mirror of
https://github.com/yasuhirokimura/db18
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Yasuhiro KIMURA
201 lines
7.8 KiB
HTML
201 lines
7.8 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>Opening the databases</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="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="transapp.html" title="Chapter 11. Berkeley DB Transactional Data Store Applications" />
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<link rel="prev" href="transapp_env_open.html" title="Opening the environment" />
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<link rel="next" href="transapp_put.html" title="Recoverability and deadlock handling" />
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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 18.1.40</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">Opening the databases</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="transapp_env_open.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 11. Berkeley DB Transactional Data Store Applications </th>
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<td width="20%" align="right"> <a accesskey="n" href="transapp_put.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="transapp_data_open"></a>Opening the databases</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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Next, we open three databases ("color" and "fruit" and
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"cats"), in the database environment. Again, our <a href="../api_reference/C/db.html" class="olink">DB</a> database
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handles are declared to be free-threaded using the <a href="../api_reference/C/dbopen.html#open_DB_THREAD" class="olink">DB_THREAD</a>
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flag, and so may be used by any number of threads we
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subsequently create.
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</p>
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<a id="prog_transapp2"></a>
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<pre class="programlisting">int
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main(int argc, char *argv[])
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{
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extern int optind;
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DB *db_cats, *db_color, *db_fruit;
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DB_ENV *dbenv;
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int ch;
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while ((ch = getopt(argc, argv, "")) != EOF)
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switch (ch) {
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case '?':
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default:
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usage();
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}
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argc -= optind;
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argv += optind;
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env_dir_create();
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env_open(&dbenv);
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...
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/* Open database: Key is fruit class; Data is specific type. */
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if (db_open(dbenv, &db_fruit, "fruit", 0))
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return (1);
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/* Open database: Key is a color; Data is an integer. */
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if (db_open(dbenv, &db_color, "color", 0))
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return (1);
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/*
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* Open database:
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* Key is a name; Data is: company name, cat breeds.
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*/
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if (db_open(dbenv, &db_cats, "cats", 1))
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return (1);
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...
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return (0);
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}
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int
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db_open(DB_ENV *dbenv, DB **dbp, char *name, int dups)
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{
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DB *db;
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int ret;
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/* Create the database handle. */
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if ((ret = db_create(&db, dbenv, 0)) != 0) {
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dbenv->err(dbenv, ret, "db_create");
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return (1);
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}
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/* Optionally, turn on duplicate data items. */
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if (dups && (ret = db->set_flags(db, DB_DUP)) != 0) {
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(void)db->close(db, 0);
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dbenv->err(dbenv, ret, "db->set_flags: DB_DUP");
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return (1);
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}
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/*
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* Open a database in the environment:
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* create if it doesn't exist
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* free-threaded handle
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* read/write owner only
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*/
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if ((ret = db->open(db, NULL, name, NULL, DB_BTREE,
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DB_AUTO_COMMIT | DB_CREATE | DB_THREAD, S_IRUSR | S_IWUSR)) != 0) {
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(void)db->close(db, 0);
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dbenv->err(dbenv, ret, "db->open: %s", name);
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return (1);
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}
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*dbp = db;
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return (0);
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}</pre>
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<p>
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After opening the database, we can use the <a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility to
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display information about a database we have created:
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</p>
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<pre class="programlisting">prompt> db_stat -h TXNAPP -d color
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53162 Btree magic number.
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8 Btree version number.
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Flags:
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2 Minimum keys per-page.
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8192 Underlying database page size.
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1 Number of levels in the tree.
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0 Number of unique keys in the tree.
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0 Number of data items in the tree.
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0 Number of tree internal pages.
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0 Number of bytes free in tree internal pages (0% ff).
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1 Number of tree leaf pages.
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8166 Number of bytes free in tree leaf pages (0.% ff).
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0 Number of tree duplicate pages.
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0 Number of bytes free in tree duplicate pages (0% ff).
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0 Number of tree overflow pages.
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0 Number of bytes free in tree overflow pages (0% ff).
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0 Number of pages on the free list.</pre>
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<p>
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The database open must be enclosed within a transaction in
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order to be recoverable. The transaction will ensure that
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created files are re-created in recovered environments (or do
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not appear at all). Additional database operations or
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operations on other databases can be included in the same
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transaction, of course. In the simple case, where the open is
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the only operation in the transaction, an application can set
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the <a href="../api_reference/C/envset_flags.html#envset_flags_DB_AUTO_COMMIT" class="olink">DB_AUTO_COMMIT</a> flag instead of creating and managing its
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own transaction handle. The <a href="../api_reference/C/envset_flags.html#envset_flags_DB_AUTO_COMMIT" class="olink">DB_AUTO_COMMIT</a> flag will
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internally wrap the operation in a transaction, simplifying
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application code.
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</p>
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<p>
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The previous example is the simplest case of transaction
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protection for database open. Obviously, additional database
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operations can be done in the scope of the same transaction.
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For example, an application maintaining a list of the
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databases in a database environment in a well-known file might
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include an update of the list in the same transaction in which
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the database is created. Or, an application might create both
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a primary and secondary database in a single
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transaction.
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</p>
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<p>
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<a href="../api_reference/C/db.html" class="olink">DB</a> handles that will later be used for transactionally
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protected database operations must be opened within a
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transaction. Specifying a transaction handle to database
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operations using <a href="../api_reference/C/db.html" class="olink">DB</a> handles not opened within a transaction
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will return an error. Similarly, not specifying a transaction
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handle to database operations that will modify the database,
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using handles that were opened within a transaction, will also
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return an error.
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</p>
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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="transapp_env_open.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="transapp.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="transapp_put.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">Opening the environment </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"> Recoverability and deadlock handling</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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