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| <h1 class="title">Concurrent access to Models</h1> |
| |
| <i> |
| All datasets provide transactions. |
| This is the preferred way to |
| handle concurrenct access to data. |
| </i> |
| <br/> |
| <br/> |
| <p>Applications need to be aware of the concurrency issues in access |
| Jena models. API operations are not thread safe by default. Thread |
| safety would simple ensure that the model data-structures remained |
| intact but would not give an application consistent access to the |
| RDF graph. It would also limit the throughput of multi-threaded |
| applications on multiprocessor machines where true concurrency can |
| lead to a reduction in response time.</p> |
| <p>For example, supposed an application wishes to read the name and |
| age of a person from model. This takes two API calls. It is more |
| convenient to be able to read that information in a consistent |
| fashion, knowing that the access to the second piece of information |
| is not being done after some model change has occurred.</p> |
| <p>Special care is needed with iterators. In general, Jena’s |
| iterators do <em>not</em> take a copy to enable safe use in the presence |
| of concurrent update. A multi-threaded application needs to be |
| aware of these issues and correctly use the mechanisms that Jena |
| provides (or manage its own concurrency itself). While not zero, |
| the application burden is not high.</p> |
| <p>There are two main cases:</p> |
| <ul> |
| <li>Multiple threads in the same JVM.</li> |
| <li>Multiple applications accessing the same persistent model |
| (typically, a database).</li> |
| </ul> |
| <p>This note describes the support for same-JVM, multi-threaded |
| applications using in-memory Jena Models.</p> |
| <h2 id="locks">Locks</h2> |
| <p>Locks provide critical section support for managing the |
| interactions of multiple threads in the same JVM. Jena provides |
| multiple-reader/single-writer concurrency support (MRSW).</p> |
| <p>The pattern general is:</p> |
| <pre><code>Model model = . . . ; |
| model.enterCriticalSection(Lock.READ) ; // or Lock.WRITE |
| try { |
| ... perform actions on the model ... |
| ... obey contract - no update operations if a read lock |
| } finally { |
| model.leaveCriticalSection() ; |
| } |
| </code></pre> |
| <p>Applications are expected to obey the lock contract, that is, they |
| must not do update operations if they have a read lock as there can |
| be other application threads reading the model concurrently.</p> |
| <h2 id="iterators">Iterators</h2> |
| <p>Care must be taken with iterators: unless otherwise stated, all |
| iterators must be assumed to be iterating over the data-structures |
| in the model or graph implementation itself. It is not possible to |
| safely pass these out of critical sections.</p> |
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