (answer.next()) {

      System.out.print(" " + answer.getObject(0));

      System.out.print(" " + answer.getObject(1));

      System.out.println(" " + answer.getObject(2));

    }

    answer.close();

    connection.close();

}}}

A working example program for issuing SPARQL queries is provided with the source code. It can be found on the following path:

  [http://mulgara.org/svn/mulgara/tags/release-2.0-alpha-final/tools/src/org/mulgara/tools/Sparql.java tools/src/org/mulgara/tools/Sparql.java]