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| <h1 class="title">TDB Transactions</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="#overview">Overview</a></li> |
| <li><a href="#limitations">Limitations</a></li> |
| <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> |
| </ul> |
| </li> |
| <li><a href="#multi-threaded-use">Multi-threaded use</a></li> |
| <li><a href="#multi-jvm">Multi JVM</a></li> |
| <li><a href="#bulk-loading">Bulk loading</a></li> |
| </ul> |
| </nav> |
| </aside> |
| <article class="flex-column me-lg-4"> |
| <p>TDB provides |
| <a href="http://en.wikipedia.org/wiki/ACID">ACID</a> |
| transaction support through the use of |
| <a href="http://en.wikipedia.org/wiki/Write-ahead_logging">write-ahead-logging</a> in TDB1 |
| and copy-on-write MVCC structures in TDB2.</p> |
| <p>Use of transactions protects a TDB dataset against data corruption, unexpected |
| process termination and system crashes.</p> |
| <p>Non-transactional use of TDB1 should be avoided; TDB2 only operates with transactions.</p> |
| <h2 id="overview">Overview</h2> |
| <p>TDB2 uses <a href="https://en.wikipedia.org/wiki/Multiversion_concurrency_control">MVCC</a> |
| via a copy-on-write mechanism. Update transactions can be of any size.</p> |
| <p>The TDB1 transaction mechanism is based on |
| <a href="http://en.wikipedia.org/wiki/Write-ahead_logging">write-ahead-logging</a>. All |
| changes made inside a write-transaction are written to |
| <a href="http://en.wikipedia.org/wiki/Journaling_file_system">journals</a>, then propagated |
| to the main database at a suitable moment. Transactions in TDB1 are limited in |
| size to a few 10’s of million triples because they retain data in-memory until |
| indexes can be updated.</p> |
| <p>Transactional TDB supports one active write transaction, and |
| multiple read transactions at the same time. Read-transactions |
| started before a write-transaction commits see the database in a |
| state without any changes visible. Any transaction starting after a |
| write-transaction commits sees the database with the changes |
| visible, whether fully propagates back to the database or not. |
| There can be active read transactions seeing the state of the |
| database before the updates, and read transactions seeing the state |
| of the database after the updates running at the same time.</p> |
| <p>Transactional TDB works with SPARQL Query, SPARQL Update, SPARQL |
| Graph Store Update as well as the full Jena API.</p> |
| <p>TDB provides |
| <a href="http://en.wikipedia.org/wiki/Isolation_(database_systems)#SERIALIZABLE">Serializable</a> |
| transactions, the highest |
| <a href="http://en.wikipedia.org/wiki/Isolation_(database_systems)">isolation level</a>.</p> |
| <h2 id="limitations">Limitations</h2> |
| <p>(some of these limitations may be removed in later versions)</p> |
| <ul> |
| <li>Bulk loads: the TDB bulk loader is not transactional</li> |
| <li><a href="http://en.wikipedia.org/wiki/Nested_transaction">Nested transactions</a> |
| are not supported.</li> |
| </ul> |
| <p>TDB2 removed the limitations of TDB1:</p> |
| <ul> |
| <li>Some active transaction state is held exclusively in-memory, |
| limiting scalability.</li> |
| <li>Long-running transactions. Read-transactions cause a build-up |
| of pending changes;</li> |
| </ul> |
| <p>If a single read transaction runs for a long time when there are |
| many updates, the TDB1 system will consume a lot of temporary |
| resources.</p> |
| <h2 id="api-for-transactions">API for Transactions</h2> |
| <p>Ths section uses the primitives of the transaction mechanism.</p> |
| <p>Better APIs are described in <a href="/documentation/txn/">the transaction API |
| documentation</a>.</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> |
| <pre><code> Location location = ... ; |
| Dataset dataset = ... ; |
| dataset.begin(ReadWrite.READ) ; |
| String qs1 = "SELECT * {?s ?p ?o} LIMIT 10" ; |
| |
| try(QueryExecution qExec = QueryExecution.dataset(dataset).query(qs1).build() ) { |
| ResultSet rs = qExec.execSelect() ; |
| ResultSetFormatter.out(rs) ; |
| } |
| |
| String qs2 = "SELECT * {?s ?p ?o} OFFSET 10 LIMIT 10" ; |
| try(QueryExecution qExec = QueryExecution.dataset(dataset).query(qs2).build() ) { |
| rs = qExec.execSelect() ; |
| ResultSetFormatter.out(rs) ; |
| } |
| </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 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> Location location = ... ; |
| Dataset dataset = ... ; |
| dataset.begin(ReadWrite.WRITE) ; |
| |
| try { |
| Model model = dataset.getDefaultModel() ; |
| // API calls to a model in the dataset |
| |
| model.add( ... ) |
| |
| // A SPARQL query will see the new statement added. |
| try (QueryExecution qExec = QueryExecution.dataset(dataset) |
| .query("SELECT (count(*) AS ?count) { ?s ?p ?o} LIMIT 10") |
| .build() ) { |
| 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="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 per thread, and so there is one transaction per thread.</p> |
| <p>Either:</p> |
| <pre><code> // Create a dataset and keep it globally. |
| Dataset dataset = TDBFactory.createDataset(location) ; |
| </code></pre> |
| <p>Thread 1:</p> |
| <pre><code> dataset.begin(ReadWrite.WRITE) ; |
| try { |
| ... |
| dataset.commit() ; |
| } finally { dataset.end() ; } |
| </code></pre> |
| <p>Thread 2:</p> |
| <pre><code> dataset.begin(ReadWrite.READ) ; |
| try { |
| ... |
| } finally { dataset.end() ; } |
| </code></pre> |
| <p>or create a dataset object on the thread:</p> |
| <p>Thread 1:</p> |
| <pre><code> Dataset dataset = TDBFactory.createDataset(location) ; |
| dataset.begin(ReadWrite.WRITE) ; |
| try { |
| ... |
| dataset.commit() ; |
| } finally { dataset.end() ; } |
| </code></pre> |
| <p>Thread 2:</p> |
| <pre><code> Dataset dataset = TDBFactory.createDataset(location) ; |
| dataset.begin(ReadWrite.READ) ; |
| try { |
| ... |
| } finally { dataset.end() ; } |
| </code></pre> |
| <p>Each thread has a separate <code>dataset</code> object; these safely share the |
| same storage. in both cases, the transactions are independent.</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. TDB includes automatic prevention against multi-JVM usage |
| which prevents this under most circumstances.</p> |
| <p>Use <a href="../fuseki2/">Fuseki</a> 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> |
| <h2 id="bulk-loading">Bulk loading</h2> |
| <p>Bulk loaders are not transactional.</p> |
| |
| </article> |
| |
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| <h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2> |
| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#overview">Overview</a></li> |
| <li><a href="#limitations">Limitations</a></li> |
| <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> |
| </ul> |
| </li> |
| <li><a href="#multi-threaded-use">Multi-threaded use</a></li> |
| <li><a href="#multi-jvm">Multi JVM</a></li> |
| <li><a href="#bulk-loading">Bulk loading</a></li> |
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