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391 lines
14 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Chapter 3. Transaction Basics</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Getting Started with Berkeley DB, Java Edition Transaction Processing" />
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<link rel="up" href="index.html" title="Getting Started with Berkeley DB, Java Edition Transaction Processing" />
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<link rel="prev" href="enabletxn.html" title="Chapter 2. Enabling Transactions" />
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<link rel="next" href="nodurabletxn.html" title="Non-Durable Transactions" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 12.2.7.5</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Chapter 3. Transaction Basics</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="enabletxn.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="nodurabletxn.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="chapter" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title"><a id="usingtxns"></a>Chapter 3. Transaction Basics</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<p>
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<b>Table of Contents</b>
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</p>
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<dl>
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<dt>
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<span class="sect1">
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<a href="usingtxns.html#commitresults">Committing a Transaction</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="nodurabletxn.html">Non-Durable Transactions</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="abortresults.html">Aborting a Transaction</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="autocommit.html">Auto Commit</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="txncursor.html">Transactional Cursors</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="txncursor.html#dplcursors">Using Transactional DPL Cursors</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="txnindices.html">Secondary Indices with Transaction Applications</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="maxtxns.html">Configuring the Transaction Subsystem</a>
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</span>
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</dt>
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</dl>
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</div>
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<p>
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Once you have enabled transactions for your environment and your databases,
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you can use them to protect your database operations. You do this by
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acquiring a transaction handle and then using that handle for any
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database operation that you want to participate in that transaction.
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</p>
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<p>
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You obtain a transaction handle using the
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<span><code class="methodname">Environment.beginTransaction()</code> method.</span>
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</p>
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<p>
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Once you have completed all of the operations that you want to include
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in the transaction, you must commit the transaction using the
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<span><code class="methodname">Transaction.commit()</code> method.</span>
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</p>
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<p>
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If, for any reason, you want to abandon the transaction, you abort
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it using
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<span><code class="methodname">Transaction.abort()</code>.</span>
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</p>
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<p>
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Any transaction handle that has been committed or aborted can no longer
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be used by your application.
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</p>
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<p>
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Finally, you must make sure that all transaction handles are either
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committed or aborted before closing your databases and environment.
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</p>
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<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
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<h3 class="title">Note</h3>
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<p>
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If you only want to transaction protect a single database write operation, you can use auto commit to
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perform the transaction administration. When you use auto commit, you do not need an explicit transaction
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handle. See <a class="xref" href="autocommit.html" title="Auto Commit">Auto Commit</a> for more information.
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</p>
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</div>
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<p>
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For example, the following example opens a transactional-enabled environment and
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store, obtains a transaction handle, and then performs a write
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operation under its protection. In the event of any failure in the
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write operation, the transaction is aborted and the store is left in a
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state as if no operations had ever been attempted in the first place.
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</p>
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<pre class="programlisting">package persist.txn;
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import com.sleepycat.je.DatabaseException;
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import com.sleepycat.je.Environment;
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import com.sleepycat.je.EnvironmentConfig;
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import com.sleepycat.je.Transaction;
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import com.sleepycat.persist.EntityStore;
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import com.sleepycat.persist.StoreConfig;
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import java.io.File;
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...
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Environment myEnv = null;
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EntityStore store = null;
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// Our convenience data accessor class, used for easy access to
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// EntityClass indexes.
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DataAccessor da;
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try {
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EnvironmentConfig myEnvConfig = new EnvironmentConfig();
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myEnvConfig.setTransactional(true);
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myEnv = new Environment(new File("/my/env/home"),
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myEnvConfig);
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StoreConfig storeConfig = new StoreConfig();
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storeConfig.setTransactional(true);
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EntityStore store = new EntityStore(myEnv,
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"EntityStore", storeConfig);
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da = new DataAccessor(store);
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// Assume that Inventory is an entity class.
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Inventory theInventory = new Inventory();
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theInventory.setItemName("Waffles");
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theInventory.setItemSku("waf23rbni");
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Transaction txn = myEnv.beginTransaction(null, null);
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try {
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// Put the object to the store using the transaction handle.
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da.inventoryBySku.put(txn, theInventory);
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// Commit the transaction. The data is now safely written to the
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// store.
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txn.commit();
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// If there is a problem, abort the transaction
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} catch (Exception e) {
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if (txn != null) {
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txn.abort();
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txn = null;
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}
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}
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} catch (DatabaseException de) {
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// Exception handling goes here
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} </pre>
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<p>
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The same thing can be done with the base API; the
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database in use is left unchanged if the write operation fails:
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</p>
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<pre class="programlisting">package je.txn;
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import com.sleepycat.je.Database;
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import com.sleepycat.je.DatabaseConfig;
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import com.sleepycat.je.DatabaseEntry;
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import com.sleepycat.je.DatabaseException;
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import com.sleepycat.je.Environment;
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import com.sleepycat.je.EnvironmentConfig;
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import com.sleepycat.je.Transaction;
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import java.io.File;
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...
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Database myDatabase = null;
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Environment myEnv = null;
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try {
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EnvironmentConfig myEnvConfig = new EnvironmentConfig();
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myEnvConfig.setTransactional(true);
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myEnv = new Environment(new File("/my/env/home"),
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myEnvConfig);
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// Open the database. Create it if it does not already exist.
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DatabaseConfig dbConfig = new DatabaseConfig();
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dbConfig.setTransactional(true);
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myDatabase = myEnv.openDatabase(null,
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"sampleDatabase",
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dbConfig);
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String keyString = "thekey";
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String dataString = "thedata";
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DatabaseEntry key =
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new DatabaseEntry(keyString.getBytes("UTF-8"));
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DatabaseEntry data =
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new DatabaseEntry(dataString.getBytes("UTF-8"));
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Transaction txn = myEnv.beginTransaction(null, null);
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try {
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myDatabase.put(txn, key, data);
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txn.commit();
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} catch (Exception e) {
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if (txn != null) {
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txn.abort();
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txn = null;
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}
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}
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} catch (DatabaseException de) {
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// Exception handling goes here
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} </pre>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="commitresults"></a>Committing a Transaction</h2>
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</div>
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</div>
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</div>
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<p>
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In order to fully understand what is happening when you commit
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a transaction, you must first understand a little about what
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JE is doing with
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<span>
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its log files.
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</span>
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Logging causes all database <span>or
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store</span> write operations to be identified in
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<span>
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log files (remember that in JE, your log files <span class="emphasis"><em>are</em></span>
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your database files; there is no difference between the two).
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Enough information is written to restore your entire BTree
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</span>
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in the event of a system or application failure, so by performing
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logging, JE ensures the integrity of your data.
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</p>
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<p>
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Remember that all write activity made to your database or
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store is
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identified in JE's logs as the writes are performed by your
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application. However, JE maintains logs in memory.
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Eventually this information is written to disk, but especially
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in the case of a transactional application this data may be
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held in memory until the transaction is committed, or
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JE runs out of buffer space for the logging information.
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</p>
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<p>
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When you commit a transaction, the following occurs:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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A commit record is written to the log. This
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indicates that the modifications made by the
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transaction are now permanent. By default, this write is performed synchronously to disk so the
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commit record arrives in the log files before any other actions are taken.
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</p>
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</li>
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<li>
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<p>
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Any log information held in memory is (by default)
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synchronously written to disk. Note that this requirement can be
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relaxed, depending on the type of commit you perform.
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See <a class="xref" href="nodurabletxn.html" title="Non-Durable Transactions">Non-Durable Transactions</a> for
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more information.
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</p>
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<p>
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Note that a transaction commit only writes the BTree's leaf nodes to JE's log files. All other
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internal BTree structures are left unwritten.
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</p>
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</li>
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<li>
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<p>
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All locks held by the transaction are released. This means
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that read operations performed by other transactions or
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threads of control can now see the modifications without
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resorting to uncommitted reads (see <a class="xref" href="isolation.html#dirtyreads" title="Reading Uncommitted Data">Reading Uncommitted Data</a> for more information).
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</p>
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</li>
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</ul>
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</div>
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<p>
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To commit a transaction, you simply call
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<span><code class="methodname">Transaction.commit()</code>.</span>
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</p>
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<p>
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Remember that transaction commit causes only the BTree leaf nodes to be written to JE's log files. Any
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other modifications made to the the BTree as a result of the transaction's activities are not written to the
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log file. This means that over time JE's normal recovery time can greatly increase (remember that JE always runs
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normal recovery when it opens an environment).
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</p>
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<p>
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For this reason, JE by default runs the checkpointer thread. This background thread runs
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a checkpoint on a periodic interval so as to ensure that the amount of data that needs to be
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recovered upon environment open is minimized. In addition, you can also run a checkpoint manually.
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For more information, see <a class="xref" href="chkpoint.html" title="Checkpoints">Checkpoints</a>.
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</p>
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<p>
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Note that once you have committed a transaction, the transaction
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handle that you used for the transaction is no longer valid. To
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perform database activities under the control of a new
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transaction, you must obtain a fresh transaction handle.
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</p>
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</div>
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</div>
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||
<div class="navfooter">
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<hr />
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||
<table width="100%" summary="Navigation footer">
|
||
<tr>
|
||
<td width="40%" align="left"><a accesskey="p" href="enabletxn.html">Prev</a> </td>
|
||
<td width="20%" align="center"> </td>
|
||
<td width="40%" align="right"> <a accesskey="n" href="nodurabletxn.html">Next</a></td>
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||
</tr>
|
||
<tr>
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||
<td width="40%" align="left" valign="top">Chapter 2. Enabling Transactions </td>
|
||
<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Non-Durable Transactions</td>
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</tr>
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||
</table>
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||
</div>
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||
</body>
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||
</html>
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