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| <h1 class="title">TDB Transactions</h1> |
| |
| <h2 id="api-for-transactions">API for Transactions</h2> |
| <h2 id="tdb1-and-tdb2">TDB1 and TDB2</h2> |
| <p><a href="/documentation/tdb">TDB1</a>, the original native TDB database for Apache |
| Jena, and <a href="documentation/tdb2">TDB2</a> are related but different systems. |
| Their transaction systems both provide |
| <a href="http://en.wikipedia.org/wiki/Isolation_%28database_systems%29#SERIALIZABLE">Serializable</a> |
| transactions, the highest |
| <a href="http://en.wikipedia.org/wiki/Isolation_%28database_systems%29">isolation level</a>. |
| with one active write transaction and multiple read |
| transactions at the same time.</p> |
| <p>TDB2 does not have the transaction size limitations of TDB1.</p> |
| <h2 id="tdb1">TDB1</h2> |
| <p>The transaction mechanism in TDB 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. This |
| design allows for read-transactions to proceed without locking or |
| other overhead over the base database.</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>TDB provides |
| <a href="http://en.wikipedia.org/wiki/Isolation_%28database_systems%29#SERIALIZABLE">Serializable</a> |
| transactions, the highest |
| <a href="http://en.wikipedia.org/wiki/Isolation_%28database_systems%29">isolation level</a>.</p> |
| <h2 id="limitations">Limitations</h2> |
| <ul> |
| <li><a href="http://en.wikipedia.org/wiki/Nested_transaction">Nested transactions</a> are not supported.</li> |
| <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 system will consume a lot of temporary |
| resources.</p> |
| <h2 id="tdb2">TDB2</h2> |
| <p>The transaction mechanism in TDB2 is based on |
| <a href="https://en.wikipedia.org/wiki/Multiversion_concurrency_control">MVCC</a> |
| using immutable datastructrures, which are known as <a href="https://en.wikipedia.org/wiki/Persistent_data_structure">“persistent data |
| structures”</a> |
| (although this name, for the functional programming community, is |
| slightly confusing).</p> |
| <h2 id="limitations-1">Limitations</h2> |
| <p>(some of these limitations may be removed in later versions)</p> |
| <ul> |
| <li>Bulk loads: the TDB2 bulk loader is not transactional</li> |
| <li><a href="http://en.wikipedia.org/wiki/Nested_transaction">Nested transactions</a> are not supported.</li> |
| </ul> |
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