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231 lines
8.4 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 environment</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_app.html" title="Architecting Transactional Data Store applications" />
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<link rel="next" href="transapp_data_open.html" title="Opening the databases" />
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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 11.2.5.3</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 environment</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_app.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 11.
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Berkeley DB Transactional Data Store Applications
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="transapp_data_open.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_env_open"></a>Opening the environment</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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Creating transaction-protected applications using the Berkeley DB
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library is quite easy. Applications first use <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> to initialize
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the database environment. Transaction-protected applications normally
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require all four Berkeley DB subsystems, so the <a href="../api_reference/C/envopen.html#envopen_DB_INIT_MPOOL" class="olink">DB_INIT_MPOOL</a>,
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<a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOCK" class="olink">DB_INIT_LOCK</a>, <a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOG" class="olink">DB_INIT_LOG</a>, and <a href="../api_reference/C/envopen.html#envopen_DB_INIT_TXN" class="olink">DB_INIT_TXN</a> flags should be
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specified.
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</p>
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<p>
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Once the application has called <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a>, it opens its databases
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within the environment. Once the databases are opened, the application
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makes changes to the databases inside of transactions. Each set of
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changes that entails a unit of work should be surrounded by the
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appropriate <a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a>, <a href="../api_reference/C/txncommit.html" class="olink">DB_TXN->commit()</a> and <a href="../api_reference/C/txnabort.html" class="olink">DB_TXN->abort()</a> calls. The Berkeley
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DB access methods will make the appropriate calls into the Lock, Log
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and Memory Pool subsystems in order to guarantee transaction semantics.
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When the application is ready to exit, all outstanding transactions
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should have been committed or aborted.
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</p>
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<p>
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Databases accessed by a transaction must not be closed during the
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transaction. Once all outstanding transactions are finished, all open
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Berkeley DB files should be closed. When the Berkeley DB database
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files have been closed, the environment should be closed by calling
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<a href="../api_reference/C/envclose.html" class="olink">DB_ENV->close()</a>.
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</p>
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<p>
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The following code fragment creates the database environment directory
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then opens the environment, running recovery. Our <a href="../api_reference/C/env.html" class="olink">DB_ENV</a> database
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environment handle is declared to be free-threaded using the
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<a href="../api_reference/C/dbopen.html#open_DB_THREAD" class="olink">DB_THREAD</a> flag, and so may be used by any number of threads that we
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may subsequently create.
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</p>
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<a id="prog_transapp1"></a>
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<pre class="programlisting">#include <sys/types.h>
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#include <sys/stat.h>
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#include <errno.h>
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#include <stdarg.h>
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#include <stdlib.h>
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#include <string.h>
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#include <unistd.h>
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#include <db.h>
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#define ENV_DIRECTORY "TXNAPP"
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void env_dir_create(void);
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void env_open(DB_ENV **);
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...
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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_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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return (0);
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}
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...
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void
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env_dir_create()
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{
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struct stat sb;
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/*
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* If the directory exists, we're done. We do not further check
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* the type of the file, DB will fail appropriately if it's the
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* wrong type.
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*/
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if (stat(ENV_DIRECTORY, &sb) == 0)
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return;
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/* Create the directory, read/write/access owner only. */
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if (mkdir(ENV_DIRECTORY, S_IRWXU) != 0) {
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fprintf(stderr,
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"txnapp: mkdir: %s: %s\n", ENV_DIRECTORY, strerror(errno));
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exit (1);
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}
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}
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void
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env_open(DB_ENV **dbenvp)
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{
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DB_ENV *dbenv;
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int ret;
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/* Create the environment handle. */
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if ((ret = db_env_create(&dbenv, 0)) != 0) {
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fprintf(stderr,
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"txnapp: db_env_create: %s\n", db_strerror(ret));
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exit (1);
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}
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/* Set up error handling. */
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dbenv->set_errpfx(dbenv, "txnapp");
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dbenv->set_errfile(dbenv, stderr);
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/*
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* Open a transactional environment:
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* create if it doesn't exist
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* free-threaded handle
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* run recovery
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* read/write owner only
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*/
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if ((ret = dbenv->open(dbenv, ENV_DIRECTORY,
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DB_CREATE | DB_INIT_LOCK | DB_INIT_LOG |
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DB_INIT_MPOOL | DB_INIT_TXN | DB_RECOVER | DB_THREAD,
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S_IRUSR | S_IWUSR)) != 0) {
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(void)dbenv->close(dbenv, 0);
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fprintf(stderr, "dbenv->open: %s: %s\n",
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ENV_DIRECTORY, db_strerror(ret));
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exit (1);
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}
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*dbenvp = dbenv;
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}</pre>
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<p>
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After running this initial program, we can use the <a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility to display
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the contents of the environment directory:
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</p>
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<pre class="programlisting">prompt> db_stat -e -h TXNAPP
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3.2.1 Environment version.
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120897 Magic number.
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0 Panic value.
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1 References.
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6 Locks granted without waiting.
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0 Locks granted after waiting.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
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Mpool Region: 4.
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264KB Size (270336 bytes).
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-1 Segment ID.
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1 Locks granted without waiting.
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0 Locks granted after waiting.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
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Log Region: 3.
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96KB Size (98304 bytes).
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-1 Segment ID.
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3 Locks granted without waiting.
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0 Locks granted after waiting.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
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Lock Region: 2.
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240KB Size (245760 bytes).
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-1 Segment ID.
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1 Locks granted without waiting.
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0 Locks granted after waiting.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
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Txn Region: 5.
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8KB Size (8192 bytes).
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-1 Segment ID.
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1 Locks granted without waiting.
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0 Locks granted after waiting.
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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="transapp_app.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_data_open.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">Architecting Transactional Data Store 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"> Opening the databases</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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