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| <h1 class="title">SDB Loading data</h1> |
| |
| |
| <main class="d-flex flex-xl-row flex-column"> |
| |
| <aside class="text-muted align-self-start mb-3 p-0 d-xl-none d-block"> |
| <h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2> |
| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#loading-with-modelread">Loading with <code>Model.read</code></a></li> |
| <li><a href="#loading-with-modeladd">Loading with <code>Model.add</code></a></li> |
| <li><a href="#how-the-loader-works">How the loader works</a></li> |
| <li><a href="#duplicate-suppression">Duplicate Suppression</a></li> |
| </ul> |
| </nav> |
| </aside> |
| <article class="flex-column me-lg-4"> |
| <hr> |
| <blockquote> |
| <p>The Apache Jena SDB module has been retired and is no longer supported.<br/> |
| The last release of Jena with this module was Apache Jena 3.17.0.<br/></p> |
| </blockquote> |
| <hr> |
| <p>There are three ways to load data into SDB:</p> |
| <ol> |
| <li>Use the command utility |
| <a href="commands.html#Loading_data" title="SDB/Commands">sdbload</a></li> |
| <li>Use one of the Jena <code>model.read</code> operations</li> |
| <li>Use the Jena <code>model.add</code></li> |
| </ol> |
| <p>The last one of these requires the application to signal the |
| beginning and end of batches.</p> |
| <h2 id="loading-with-modelread">Loading with <code>Model.read</code></h2> |
| <p>A Jena Model obtained from SDB via:</p> |
| <pre><code>SDBFactory.connectModel(store) |
| </code></pre> |
| <p>will automatically bulk load data for each call of one of the |
| <code>Model.read</code> operations.</p> |
| <h2 id="loading-with-modeladd">Loading with <code>Model.add</code></h2> |
| <p>The <code>Model.add</code> operations, in any form or combination of forms, |
| whether loading a single statement, list of statements, or another |
| model, will invoke the bulk loader if previously notified before an |
| add operation.</p> |
| <p>You can also explicitly delimit bulk operations:</p> |
| <pre><code> model.notifyEvent(GraphEvents.startRead) |
| ... do add/remove operations ... |
| model.notifyEvent(GraphEvents.finishRead) |
| </code></pre> |
| <p><strong>Failing to notify the end of the operations will result in data loss</strong>.</p> |
| <p>A try/finally block can ensure that the finish is notified.</p> |
| <pre><code> model.notifyEvent(GraphEvents.startRead) ; |
| try { |
| ... do add/remove operations ... |
| } finally { |
| model.notifyEvent(GraphEvents.finishRead) ; |
| } |
| </code></pre> |
| <p>The <code>model.read</code> operations do this automatically.</p> |
| <p>The bulk loader will automatically chunk large sequences of |
| additions to sizes appropriate to the underlying database. The bulk |
| loader is threaded with double-buffered; loading to the database |
| happens in parallel to the application thread and any RDF parsing.</p> |
| <h2 id="how-the-loader-works">How the loader works</h2> |
| <p>Loading consists of two phases: in the java VM, and on the database |
| itself. The SDB loader takes incoming triples and breaks them down |
| into components ready for the database. These prepared triples are |
| added to a queue for the database phase, which (by default) takes |
| place on a separate thread. When the number of triples reaches a |
| limit (default 20,000), or finish update is signalled, the triples |
| are passed to the database.</p> |
| <p>You can configure whether to use threading and the ‘chunk size’ – |
| the number of triples per load event – via <code>StoreLoader</code>.</p> |
| <pre><code>Store store; // SDB Store |
| ... |
| store.getLoader().setChunkSize(5000); // |
| store.getLoader().setUseThreading(false); // Don't thread |
| </code></pre> |
| <p>You should set these <em>before</em> the loader has been used.</p> |
| <p>Each loader sets up two temporary tables (<code>NNode</code> and <code>NTrip</code>) that |
| mirror <code>Nodes</code> and <code>Triples</code> tables. These tables are virtually |
| identical, except that a) they are not indexed and b) for the index |
| variant there is no index column for nodes.</p> |
| <p>When loading prepared triples – triples that have been broken down |
| ready for the database – are passed to the loader core (normally |
| running on a different thread). When the chunk size is reached, or |
| we are out of triples, the following happens:</p> |
| <ul> |
| <li>Prepared nodes are added in one go to <code>NNode</code>. Duplicate nodes |
| within a chunk are suppressed on the java side (this is worth doing |
| since they are quite common, e.g. properties).</li> |
| <li>Prepared triples are added in one go to <code>NTrip</code>.</li> |
| <li>New nodes are added to the node table (duplicate suppression is |
| explained below).</li> |
| <li>New triples are added to the triple table (once again |
| suppressing dupes). For the index case this involves joining on the |
| node table to do a hash to index lookup.</li> |
| <li>We commit.</li> |
| <li>If anything goes wrong the transaction (the chunk) is rolled |
| back, and an exception is thrown (or readied for throwing on the |
| calling thread).</li> |
| </ul> |
| <p>Thus there are five calls to the database for every chunk. The |
| database handles almost all of the work uninterrupted (duplicate |
| suppression, hash to index lookup), which makes loading reasonably |
| quick.</p> |
| <h2 id="duplicate-suppression">Duplicate Suppression</h2> |
| <p>MySQL has a very useful <code>INSERT IGNORE</code>, which will keep going, |
| skipping an offending row if a uniqueness constraint is violated. |
| For other databases we need something else.</p> |
| <p>Having tried a number of options the best seems to be to <code>INSERT</code> |
| new items by <code>LEFT JOIN</code> new items to existing items, then |
| filtering <code>WHERE (existing item feature) IS NULL</code>. Specifically, |
| for the triple hash case (where no id lookups are needed):</p> |
| <pre><code>INSERT INTO Triples |
| SELECT DISTINCT NTrip.s, NTrip.p, NTrip.o -- DISTINCT because new triples may contain duplicates (not so for nodes) |
| NTrip LEFT JOIN Triples ON (NTrip.s=Triples.s AND NTrip.p=Triples.p AND NTrip.o=Triples.o) |
| WHERE Triples.s IS NULL OR Triples.p IS NULL OR Triples.o IS NULL |
| </code></pre> |
| |
| </article> |
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| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#loading-with-modelread">Loading with <code>Model.read</code></a></li> |
| <li><a href="#loading-with-modeladd">Loading with <code>Model.add</code></a></li> |
| <li><a href="#how-the-loader-works">How the loader works</a></li> |
| <li><a href="#duplicate-suppression">Duplicate Suppression</a></li> |
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