<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <title>Opening a Transactional Environment and Database </title> <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> <meta name="generator" content="DocBook XSL Stylesheets V1.62.4" /> <link rel="home" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" /> <link rel="up" href="enabletxn.html" title="Chapter 2. Enabling Transactions" /> <link rel="previous" href="enabletxn.html" title="Chapter 2. Enabling Transactions" /> <link rel="next" href="usingtxns.html" title="Chapter 3. Transaction Basics" /> </head> <body> <div class="navheader"> <table width="100%" summary="Navigation header"> <tr> <th colspan="3" align="center">Opening a Transactional Environment and Database </th> </tr> <tr> <td width="20%" align="left"><a accesskey="p" href="enabletxn.html">Prev</a> </td> <th width="60%" align="center">Chapter 2. Enabling Transactions</th> <td width="20%" align="right"> <a accesskey="n" href="usingtxns.html">Next</a></td> </tr> </table> <hr /> </div> <div class="sect1" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h2 class="title" style="clear: both"><a id="envopen"></a>Opening a Transactional Environment and <span>Database</span> </h2> </div> </div> <div></div> </div> <p> To enable transactions for your environment, you must initialize the transactional subsystem. Note that doing this also initializes the logging subsystem. In addition, you must initialize the memory pool (in-memory cache). Frequently, but not always, you will also initialize the locking subsystem. <span> For example: </span> </p> <pre class="programlisting">#include "db_cxx.h" ... int main(void) { u_int32_t env_flags = DB_CREATE | // If the environment does not // exist, create it. DB_INIT_LOCK | // Initialize locking DB_INIT_LOG | // Initialize logging DB_INIT_MPOOL | // Initialize the cache DB_INIT_TXN; // Initialize transactions std::string envHome("/export1/testEnv"); DbEnv myEnv(0); try { myEnv.open(envHome.c_str(), env_flags, 0); } catch(DbException &e) { std::cerr << "Error opening database environment: " << envHome << std::endl; std::cerr << e.what() << std::endl; return (EXIT_FAILURE); } try { myEnv.close(0); } catch(DbException &e) { std::cerr << "Error closing database environment: " << envHome << std::endl; std::cerr << e.what() << std::endl; return (EXIT_FAILURE); } return (EXIT_SUCCESS); } </pre> <p> You then create and open your database(s) as you would for a non-transactional system. <span> The only difference is that you must pass the environment handle to the <span> <tt class="methodname">DbEnv::open()</tt> method, </span> and you must open the database within a transaction. Typically auto commit is used for this purpose. To do so, pass <tt class="literal">DB_AUTO_COMMIT</tt> to the database open command. Also, make sure you close all your databases before you close your environment. For example: </span> </p> <pre class="programlisting">#include "db_cxx.h" ... int main(void) { u_int32_t env_flags = DB_CREATE | // If the environment does not // exist, create it. DB_INIT_LOCK | // Initialize locking DB_INIT_LOG | // Initialize logging DB_INIT_MPOOL | // Initialize the cache DB_INIT_TXN; // Initialize transactions <b class="userinput"><tt>u_int32_t db_flags = DB_CREATE | DB_AUTO_COMMIT; Db *dbp = NULL; const char *file_name = "mydb.db";</tt></b> std::string envHome("/export1/testEnv"); DbEnv myEnv(0); try { myEnv.open(envHome.c_str(), env_flags, 0); <b class="userinput"><tt>dbp = new Db(&myEnv, 0); dbp->open(NULL, // Txn pointer file_name, // File name NULL, // Logical db name DB_BTREE, // Database type (using btree) db_flags, // Open flags 0); // File mode. Using defaults </tt></b> } catch(DbException &e) { std::cerr << "Error opening database and environment: " << file_name << ", " << envHome << std::endl; std::cerr << e.what() << std::endl; } try { <b class="userinput"><tt>dbp->close(0);</tt></b> myEnv.close(0); } catch(DbException &e) { std::cerr << "Error closing database and environment: " << file_name << ", " << envHome << std::endl; std::cerr << e.what() << std::endl; return (EXIT_FAILURE); } return (EXIT_SUCCESS); } </pre> <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"> <h3 class="title">Note</h3> <p> Never close a database that has active transactions. Make sure all transactions are resolved (either committed or aborted) before closing the database. </p> </div> </div> <div class="navfooter"> <hr /> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"><a accesskey="p" href="enabletxn.html">Prev</a> </td> <td width="20%" align="center"> <a accesskey="u" href="enabletxn.html">Up</a> </td> <td width="40%" align="right"> <a accesskey="n" href="usingtxns.html">Next</a></td> </tr> <tr> <td width="40%" align="left" valign="top">Chapter 2. Enabling Transactions </td> <td width="20%" align="center"> <a accesskey="h" href="index.html">Home</a> </td> <td width="40%" align="right" valign="top"> Chapter 3. Transaction Basics</td> </tr> </table> </div> </body> </html>