251 lines
8.5 KiB
HTML
251 lines
8.5 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 a Transactional Environment and
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Database
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</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 V1.62.4" />
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<link rel="home" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" />
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<link rel="up" href="enabletxn.html" title="Chapter 2. Enabling Transactions" />
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<link rel="previous" href="enabletxn.html" title="Chapter 2. Enabling Transactions" />
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<link rel="next" href="usingtxns.html" title="Chapter 3. Transaction Basics" />
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</head>
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<body>
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<div class="navheader">
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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 a Transactional Environment and
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Database
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</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="enabletxn.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 2. Enabling Transactions</th>
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<td width="20%" align="right"> <a accesskey="n" href="usingtxns.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="envopen"></a>Opening a Transactional Environment and
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<span>Database</span>
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</h2>
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</div>
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</div>
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<div></div>
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</div>
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<p>
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To enable transactions for your environment, you must initialize the
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transactional subsystem. Note that doing this also initializes the
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logging subsystem. In addition, you must initialize the memory pool
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(in-memory cache). Frequently, but not always, you will also
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initialize the locking subsystem.
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<span>
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For example:
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</span>
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</p>
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<p>
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Notice in the following example that you create your environment
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handle using the <tt class="function">db_env_create()</tt> function before you open
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the environment:
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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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int ret, ret_c;
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u_int32_t env_flags;
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DB_ENV *envp;
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const char *db_home_dir = "/tmp/myEnvironment";
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envp = 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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err:
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/* Close the environment */
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if (envp != NULL) {
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ret_c = envp->close(envp, 0);
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if (ret_c != 0) {
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fprintf(stderr, "environment close failed: %s\n",
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db_strerror(ret_c));
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ret = ret_c;
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}
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}
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return (ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
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} </pre>
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<p>
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You then create and open your database(s) as you would for a non-transactional system.
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<span>
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The only difference is that you must pass the environment handle to
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the
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<span>
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<tt class="function">db_create()</tt> function,
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</span>
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and you must open the database within a transaction.
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Typically auto commit is used for this purpose. To do so, pass
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<tt class="literal">DB_AUTO_COMMIT</tt> to the database open command.
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Also, make sure you close all your databases before you close
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your environment.
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For example:
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</span>
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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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int ret, ret_c;
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u_int32_t <b class="userinput"><tt>db_flags,</tt></b> env_flags;
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<b class="userinput"><tt>DB *dbp;</tt></b>
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DB_ENV *envp;
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const char *db_home_dir = "/tmp/myEnvironment";
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<b class="userinput"><tt>const char *file_name = "mydb.db";
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dbp = NULL;</tt></b>
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envp = 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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<b class="userinput"><tt>/* 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 | DB_AUTO_COMMIT;
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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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}</tt></b>
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err:
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<b class="userinput"><tt>/* Close the database */
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if (dbp != NULL) {
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ret_c = dbp->close(dbp, 0);
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if (ret_c != 0) {
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envp->err(envp, ret_c, "Database close failed.");
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ret = ret_c
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}
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}</tt></b>
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/* Close the environment */
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if (envp != NULL) {
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ret_c = envp->close(envp, 0);
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if (ret_c != 0) {
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fprintf(stderr, "environment close failed: %s\n",
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db_strerror(ret_c));
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ret = ret_c;
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}
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}
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return (ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
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} </pre>
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<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
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<h3 class="title">Note</h3>
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<p>
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Never close a database that has active transactions. Make sure
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all transactions are resolved (either committed or aborted)
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before closing the database.
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</p>
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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="enabletxn.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="enabletxn.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="usingtxns.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">Chapter 2. Enabling Transactions </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 3. Transaction Basics</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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