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| <h2 id="Transactions">Transactions</h2> |
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| <div class="paragraph"> |
| <p>When managing the persistence of objects using a <a href="pm.html">PersistenceManager</a> it is common to handle all datastore |
| operations in a transaction. |
| For this reason each <em>PersistenceManager</em> has its own transaction. |
| Consequently a typical JDO persistence method will look something like this</p> |
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| <pre class="CodeRay highlight"><code data-lang="java">PersistenceManager pm = pmf.getPersistenceManager(); |
| Transaction tx = pm.currentTransaction(); |
| <span class="keyword">try</span> |
| { |
| tx.begin(); <span class="comment">// Start the PM transaction</span> |
| |
| ... perform some persistence operations |
| |
| tx.commit(); <span class="comment">// Commit the PM transaction</span> |
| } |
| <span class="keyword">finally</span> |
| { |
| <span class="keyword">if</span> (tx.isActive()) |
| { |
| tx.rollback(); <span class="comment">// Error occurred so rollback the PM transaction</span> |
| } |
| }</code></pre> |
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| <div class="sect2"> |
| <h3 id="locking">Locking</h3> |
| <div class="paragraph"> |
| <p>JDO supports the two main forms of transaction locking</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Pessimistic (or datastore) locking is where you lock all records in a datastore affected by a transaction and keep them locked |
| until you are ready to commit their changes. This is suitable for short lived operations where no user interaction is taking place.</p> |
| </li> |
| <li> |
| <p>Optimistic locking is where you can simply assume that things in the datastore will not change until they are ready to commit, |
| not lock any records and then just before committing make a check for changes. |
| This is suitable for longer lived operations maybe where user interaction is taking place and where it would be undesirable to block access to |
| datastore entities for the duration of the transaction</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>You select the type of locking used by the transaction of a <em>PersistenceManager</em> (PM) |
| either by setting the persistence property <strong>javax.jdo.option.Optimistic</strong>, or on the transaction you call</p> |
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| <pre class="CodeRay highlight"><code data-lang="java">pm.currentTransaction().setOptimistic(<span class="predefined-constant">true</span>);</code></pre> |
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| <div class="sect2"> |
| <h3 id="nontransactional">Non-Transactional</h3> |
| <div class="paragraph"> |
| <p>The JDO API also allows the ability to operate without transactions. |
| You have 2 persistence properties <strong>javax.jdo.option.NontransactionalRead</strong> and <strong>javax.jdo.option.NontransactionalWrite</strong> to enable this mode of operation |
| assuming your JDO provider supports it. |
| Use of these two properties mean that you can then read objects and make updates outside of transactions. |
| This is effectively an "auto-commit" mode.</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="CodeRay highlight"><code data-lang="java">PersistenceManager pm = pmf.getPersistenceManager(); |
| |
| {users code to persist objects} |
| |
| pm.close();</code></pre> |
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