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212 lines
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HTML
212 lines
9 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>Configuring the Transaction Subsystem</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="usingtxns.html" title="Chapter 3. Transaction Basics" />
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<link rel="prev" href="txnindices.html" title="Secondary Indices with Transaction Applications" />
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<link rel="next" href="txnconcurrency.html" title="Chapter 4. Concurrency" />
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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">Configuring the Transaction Subsystem</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="txnindices.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 3. Transaction Basics</th>
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<td width="20%" align="right"> <a accesskey="n" href="txnconcurrency.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="maxtxns"></a>Configuring the Transaction Subsystem</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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Most of the configuration activities that you need to perform
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for your transactional DB application will involve the
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locking and logging subsystems. See
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<a class="xref" href="txnconcurrency.html" title="Chapter 4. Concurrency">Concurrency</a>
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and
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<a class="xref" href="filemanagement.html" title="Chapter 5. Managing DB Files">Managing DB Files</a>
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for details.
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</p>
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<p>
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However, there are a couple of things that you can do to
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configure your transaction subsystem directly. These things
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are:
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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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<span>
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<p>
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<span>
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Configure
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</span>
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the maximum number of simultaneous
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transactions needed by your application.
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In general, you should not need to do this unless you
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use deeply nested transactions or you have many threads all
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of which have active transactions. In addition, you may
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need to configure a higher maximum number of transactions if you
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are using snapshot isolation. See
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<a class="xref" href="isolation.html#sisolation_maxtxn" title="Snapshot Isolation Transactional Requirements">Snapshot Isolation Transactional Requirements</a>
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for details.
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</p>
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<p>
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By default, your application can support 20 active
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transactions.
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</p>
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<p>
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You can set the maximum number of simultaneous
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transactions supported by your application using
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<span>
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the
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<code class="methodname">DbEnv::set_tx_max()</code>
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method. Note that this method must be called
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before the environment has been opened.
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</span>
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</p>
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<p>
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If your application has exceeded this maximum value,
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then any attempt to begin a new transaction will fail.
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</p>
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<p>
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This value can also be set using the
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<code class="literal">DB_CONFIG</code> file's
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<code class="literal">set_tx_max</code> parameter. Remember that
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the <code class="literal">DB_CONFIG</code> must reside in your
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environment home directory.
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</p>
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</span>
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</li>
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<li>
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<span>
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<p>
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<span>
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Configure the timeout value for your transactions.
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</span>
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This value represents the longest period of time a
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transaction can be active. Note, however, that
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transaction timeouts are checked only when DB
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examines its lock tables for blocked locks
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(see <a class="xref" href="blocking_deadlocks.html" title="Locks, Blocks, and Deadlocks">Locks, Blocks, and Deadlocks</a>
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for more information). Therefore, a transaction's timeout can
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have expired, but the application will not be notified until DB
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has a reason to examine its lock tables.
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</p>
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<p>
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Be aware that some transactions may be
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inappropriately timed out before the transaction has a
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chance to complete. You should therefore use this
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mechanism only if you know your application
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might have unacceptably long transactions and
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you want to make sure your application will
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not stall during their execution.
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(This might happen if, for example, your
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transaction blocks or requests too much
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data.)
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</p>
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<p>
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Note that by default transaction timeouts are set to 0 seconds, which means that they never time
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out.
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</p>
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<p>
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To set the maximum timeout value for your transactions,
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use the
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<span><code class="methodname">DbEnv::set_timeout()</code></span>
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method. This method configures the entire
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environment; not just the handle used to set the
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configuration. Further, this value may
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be set at any time during the application's
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lifetime.
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</p>
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<p>
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This value can also be set using the
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<code class="literal">DB_CONFIG</code> file's
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<code class="literal">set_txn_timeout</code> parameter.
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</p>
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</span>
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</li>
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</ul>
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</div>
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<p>
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For example:
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</p>
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<pre class="programlisting">#include "db_cxx.h"
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...
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int main(void)
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{
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u_int32_t env_flags = DB_CREATE | // If the environment does not
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// exist, create it.
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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 cache
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DB_THREAD | // Free-thread the env handle
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DB_INIT_TXN; // Initialize transactions
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std::string envHome("/export1/testEnv");
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DbEnv myEnv(0);
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try {
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// Configure a maximum transaction timeout of 1 second.
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myEnv.set_timeout(1000000, DB_SET_TXN_TIMEOUT);
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// Configure 40 maximum transactions.
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myEnv.set_tx_max(40);
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myEnv.open(envHome.c_str(), env_flags, 0);
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// From here, you open your databases, proceed with your
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// database operations, and respond to deadlocks as
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// is normal (omitted for brevity).
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...</pre>
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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="txnindices.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="usingtxns.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="txnconcurrency.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">Secondary Indices with Transaction Applications </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"> Chapter 4. Concurrency</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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