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212 lines
8.2 KiB
HTML
212 lines
8.2 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 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 11.2.5.2</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.
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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.
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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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Either way, you must first configure your database
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handle to support
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uncommitted reads before you can configure your transactions or
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your cursors to use them.
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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">#include <stdio.h>
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#include <stdlib.h>
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#include "db.h"
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int
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main(void)
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{
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DB *dbp;
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DBC *cursorp;
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DB_ENV *envp;
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DB_TXN *txn;
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int ret, c_ret;
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char *replacementString = "new string";
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dbp = NULL;
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envp = NULL;
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txn = NULL;
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/* Open the environment */
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ret = db_env_create(&envp, 0);
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if (ret != 0) {
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fprintf(stderr, "Error creating environment handle: %s\n",
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db_strerror(ret));
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return (EXIT_FAILURE);
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}
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env_flags = DB_CREATE | /* Create the environment if it does
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* not already exist. */
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DB_INIT_TXN | /* Initialize transactions */
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DB_INIT_LOCK | /* Initialize locking. */
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DB_INIT_LOG | /* Initialize logging */
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DB_INIT_MPOOL; /* Initialize the in-memory cache. */
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ret = envp->open(envp, db_home_dir, env_flags, 0);
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if (ret != 0) {
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fprintf(stderr, "Error opening environment: %s\n",
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db_strerror(ret));
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goto err;
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}
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/* Initialize the DB handle */
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ret = db_create(&dbp, envp, 0);
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if (ret != 0) {
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envp->err(envp, ret, "Database creation failed");
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goto err;
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}
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db_flags = DB_CREATE | /* Create the db if it does not
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* exist */
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DB_AUTO_COMMIT | /* Enable auto commit */
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DB_READ_UNCOMMITTED; /* Enable uncommitted reads */
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ret = dbp->open(dbp, /* Pointer to the database */
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NULL, /* Txn pointer */
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file_name, /* File name */
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NULL, /* Logical db name */
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DB_BTREE, /* Database type (using btree) */
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db_flags, /* Open flags */
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0); /* File mode. Using defaults */
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if (ret != 0) {
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envp->err(envp, ret, "Database '%s' open failed",
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file_name);
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goto err;
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}
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/* Get the txn handle */
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/* Note that this is a degree 3 transaction */
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ret = envp->txn_begin(envp, NULL, &txn, 0);
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if (ret != 0) {
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envp->err(envp, ret, "Transaction begin failed.");
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goto err;
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}
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/* Get the cursor, supply the txn handle at that time */
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/* Cause the cursor to perform uncommitted reads */
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ret = dbp->cursor(dbp, txn, &cursorp, DB_READ_UNCOMMITTED);
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if (ret != 0) {
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envp->err(envp, ret, "Cursor open failed.");
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txn->abort(txn);
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goto err;
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}
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/*
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* From here, you perform your cursor reads and writes as normal,
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* committing and aborting the transactions as is necessary, and
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* testing for deadlock exceptions as normal (omitted for brevity).
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*/
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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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