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227 lines
8.7 KiB
HTML
227 lines
8.7 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>Transactional Cursors and Concurrent Applications</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="txnconcurrency.html" title="Chapter 4. Concurrency" />
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<link rel="prev" href="isolation.html" title="Isolation" />
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<link rel="next" href="readmodifywrite.html" title="Read/Modify/Write" />
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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">Transactional Cursors and Concurrent Applications</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="isolation.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 4. Concurrency</th>
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<td width="20%" align="right"> <a accesskey="n" href="readmodifywrite.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="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="txn_ccursor"></a>Transactional Cursors and Concurrent Applications</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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<dl>
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<dt>
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<span class="sect2">
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<a href="txn_ccursor.html#cursordirtyreads">Using Cursors with Uncommitted Data</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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When you use transactional cursors with a concurrent application, remember that
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in the event of a deadlock you must make sure that you close your cursor before you abort and retry your
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transaction. <span>This is true of both
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base API and DPL cursors.</span>
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</p>
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<p>
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Also, remember that when you are using the default isolation level,
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every time your cursor reads a record it locks
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that record until the encompassing transaction is resolved. This
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means that walking your database with a transactional cursor
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increases the chance of lock contention.
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</p>
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<p>
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For this reason, if you must routinely walk your database with a
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transactional cursor, consider using a reduced isolation level
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such as read committed. <span>This is
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true of both base API and DPL cursors.</span>
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</p>
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<div class="sect2" 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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<h3 class="title"><a id="cursordirtyreads"></a>Using Cursors with Uncommitted Data</h3>
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</div>
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</div>
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</div>
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<p>
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As described in <a class="xref" href="isolation.html#dirtyreads" title="Reading Uncommitted Data">Reading Uncommitted Data</a>
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above, it is possible to relax your transaction's isolation
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level such that it can read data modified but not yet committed
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by another transaction. You can configure this when you create
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your transaction handle, and when you do so then all cursors opened
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inside that transaction will automatically use uncommitted reads.
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</p>
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<p>
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You can also do this when you create a cursor handle from within
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a serializable transaction. When you do this, only those
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cursors configured for uncommitted reads uses uncommitted reads.
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</p>
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<p>
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The following example shows how to configure an individual cursor handle
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to read uncommitted data from within a serializable (full isolation) transaction.
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For an example of
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configuring a transaction to perform uncommitted reads in
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general, see <a class="xref" href="isolation.html#dirtyreads" title="Reading Uncommitted Data">Reading Uncommitted Data</a>.
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</p>
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<pre class="programlisting">package je.txn;
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import com.sleepycat.je.Cursor;
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import com.sleepycat.je.CursorConfig;
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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.Environment;
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import com.sleepycat.je.EnvironmentConfig;
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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.
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DatabaseConfig dbConfig = new DatabaseConfig();
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dbConfig.setTransactional(true);
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myDatabase = myEnv.openDatabase(null, // txn handle
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"sampleDatabase", // db file name
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dbConfig);
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// Open the transaction. Note that this is a repeatable
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// read transaction.
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Transaction txn = myEnv.beginTransaction(null, null);
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Cursor cursor = null;
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try {
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// Use the transaction handle here
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// Get our cursor. Note that we pass the transaction
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// handle here. Note also that we cause the cursor
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// to perform uncommitted reads.
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CursorConfig cconfig = new CursorConfig();
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cconfig.setReadUncommitted(true);
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cursor = db.openCursor(txn, cconfig);
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// From here, you perform your cursor reads and writes
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// as normal, committing and aborting the transactions as
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// is necessary, and testing for deadlock exceptions as
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// normal (omitted for brevity).
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... </pre>
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<p>
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If you are using the DPL:
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</p>
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<pre class="programlisting">package persist.txn;
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import com.sleepycat.je.CursorConfig;
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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.persist.EntityCursor;
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import com.sleepycat.persist.EntityStore;
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import com.sleepycat.persist.PrimaryIndex;
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import com.sleepycat.persist.StoreConfig;
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import java.util.Iterator;
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import java.io.File;
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...
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EntityStore myStore = null;
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Environment myEnv = null;
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PrimaryIndex<String,AnEntityClass> pKey;
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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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// Set up the entity store
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StoreConfig myStoreConfig = new StoreConfig();
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myStoreConfig.setAllowCreate(true);
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myStoreConfig.setTransactional(true);
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// Instantiate the store
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myStore = new EntityStore(myEnv, storeName, myStoreConfig);
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// Open the transaction. Note that this is a repeatable
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// read transaction.
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Transaction txn = myEnv.beginTransaction(null, null);
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//Configure our cursor for uncommitted reads.
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CursorConfig cconfig = new CursorConfig();
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cconfig.setReadUncommitted(true);
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// Get our cursor. Note that we pass the transaction
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// handle here. Note also that we cause the cursor
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// to perform uncommitted reads.
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EntityCursor<AnEntityClass> cursor = pKey.entities(txn, cconfig);
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try {
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// From here, you perform your cursor reads and writes
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// as normal, committing and aborting the transactions as
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// is necessary, and testing for deadlock exceptions as
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// normal (omitted for brevity).
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... </pre>
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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">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="isolation.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="txnconcurrency.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="readmodifywrite.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Isolation </td>
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<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"> Read/Modify/Write</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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