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<th colspan="3" align="center">Configuring the Transaction Subsystem</th>
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<td width="20%" align="left"><a accesskey="p" href="txnindices.html">Prev</a> </td>
<th width="60%" align="center">Chapter 3. Transaction Basics</th>
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<div>
<h2 class="title" style="clear: both"><a id="maxtxns"></a>Configuring the Transaction Subsystem</h2>
</div>
</div>
</div>
<p>
Most of the configuration activities that you need to perform
for your transactional DB application will involve the
locking and logging subsystems. See
<a class="xref" href="txnconcurrency.html" title="Chapter 4. Concurrency">Concurrency</a>
and
<a class="xref" href="filemanagement.html" title="Chapter 5. Managing DB Files">Managing DB Files</a>
for details.
</p>
<p>
However, there are a couple of things that you can do to
configure your transaction subsystem directly. These things
are:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<span>
<p>
<span>
Configure
</span>
the maximum number of simultaneous
transactions needed by your application.
In general, you should not need to do this unless you
use deeply nested transactions or you have many threads all
of which have active transactions. In addition, you may
need to configure a higher maximum number of transactions if you
are using snapshot isolation. See
<a class="xref" href="isolation.html#sisolation_maxtxn" title="Snapshot Isolation Transactional Requirements">Snapshot Isolation Transactional Requirements</a>
for details.
</p>
<p>
By default, your application can support 20 active
transactions.
</p>
<p>
You can set the maximum number of simultaneous
transactions supported by your application using
<span>
<code class="methodname">EnvironmentConfig.setTxnMaxActive()</code>.
</span>
</p>
<p>
If your application has exceeded this maximum value,
then any attempt to begin a new transaction will fail.
</p>
<p>
This value can also be set using the
<code class="literal">DB_CONFIG</code> file's
<code class="literal">set_tx_max</code> parameter. Remember that
the <code class="literal">DB_CONFIG</code> must reside in your
environment home directory.
</p>
</span>
</li>
<li>
<span>
<p>
<span>
Configure the timeout value for your transactions.
</span>
This value represents the longest period of time a
transaction can be active. Note, however, that
transaction timeouts are checked only when DB
examines its lock tables for blocked locks
(see <a class="xref" href="blocking_deadlocks.html" title="Locks, Blocks, and Deadlocks">Locks, Blocks, and Deadlocks</a>
for more information). Therefore, a transaction's timeout can
have expired, but the application will not be notified until DB
has a reason to examine its lock tables.
</p>
<p>
Be aware that some transactions may be
inappropriately timed out before the transaction has a
chance to complete. You should therefore use this
mechanism only if you know your application
might have unacceptably long transactions and
you want to make sure your application will
not stall during their execution.
(This might happen if, for example, your
transaction blocks or requests too much
data.)
</p>
<p>
Note that by default transaction timeouts are set to 0 seconds, which means that they never time
out.
</p>
<p>
To set the maximum timeout value for your transactions,
use the
<span><code class="methodname">EnvironmentConfig.setTxnTimeout()</code></span>
method. This method configures the entire
environment; not just the handle used to set the
configuration. Further, this value may
be set at any time during the application's
lifetime. <span>(Use
<code class="methodname">Environment.setConfig()</code> to
set this value after the environment has been
opened.)</span>
</p>
<p>
This value can also be set using the
<code class="literal">DB_CONFIG</code> file's
<code class="literal">set_txn_timeout</code> parameter.
</p>
</span>
</li>
</ul>
</div>
<p>
For example:
</p>
<pre class="programlisting">package db.txn;
import com.sleepycat.db.Environment;
import com.sleepycat.db.EnvironmentConfig;
import com.sleepycat.db.LockDetectMode;
import java.io.File;
import java.io.FileNotFoundException;
...
Environment myEnv = null;
try {
EnvironmentConfig myEnvConfig = new EnvironmentConfig();
myEnvConfig.setTransactional(true);
myEnvConfig.setInitializeCache(true);
myEnvConfig.setInitializeLocking(true);
myEnvConfig.setInitializeLogging(true);
// Configure a maximum transaction timeout of 1 second.
myEnvConfig.setTxnTimeout(1000000);
// Configure 40 maximum transactions.
myEnv.setTxnMaxActive(40);
myEnv = new Environment(new File("/my/env/home"),
myEnvConfig);
// From here, you open your databases (or store), proceed with your
// database or store operations, and respond to deadlocks as is
// normal (omitted for brevity).
...</pre>
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