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<h2 class="title" style="clear: both"><a id="usingtransactions"></a>
Using Transactions
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<p>
DB transactional applications have standard
transactional characteristics: recoverability, atomicity and
integrity (this is sometimes also referred to generically as <span class="emphasis"><em>ACID
properties</em></span>). The DB Java Collections API provides these
transactional capabilities using a <span class="emphasis"><em>transaction-per-thread</em></span>
model. Once a transaction is begun, it is implicitly associated
with the current thread until it is committed or aborted. This
model is used for the following reasons.
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
The transaction-per-thread model is commonly used in other Java
APIs such as J2EE.
</p>
</li>
<li>
<p>
Since the Java collections API is used for data access, there
is no way to pass a transaction object to methods such
as
<a class="ulink" href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.html#put" target="_top">Map.put</a>.
</p>
</li>
</ul>
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<p>
The DB Java Collections API provides two transaction APIs. The
lower-level API is the
<a class="ulink" href="../../java/com/sleepycat/collections/CurrentTransaction.html" target="_top">CurrentTransaction</a>
class. It provides a way to get the transaction for the current
thread, and to begin, commit and abort transactions. It also
provides access to the Berkeley DB core API
<a class="ulink" href="../../java/com/sleepycat/db/Transaction.html" target="_top">Transaction</a>
object. With
<a class="ulink" href="../../java/com/sleepycat/collections/CurrentTransaction.html" target="_top">CurrentTransaction</a>,
just as in the
<span>com.sleepycat.db</span>
API, the application is responsible
for beginning, committing and aborting transactions, and for
handling deadlock exceptions and retrying operations. This API may
be needed for some applications, but it is not used in the
example.
</p>
<p>
The example uses the higher-level
<a class="ulink" href="../../java/com/sleepycat/collections/TransactionRunner.html" target="_top">TransactionRunner</a>
and
<a class="ulink" href="../../java/com/sleepycat/collections/TransactionWorker.html" target="_top">TransactionWorker</a>
APIs, which are build on top of
<a class="ulink" href="../../java/com/sleepycat/collections/CurrentTransaction.html" target="_top">CurrentTransaction</a>.
<code class="methodname">TransactionRunner.run()</code> automatically begins a transaction and
then calls the <code class="methodname">TransactionWorker.doWork()</code> method, which is
implemented by the application.
</p>
<p>
The <code class="methodname">TransactionRunner.run()</code> method automatically detects
deadlock exceptions and performs retries by repeatedly calling the
<code class="methodname">TransactionWorker.doWork()</code> method until the operation succeeds
or the maximum retry count is reached. If the maximum retry count
is reached or if another exception (other than
<span>
<a class="ulink" href="../../java/com/sleepycat/db/DeadlockException.html" target="_top">DeadlockException</a>)
</span>
is thrown by <code class="methodname">TransactionWorker.doWork()</code>, then the transaction
will be automatically aborted. Otherwise, the transaction will be
automatically committed.
</p>
<p>
Using this high-level API, if <code class="methodname">TransactionRunner.run()</code>
throws an exception, the application can assume that the operation
failed and the transaction was aborted; otherwise, when an
exception is not thrown, the application can assume the operation
succeeded and the transaction was committed.
</p>
<p>
The <code class="methodname">Sample.run()</code> method creates a <code class="classname">TransactionRunner</code>
object and calls its <code class="methodname">run()</code> method.
</p>
<a id="cb_sample1"></a>
<pre class="programlisting"><strong class="userinput"><code>import com.sleepycat.collections.TransactionRunner;
import com.sleepycat.collections.TransactionWorker;</code></strong>
...
public class Sample
{
private SampleDatabase db;
...
<strong class="userinput"><code> private void run()
throws Exception
{
TransactionRunner runner = new TransactionRunner(db.getEnvironment());
runner.run(new PopulateDatabase());
runner.run(new PrintDatabase());
}
...
private class PopulateDatabase implements TransactionWorker
{
public void doWork()
throws Exception
{
}
}
private class PrintDatabase implements TransactionWorker
{
public void doWork()
throws Exception
{
}
}</code></strong>
} </pre>
<p>
The <code class="methodname">run()</code> method is called by <code class="methodname">main()</code> and was outlined
in the previous section. It first creates a
<code class="classname">TransactionRunner</code>, passing the database environment to its
constructor.
</p>
<p>
It then calls <code class="methodname">TransactionRunner.run()</code> to execute two
transactions, passing instances of the application-defined
<code class="classname">PopulateDatabase</code> and
<code class="classname">PrintDatabase</code> nested classes.
These classes implement the <code class="methodname">TransactionWorker.doWork()</code> method
and will be fully described in the next two sections.
</p>
<p>
For each call to <code class="methodname">TransactionRunner.run()</code>, a separate
transaction will be performed. The use of two transactions in the
example — one for populating the database and another for printing
its contents — is arbitrary. A real-life application should be
designed to create transactions for each group of operations that
should have ACID properties, while also
taking into account the impact of transactions on performance.
</p>
<p>
The advantage of using <code class="classname">TransactionRunner</code> is that deadlock
retries and transaction begin, commit and abort are handled
automatically. However, a <code class="classname">TransactionWorker</code> class must be
implemented for each type of transaction. If desired, anonymous
inner classes can be used to implement the <code class="classname">TransactionWorker</code>
interface.
</p>
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