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        <h1 id="examples">Examples</h1>
<p>This chapters shows some real examples of usage of the Tracker
SPARQL Library.</p>
<h2 id="querying-a-remote-endpoint">Querying a remote endpoint</h2>
<p>All SPARQL queries happen on a <a href="tracker-sparql-connection.html#TrackerSparqlConnection" data-gi-href-javascript="tracker-sparql-connection.html#TrackerSparqlConnection" data-gi-title-javascript="Tracker.SparqlConnection" data-gi-href-python="tracker-sparql-connection.html#TrackerSparqlConnection" data-gi-title-python="Tracker.SparqlConnection">TrackerSparqlConnection</a>, often these
connections represent a remote endpoints maintained by another process or
server.</p>
<p>This example demonstrates the use of these connections on a remote
endpoint. Concretely creating a D-Bus <a href="tracker-sparql-connection.html#TrackerSparqlConnection" data-gi-href-javascript="tracker-sparql-connection.html#TrackerSparqlConnection" data-gi-title-javascript="Tracker.SparqlConnection" data-gi-href-python="tracker-sparql-connection.html#TrackerSparqlConnection" data-gi-title-python="Tracker.SparqlConnection">TrackerSparqlConnection</a>,
creating a prepared statement from a SPARQL query string, executing
the query, and obtaining the query results from the cursor.</p>
<p>The <a href="tracker-sparql-connection.html#tracker_sparql_connection_query_statement" data-gi-href-javascript="tracker-sparql-connection.html#tracker_sparql_connection_query_statement" data-gi-title-javascript="Tracker.SparqlConnection.prototype.query_statement" data-gi-href-python="tracker-sparql-connection.html#tracker_sparql_connection_query_statement" data-gi-title-python="Tracker.SparqlConnection.query_statement">tracker_sparql_connection_query_statement</a> function can be used
to obtain a <a href="tracker-sparql-statement.html#TrackerSparqlStatement" data-gi-href-javascript="tracker-sparql-statement.html#TrackerSparqlStatement" data-gi-title-javascript="Tracker.SparqlStatement" data-gi-href-python="tracker-sparql-statement.html#TrackerSparqlStatement" data-gi-title-python="Tracker.SparqlStatement">TrackerSparqlStatement</a> object holding a prepared SPARQL
query that can then be executed with <a href="tracker-sparql-statement.html#tracker_sparql_statement_execute" data-gi-href-javascript="tracker-sparql-statement.html#tracker_sparql_statement_execute" data-gi-title-javascript="Tracker.SparqlStatement.prototype.execute" data-gi-href-python="tracker-sparql-statement.html#tracker_sparql_statement_execute" data-gi-title-python="Tracker.SparqlStatement.execute">tracker_sparql_statement_execute</a>.
The query string can contain <code>~name</code> placeholders which can be replaced with
arbitrary values before query execution with
<a href="tracker-sparql-statement.html#tracker_sparql_statement_bind_string" data-gi-href-javascript="tracker-sparql-statement.html#tracker_sparql_statement_bind_string" data-gi-title-javascript="Tracker.SparqlStatement.prototype.bind_string" data-gi-href-python="tracker-sparql-statement.html#tracker_sparql_statement_bind_string" data-gi-title-python="Tracker.SparqlStatement.bind_string">tracker_sparql_statement_bind_string</a> and similar functions.
This allows parsing the query string only once and to execute it multiple
times with different parameters with potentially significant performance gains.</p>
<p>Multiple functions offer asynchronous variants, so the application
main loop is not blocked while these operations are executed.</p>
<p>Once you end up with the query, remember to call <a href="tracker-sparql-cursor.html#tracker_sparql_cursor_close" data-gi-href-javascript="tracker-sparql-cursor.html#tracker_sparql_cursor_close" data-gi-title-javascript="Tracker.SparqlCursor.prototype.close" data-gi-href-python="tracker-sparql-cursor.html#tracker_sparql_cursor_close" data-gi-title-python="Tracker.SparqlCursor.close">tracker_sparql_cursor_close</a>.
The same applies to <a href="tracker-sparql-connection.html#tracker_sparql_connection_close" data-gi-href-javascript="tracker-sparql-connection.html#tracker_sparql_connection_close" data-gi-title-javascript="Tracker.SparqlConnection.prototype.close" data-gi-href-python="tracker-sparql-connection.html#tracker_sparql_connection_close" data-gi-title-python="Tracker.SparqlConnection.close">tracker_sparql_connection_close</a> when no longer needed.</p>
<div class="gi-lang-c">
<pre><code class="language-c">#include &lt;libtracker-sparql/tracker-sparql.h&gt;

#define REMOTE_NAME "org.freedesktop.Tracker3.Miner.Files"

int main (int argc, const char **argv)
{
  g_autoptr (GError) error = NULL;
  g_autoptr (TrackerSparqlConnection) connection = NULL;
  g_autoptr (TrackerSparqlCursor) cursor = NULL;
  g_autoptr (TrackerSparqlStatement) stmt = NULL;
  const char *query = "SELECT nie:url(?u) WHERE { ?u a nfo:FileDataObject ; nfo:fileName ~name }";
  const char *name = NULL;
  int i = 0;

  connection = tracker_sparql_connection_bus_new (REMOTE_NAME, NULL, NULL, &amp;error);
  if (!connection) {
    g_printerr ("Couldn't obtain a connection to the Tracker store: %s",
                error ? error-&gt;message : "unknown error");
    return 1;
  }

  /* Create a prepared statement */
  stmt = tracker_sparql_connection_query_statement (connection,
                                                    query,
                                                    NULL,
                                                    &amp;error);

  if (!stmt) {
    g_printerr ("Couldn't create a prepared statement: '%s'",
                error-&gt;message);
    return 1;
  }

  /* Bind a value to the query variables */
  name = argv[1] ? argv[1] : "";
  tracker_sparql_statement_bind_string (stmt, "name", name);

  /* Executes the SPARQL query with the currently bound values and get new cursor */
  cursor = tracker_sparql_statement_execute (stmt, NULL, &amp;error);
  if (!cursor) {
    g_printerr ("Couldn't execute query: '%s'",
                error-&gt;message);
    return 1;
  }

  /* Iterate synchronously the results */
  while (tracker_sparql_cursor_next (cursor, NULL, &amp;error)) {
    g_print ("Result [%d]: %s\n",
             i++,
             tracker_sparql_cursor_get_string (cursor, 0, NULL));
  }

  g_print ("A total of '%d' results were found\n", i);

  tracker_sparql_cursor_close (cursor);

  tracker_sparql_connection_close (connection);

  return 0;
}

</code></pre>
</div>
<div class="gi-lang-python">
<pre><code class="language-python">#!/usr/bin/python3

import gi, sys
from gi.repository import GLib, Gio, Tracker

try:
    connection = Tracker.SparqlConnection.bus_new(
        'org.freedesktop.Tracker3.Miner.Files',
        None, None)

    stmt = connection.query_statement (
        'SELECT DISTINCT nie:url(?u) WHERE { ' +
        '  ?u a nfo:FileDataObject ; ' +
        '     nfo:fileName ~name ' +
        '}', None)

    stmt.bind_string('name', sys.argv[1])

    cursor = stmt.execute()
    i = 0;

    while cursor.next():
        i += 1
        print('Result {0}: {1}'.format(i, cursor.get_string(0)[0]))

    print('A total of {0} results were found\n'.format(i))

    cursor.close()
    connection.close()

except Exception as e:
    print('Error: {0}'.format(e))
    sys.exit(-1)

</code></pre>
</div>
<div class="gi-lang-javascript">
<pre><code class="language-javascript">#!/usr/bin/gjs

const { GLib, Gio, Tracker } = imports.gi

try {
    let connection = Tracker.SparqlConnection.bus_new(
        'org.freedesktop.Tracker3.Miner.Files',
        null, null);

    let stmt = connection.query_statement (
        'SELECT DISTINCT nie:url(?u) WHERE { ' +
        '  ?u a nfo:FileDataObject ; ' +
        '     nfo:fileName ~name ' +
        '}', null);

    stmt.bind_string('name', ARGV[0]);

    let cursor = stmt.execute(null);
    let i = 0;

    while (cursor.next(null)) {
        i++;
        print(`Result ${i}: ${cursor.get_string(0)[0]}`);
    }

    print(`A total of ${i} results were found`);

    cursor.close();
    connection.close();
} catch (e) {
    printerr(`Error: ${e.message}`)
}

</code></pre>
</div>
<h2 id="creating-a-private-database">Creating a private database</h2>
<p>Applications may create private stores via the <a href="tracker-sparql-connection.html#tracker_sparql_connection_new" data-gi-href-javascript="tracker-sparql-connection.html#tracker_sparql_connection_new" data-gi-title-javascript="Tracker.SparqlConnection.prototype.new" data-gi-href-python="tracker-sparql-connection.html#tracker_sparql_connection_new" data-gi-title-python="Tracker.SparqlConnection.new">tracker_sparql_connection_new</a>
constructor.</p>
<p>This example demonstrates the creation of a private store, for simplicity the
example uses the builtin Nepomuk ontology, but the data structures may be defined
by the application, see the documentation on
<a href="defining-ontologies.html">defining ontologies</a> for more information about this.</p>
<p>The example also demonstrates the use of <a href="tracker-resource.html#TrackerResource" data-gi-href-javascript="tracker-resource.html#TrackerResource" data-gi-title-javascript="Tracker.Resource" data-gi-href-python="tracker-resource.html#TrackerResource" data-gi-title-python="Tracker.Resource">TrackerResource</a> and <a href="tracker-batch.html#TrackerBatch" data-gi-href-javascript="tracker-batch.html#TrackerBatch" data-gi-title-javascript="Tracker.Batch" data-gi-href-python="tracker-batch.html#TrackerBatch" data-gi-title-python="Tracker.Batch">TrackerBatch</a>
for insertion of RDF data. It is also possible the direct use of SPARQL update
strings via <a href="tracker-sparql-connection.html#tracker_sparql_connection_update" data-gi-href-javascript="tracker-sparql-connection.html#tracker_sparql_connection_update" data-gi-title-javascript="Tracker.SparqlConnection.prototype.update" data-gi-href-python="tracker-sparql-connection.html#tracker_sparql_connection_update" data-gi-title-python="Tracker.SparqlConnection.update">tracker_sparql_connection_update</a>.</p>
<p>Multiple functions offer asynchronous variants, so the application
main loop is not blocked while these operations are executed.</p>
<p>Once you no longer need the connection, remember to call
<a href="tracker-sparql-connection.html#tracker_sparql_connection_close" data-gi-href-javascript="tracker-sparql-connection.html#tracker_sparql_connection_close" data-gi-title-javascript="Tracker.SparqlConnection.prototype.close" data-gi-href-python="tracker-sparql-connection.html#tracker_sparql_connection_close" data-gi-title-python="Tracker.SparqlConnection.close">tracker_sparql_connection_close</a> on the <a href="tracker-sparql-connection.html#TrackerSparqlConnection" data-gi-href-javascript="tracker-sparql-connection.html#TrackerSparqlConnection" data-gi-title-javascript="Tracker.SparqlConnection" data-gi-href-python="tracker-sparql-connection.html#TrackerSparqlConnection" data-gi-title-python="Tracker.SparqlConnection">TrackerSparqlConnection</a>.</p>
<div class="gi-lang-c">
<pre><code class="language-c">#include &lt;libtracker-sparql/tracker-sparql.h&gt;

int main (int argc, const char **argv)
{
  g_autoptr (GError) error = NULL;
  g_autoptr (TrackerSparqlConnection) connection = NULL;
  g_autoptr (TrackerResource) resource = NULL;
  g_autoptr (GFile) ontology = NULL;

  /* Create a private SPARQL store */
  ontology = tracker_sparql_get_ontology_nepomuk ();
  connection = tracker_sparql_connection_new (TRACKER_SPARQL_CONNECTION_FLAGS_NONE,
					      NULL, /* Database location, NULL creates it in-memory */
					      ontology, /* Ontology location */
					      NULL,
					      &amp;error);
  if (connection) {
    g_printerr ("Couldn't create a Tracker store: %s",
                error-&gt;message);
    return 1;
  }

  /* Create a resource containing RDF data */
  resource = tracker_resource_new (NULL);
  tracker_resource_set_uri (resource, "rdf:type", "nmm:MusicPiece");

  /* Create a batch, and add the resource to it */
  batch = tracker_sparql_connection_create_batch (connection);
  tracker_batch_add_resource (batch, NULL, resource);

  /* Execute the batch to insert the data */
  if (!tracker_batch_execute (batch, NULL, &amp;error)) {
    g_printerr ("Couldn't insert batch of resources: %s",
                error-&gt;message);
    return 1;
  }

  tracker_sparql_connection_close (connection);

  return 0;
}

</code></pre>
</div>
<div class="gi-lang-python">
<pre><code class="language-python">#!/usr/bin/python3

import gi, sys
from gi.repository import GLib, Gio, Tracker

try:
    connection = Tracker.SparqlConnection.new(
        Tracker.SparqlConnectionFlags.NONE,
        None, # Database location, None creates it in-memory
        Tracker.sparql_get_ontology_nepomuk(), # Ontology location
        None)

    # Create a resource containing RDF data
    resource = Tracker.Resource.new(None)
    resource.set_uri('rdf:type', 'nmm:MusicPiece')

    # Create a batch, and add the resource to it
    batch = connection.create_batch()
    batch.add_resource(None, resource)

    # Execute the batch to insert the data
    batch.execute()

    connection.close()

except Exception as e:
    print('Error: {0}'.format(e))
    sys.exit(-1)

</code></pre>
</div>
<div class="gi-lang-javascript">
<pre><code class="language-javascript">#!/usr/bin/gjs

const { GLib, Gio, Tracker } = imports.gi

try {
    let connection = Tracker.SparqlConnection.new(
        Tracker.SparqlConnectionFlags.NONE,
        null, // Database location, None creates it in-memory
        Tracker.sparql_get_ontology_nepomuk(), // Ontology location
        null);

    // Create a resource containing RDF data
    let resource = Tracker.Resource.new(null)
    resource.set_uri('rdf:type', 'nmm:MusicPiece')

    // Create a batch, and add the resource to it
    let batch = connection.create_batch()
    batch.add_resource(null, resource)

    // Execute the batch to insert the data
    batch.execute(null)

    connection.close();
} catch (e) {
    printerr(`Error: ${e.message}`)
}

</code></pre>
</div>
<h2 id="creating-a-sparql-endpoint">Creating a SPARQL endpoint</h2>
<p>For some applications and services, it might be desirable to export a
SPARQL store as an endpoint. Making it possible for other applications to
query the data they hold.</p>
<p>This example demonstrates the use of <a href="tracker-endpoint.html#TrackerEndpoint" data-gi-href-javascript="tracker-endpoint.html#TrackerEndpoint" data-gi-title-javascript="Tracker.Endpoint" data-gi-href-python="tracker-endpoint.html#TrackerEndpoint" data-gi-title-python="Tracker.Endpoint">TrackerEndpoint</a> subclasses,
concretely the creation of a D-Bus endpoint, that other applications
may query e.g. through a connection created with
<a href="tracker-sparql-connection.html#tracker_sparql_connection_bus_new" data-gi-href-javascript="tracker-sparql-connection.html#tracker_sparql_connection_bus_new" data-gi-title-javascript="Tracker.SparqlConnection.prototype.bus_new" data-gi-href-python="tracker-sparql-connection.html#tracker_sparql_connection_bus_new" data-gi-title-python="Tracker.SparqlConnection.bus_new">tracker_sparql_connection_bus_new</a>.</p>
<div class="gi-lang-c">
<pre><code class="language-c">#include &lt;libtracker-sparql/tracker-sparql.h&gt;

int main (int argc, const char **argv)
{
  g_autoptr (GError) error = NULL;
  g_autoptr (TrackerSparqlConnection) connection = NULL;
  g_autoptr (GFile) ontology = NULL;
  g_autoptr (GMainLoop) main_loop = NULL;

  /* Create a SPARQL store */
  ontology = tracker_sparql_get_ontology_nepomuk ();
  connection = tracker_sparql_connection_new (TRACKER_SPARQL_CONNECTION_FLAGS_NONE,
					      NULL, /* Database location, NULL creates it in-memory */
					      ontology, /* Ontology location */
					      NULL,
					      &amp;error);
  if (connection) {
    g_printerr ("Couldn't create a Tracker store: %s",
                error-&gt;message);
    return 1;
  }

  /* Make the connection available via D-Bus, other applications would be
   * able to make connections to this application's bus name
   */
  endpoint = tracker_endpoint_dbus_new (connection, NULL, NULL, NULL, &amp;error);
  if (!endpoint) {
    g_printerr ("Couldn't create D-Bus endpoint: %s",
                error-&gt;message);
    return 1;
  }

  main_loop = g_main_loop_new (NULL, FALSE);
  g_main_loop_run (main_loop);

  return 0;
}

</code></pre>
</div>
<div class="gi-lang-python">
<pre><code class="language-python">#!/usr/bin/python3

import gi, sys
from gi.repository import GLib, Gio, Tracker

try:
    connection = Tracker.SparqlConnection.new(
        Tracker.SparqlConnectionFlags.NONE,
        None, # Database location, None creates it in-memory
        Tracker.sparql_get_ontology_nepomuk(), # Ontology location
        None)

    bus = Gio.bus_get_sync(Gio.BusType.SESSION, None)

    endpoint = Tracker.EndpointDBus.new(
        connection, bus, None, None)

    loop = GLib.MainLoop.new(None, False)
    loop.run()

    connection.close()

except Exception as e:
    print('Error: {0}'.format(e))
    sys.exit(-1)

</code></pre>
</div>
<div class="gi-lang-javascript">
<pre><code class="language-javascript">#!/usr/bin/gjs

const { GLib, Gio, Tracker } = imports.gi

try {
    let connection = Tracker.SparqlConnection.new(
        Tracker.SparqlConnectionFlags.NONE,
        null, // Database location, None creates it in-memory
        Tracker.sparql_get_ontology_nepomuk(), // Ontology location
        null);

    let bus = Gio.bus_get_sync(Gio.BusType.SESSION, null)

    let endpoint = Tracker.EndpointDBus.new(
        connection, bus, null, null);

    let loop = GLib.MainLoop.new(null, false);
    loop.run();

    connection.close();
} catch (e) {
    printerr(`Error: ${e.message}`)
}

</code></pre>
</div>
<h2 id="receiving-notification-on-changes">Receiving notification on changes</h2>
<p>As an additional feature over SPARQL endpoints, Tracker allows for
users of private and D-Bus SPARQL connections to receive notifications
on changes of certain RDF classes (Those with the
<a href="nrl-ontology.html#nrl:notify">nrl:notify</a> property, like
<a href="nmm-ontology.html#nmm:MusicPiece">nmm:MusicPiece</a>).</p>
<p>This example demonstrates the use of <a href="tracker-notifier.html#TrackerNotifier" data-gi-href-javascript="tracker-notifier.html#TrackerNotifier" data-gi-title-javascript="Tracker.Notifier" data-gi-href-python="tracker-notifier.html#TrackerNotifier" data-gi-title-python="Tracker.Notifier">TrackerNotifier</a> to receive
notifications on database updates.</p>
<div class="gi-lang-c">
<pre><code class="language-c">#include &lt;libtracker-sparql/tracker-sparql.h&gt;

#define REMOTE_NAME "org.freedesktop.Tracker3.Miner.Files"

void on_events (TrackerNotifier *notifier,
                const char      *service,
                const char      *graph,
                GPtrArray       *events,
                gpointer         user_data)
{
  int i;

  for (i = 0; i &lt; events-&gt;len; i++) {
    TrackerNotifierEvent *event = g_ptr_array_index (events, i);

    g_print ("Event %d on %s\n",
             tracker_notifier_event_get_event_type (event),
             tracker_notifier_event_get_urn (event));
  }
}

int main (int   argc,
          char *argv[])
{
  g_autoptr (GError) error = NULL;
  g_autoptr (TrackerSparqlConnection) connection = NULL;
  g_autoptr (TrackerNotifier) notifier = NULL;
  g_autoptr (GMainLoop) main_loop = NULL;

  /* Create connection to remote service */
  connection = tracker_sparql_connection_bus_new (REMOTE_NAME, NULL, NULL, &amp;error);
  if (!connection) {
    g_printerr ("Couldn't obtain a connection to the Tracker store: %s",
                error ? error-&gt;message : "unknown error");
    return 1;
  }

  /* Create notifier, and connect to its events signal */
  notifier = tracker_sparql_connection_create_notifier (connection);
  g_signal_connect (notifier, "events", G_CALLBACK (on_events), NULL);

  main_loop = g_main_loop_new (NULL, FALSE);
  g_main_loop_run (main_loop);

  tracker_sparql_connection_close (connection);

  return 0;
}

</code></pre>
</div>
<div class="gi-lang-python">
<pre><code class="language-python">#!/usr/bin/python3

import gi, sys
from gi.repository import GLib, Gio, Tracker

def callback(service, graph, events):
    for event in events:
        print('Event {0} on {1}\n'.format(
            event.get_event_type(), event.get_urn()))

try:
    connection = Tracker.SparqlConnection.bus_new(
        'org.freedesktop.Tracker3.Miner.Files',
        None, None)

    notifier = connection.create_notifier()
    notifier.connect('events', callback)

    loop = GLib.MainLoop.new(None, False)
    loop.run()

    connection.close()

except Exception as e:
    print('Error: {0}'.format(e))
    sys.exit(-1)

</code></pre>
</div>
<div class="gi-lang-javascript">
<pre><code class="language-javascript">#!/usr/bin/gjs

const { GLib, Gio, Tracker } = imports.gi

try {
    let connection = Tracker.SparqlConnection.bus_new(
        'org.freedesktop.Tracker3.Miner.Files',
        null, null);

    let notifier = connection.create_notifier();
    notifier.connect('events', (service, graph, events) =&gt; {
        for (let event in events)
            print (`Event ${event.get_event_type()} on ${event.get_urn()}`);
    });

    let loop = GLib.MainLoop.new(null, false);
    loop.run();

    connection.close();
} catch (e) {
    printerr(`Error: ${e.message}`)
}

</code></pre>
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