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| <h1 class="title">Jena Transactions API</h1> |
| |
| <ul> |
| <li><a href="#api-for-transactions">API for Transactions</a> |
| <ul> |
| <li><a href="#read-transactions">Read transactions</a></li> |
| <li><a href="#write-transactions">Write transactions</a></li> |
| <li><a href="#types-modes-promotion">Transaction promotion</a></li> |
| </ul> |
| </li> |
| <li><a href="txn.html">Txn</a> - A higher level API to transactions</li> |
| </ul> |
| <h2 id="api-for-transactions">API for Transactions</h2> |
| <p>This page describes the basic transaction API in Jena (3.1.0 and later).</p> |
| <p>There is also a <a href="txn.html">higher-level API</a> useful in many situations |
| but sometimes it is necessary to use the basic transaction API described here.</p> |
| <h3 id="read-transactions">Read transactions</h3> |
| <p>These are used for SPARQL queries and code using the Jena API |
| actions that do not change the data. The general pattern is:</p> |
| <pre><code> dataset.begin(ReadWrite.READ) ; |
| try { |
| ... |
| } finally { dataset.end() ; } |
| </code></pre> |
| <p>The <code>dataset.end()</code> declares the end of the read transaction. Applications may also call |
| <code>dataset.commit()</code> or <code>dataset.abort()</code> which all have the same effect for a read transaction.</p> |
| <p>This example has two queries - no updates between or during the queries will be seen by |
| this code even if another thread commits changes in the lifetime of this transaction.</p> |
| <pre><code> Dataset dataset = ... ; |
| dataset.begin(ReadWrite.READ) ; |
| try { |
| String qs1 = "SELECT * {?s ?p ?o} LIMIT 10" ; |
| try(QueryExecution qExec = QueryExecution.create(qs1, dataset)) { |
| ResultSet rs = qExec.execSelect() ; |
| ResultSetFormatter.out(rs) ; |
| } |
| |
| String qs2 = "SELECT * {?s ?p ?o} OFFSET 10 LIMIT 10" ; |
| try(QueryExecution qExec = QueryExecution.create(qs2, dataset)) { |
| rs = qExec.execSelect() ; |
| ResultSetFormatter.out(rs) ; |
| } |
| } finally { dataset.end() ; } |
| </code></pre> |
| <h3 id="write-transactions">Write transactions</h3> |
| <p>These are used for SPARQL queries, SPARQL updates and any Jena API |
| actions that modify the data. Beware that large <code>model.read</code> |
| operations to change a dataset may consume large amounts of temporary space.</p> |
| <p>The general pattern is:</p> |
| <pre><code> dataset.begin(ReadWrite.WRITE) ; |
| try { |
| ... |
| dataset.commit() ; |
| } finally { |
| dataset.end() ; |
| } |
| </code></pre> |
| <p>The <code>dataset.end()</code> will abort the transaction is there was no call to |
| <code>dataset.commit()</code> or <code>dataset.abort()</code> inside the write transaction.</p> |
| <p>Once <code>dataset.commit()</code> or <code>dataset.abort()</code> is called, the application |
| needs to start a new transaction to perform further operations on the |
| dataset.</p> |
| <pre><code> Dataset dataset = ... ; |
| dataset.begin(TxnType.WRITE) ; |
| |
| try { |
| Model model = dataset.getDefaultModel() ; |
| // API calls to a model in the dataset |
| |
| // Make some changes via the model ... |
| model.add( ... ) |
| |
| // A SPARQL query will see the new statement added. |
| try (QueryExecution qExec = QueryExecution.create( |
| "SELECT (count(?s) AS ?count) { ?s ?p ?o} LIMIT 10", |
| dataset)) { |
| ResultSet rs = qExec.execSelect() ; |
| ResultSetFormatter.out(rs) ; |
| } |
| |
| // ... perform a SPARQL Update |
| String sparqlUpdateString = StrUtils.strjoinNL( |
| "PREFIX . <http://example/>", |
| "INSERT { :s :p ?now } WHERE { BIND(now() AS ?now) }" |
| ) ; |
| |
| UpdateRequest request = UpdateFactory.create(sparqlUpdateString) ; |
| UpdateExecution.dataset(dataset).update(request).execute(); |
| |
| // Finally, commit the transaction. |
| dataset.commit() ; |
| // Or call .abort() |
| } finally { |
| dataset.end() ; |
| } |
| </code></pre> |
| <h2 id="types-modes-promotion">Transaction Types, Modes and Promotion.</h2> |
| <p>Transaction have a type (enum <code>TxnType</code>) and a mode (enum <code>ReadWrite</code>). |
| <code>TxnType.READ</code> and <code>TxnType.Write</code> start the transaction in |
| that mode and the mode is fixed for the transaction’s lifetime. A <code>READ</code> |
| transaction can never update the data of the transactional object it is |
| acting on.</p> |
| <p>Transactions can have type <code>TxnType.READ_PROMOTE</code> or |
| <code>TxnType.READ_COMMITTED_PROMOTE</code>. These start in mode <code>READ</code> but can |
| become mode <code>WRITE</code>, either implicitly by attempting an update, or |
| explicitly by calling <code>promote</code>.</p> |
| <p><code>READ_PROMOTE</code> only succeeds if no writer has made any changes since |
| this transaction started. It gives full isolation.</p> |
| <p><code>READ_COMMITTED_PROMOTE</code> always succeeds because it changes the view of |
| the data to include any changes made up to that point (it is “read |
| committed”). Applications should be aware that data they have read up |
| until the point of promotion (the first call or <code>.promote</code> or first |
| update made) may now be invalid. For this reason, <code>READ_PROMOTE</code> is preferred.</p> |
| <p><code>begin()</code>, the method with no arguments, is equivalent to |
| <code>begin(TxnType.READ_PROMOTE)</code>.</p> |
| <h2 id="multi-threaded-use">Multi-threaded use</h2> |
| <p>Each dataset object has one transaction active at a time per thread. |
| A dataset object can be used by different threads, with independent transactions.</p> |
| <p>The usual idiom within multi-threaded applications is to have |
| one dataset, and so there is one transaction per thread.</p> |
| <p>Either:</p> |
| <pre><code> // Create a dataset and keep it globally. |
| static Dataset dataset = TDBFactory.createDataset(location) ; |
| </code></pre> |
| <p>Thread 1:</p> |
| <pre><code> dataset.begin(TxnType.WRITE) ; |
| try { |
| ... |
| dataset.commit() ; |
| } finally { dataset.end() ; } |
| </code></pre> |
| <p>Thread 2:</p> |
| <pre><code> dataset.begin(TxnType.READ) ; |
| try { |
| ... |
| } finally { dataset.end() ; } |
| </code></pre> |
| <p>It is possible (in TDB) to create different dataset objects to the same location.</p> |
| <p>Thread 1:</p> |
| <pre><code> Dataset dataset = TDBFactory.createDataset(location) ; |
| dataset.begin(TxnType.WRITE) ; |
| try { |
| ... |
| dataset.commit() ; |
| } finally { dataset.end() ; } |
| </code></pre> |
| <p>Thread 2:</p> |
| <pre><code> Dataset dataset = TDBFactory.createDataset(location) ; |
| dataset.begin(TxnType.READ) ; |
| try { |
| ... |
| } finally { dataset.end() ; } |
| </code></pre> |
| <p>Each thread has a separate <code>dataset</code> object; these safely share the |
| same storage and have independent transactions.</p> |
| <h2 id="multi-jvm">Multi JVM</h2> |
| <p>Multiple applications, running in multiple JVMs, using the same file |
| databases is not supported and has a high risk of data corruption. Once |
| corrupted a database cannot be repaired and must be rebuilt from the |
| original source data. Therefore there <strong>must</strong> be a single JVM |
| controlling the database directory and files. From 1.1.0 onwards TDB |
| includes automatic prevention against multi-JVM which prevents this |
| under most circumstances.</p> |
| <p>Use our <a href="../fuseki2/">Fuseki</a> component to provide a |
| database server for multiple applications. Fuseki supports |
| <a href="http://www.w3.org/TR/sparql11-query/">SPARQL Query</a>, |
| <a href="http://www.w3.org/TR/sparql11-update/">SPARQL Update</a> and the |
| <a href="http://www.w3.org/TR/sparql11-http-rdf-update/">SPARQL Graph Store protocol</a>.</p> |
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