179 lines
6.8 KiB
HTML
179 lines
6.8 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>Managing Databases in Environments</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" />
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<link rel="up" href="DB.html" title="Chapter 2. Databases" />
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<link rel="previous" href="dbErrorReporting.html" title="Error Reporting Functions" />
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<link rel="next" href="CoreDbCXXUsage.html" title="Database Example" />
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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">Managing Databases in Environments</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="dbErrorReporting.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 2. Databases</th>
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<td width="20%" align="right"> <a accesskey="n" href="CoreDbCXXUsage.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="CoreEnvUsage"></a>Managing Databases in Environments</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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In
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<span>
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<a href="environments.html">Environments</a>,
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</span>
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we introduced
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environments. While environments are not used in the example built in this book,
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they are so commonly used for a wide class of DB applications that it is
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necessary to show their basic usage, if only from a completeness perspective.
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</p>
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<p>
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To use an environment, you must first
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open it. At open time, you must identify the directory in
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which it resides. This directory must exist prior to the open attempt.
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You can also identify open properties, such as whether the environment can be
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created if it does not already exist.
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</p>
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<p>
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You will also need to initialize the in-memory cache when you open your environment.
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</p>
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<p>
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For example, to
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<span>create an environment handle and</span>
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open an environment:
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</p>
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<a id="cxx_env1"></a>
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<pre class="programlisting">#include <db_cxx.h>
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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_MPOOL; // Initialize the in-memory cache.
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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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myEnv.open(envHome.c_str(), env_flags, 0);
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} catch(DbException &e) {
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std::cerr << "Error opening database environment: "
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<< envHome << std::endl;
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std::cerr << e.what() << std::endl;
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exit( -1 );
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} catch(std::exception &e) {
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std::cerr << "Error opening database environment: "
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<< envHome << std::endl;
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std::cerr << e.what() << std::endl;
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exit( -1 );
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} </pre>
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<p>
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Once an environment is opened, you can open databases in it. Note that by default databases
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are stored in the environment's home directory, or relative to that directory if you
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provide any sort of a path in the database's file name:
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</p>
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<a id="cxx_env2"></a>
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<pre class="programlisting">#include <db_cxx.h>
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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_MPOOL; // Initialize the in-memory cache.
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std::string envHome("/export1/testEnv");
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<b class="userinput"><tt>u_int32_t db_flags = DB_CREATE; // If the database does not
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// exist, create it.</tt></b>
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<b class="userinput"><tt>std::string dbName("mydb.db");</tt></b>
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DbEnv myEnv(0);
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<b class="userinput"><tt>Db *myDb;</tt></b>
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try {
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myEnv.open(envHome.c_str(), env_flags, 0);
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<b class="userinput"><tt>myDb = new Db(&myEnv, 0);
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myDb->open(NULL,
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dbName.c_str(),
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NULL,
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DB_BTREE,
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db_flags,
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0);</tt></b>
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} catch(DbException &e) {
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std::cerr << "Error opening database environment: "
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<< envHome
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<< " and database "
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<< dbName << std::endl;
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std::cerr << e.what() << std::endl;
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exit( -1 );
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} catch(std::exception &e) {
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std::cerr << "Error opening database environment: "
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<< envHome
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<< " and database "
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<< dbName << std::endl;
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std::cerr << e.what() << std::endl;
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exit( -1 );
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} </pre>
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<p>
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When you are done with an environment, you must close it. Before you close an environment,
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make sure you close any opened databases.
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</p>
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<a id="cxx_env3"></a>
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<pre class="programlisting">try {
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if (myDb != NULL) {
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myDb->close(0);
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}
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myEnv.close(0);
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} catch(DbException &e) {
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std::cerr << "Error closing database environment: "
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<< envHome
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<< " or database "
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<< dbName << std::endl;
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std::cerr << e.what() << std::endl;
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exit( -1 );
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} catch(std::exception &e) {
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std::cerr << "Error closing database environment: "
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<< envHome
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<< " or database "
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<< dbName << std::endl;
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std::cerr << e.what() << std::endl;
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exit( -1 );
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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="dbErrorReporting.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="DB.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="CoreDbCXXUsage.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">Error Reporting Functions </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"> Database Example</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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