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199 lines
7.7 KiB
HTML
199 lines
7.7 KiB
HTML
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<?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>Auto Commit</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="Getting Started with Berkeley DB Transaction Processing" />
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<link rel="up" href="usingtxns.html" title="Chapter 3. Transaction Basics" />
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<link rel="prev" href="abortresults.html" title="Aborting a Transaction" />
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<link rel="next" href="nestedtxn.html" title="Nested Transactions" />
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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 11.2.5.2</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">Auto Commit</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="abortresults.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 3. Transaction Basics</th>
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<td width="20%" align="right"> <a accesskey="n" href="nestedtxn.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="autocommit"></a>Auto Commit</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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While transactions are frequently used to provide atomicity to
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multiple database
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operations, it is sometimes necessary to perform
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a single database
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operation under the control of a transaction.
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Rather than force you to obtain a transaction, perform the single
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write operation, and then either commit or abort the transaction,
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you can automatically group this sequence of events using
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<span class="emphasis"><em>auto commit</em></span>.
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</p>
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<p>
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To use auto commit:
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</p>
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<div class="orderedlist">
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<ol type="1">
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<li>
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<p>
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Open your environment and your databases
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so that they support
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transactions. See <a class="xref" href="enabletxn.html" title="Chapter 2. Enabling Transactions">Enabling Transactions</a>
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for details.
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</p>
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<p>
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Note that frequently auto commit is used for the environment
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or database open. To use auto commit for either your
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environment or database open, specify
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<code class="literal">DB_AUTO_COMMIT</code> to the
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<code class="methodname">DbEnv::set_flags()</code>
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or
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<code class="methodname">Db::open()</code>
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method. If you specify auto commit for the environment
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open, then you do not need to also specify auto commit
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for the database open.
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</p>
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</li>
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<li>
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<p>
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Do not provide a transactional handle to the method that is
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performing the database
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write operation.
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</p>
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</li>
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</ol>
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</div>
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<p>
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Note that auto commit is not available for cursors. You must always
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open your cursor using a transaction if you want the cursor's
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operations to be transactional protected. See
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<a class="xref" href="txncursor.html" title="Transactional Cursors">Transactional Cursors</a> for details on using
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transactional cursors.
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</p>
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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 have more than one active transaction in your thread
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at a time. This is especially a problem if you mix an
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explicit transaction with another operation that uses auto
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commit. Doing so can result in undetectable deadlocks.
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</p>
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</div>
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<p>
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For example, the following uses auto commit to perform the database write operation:
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</p>
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<pre class="programlisting">#include "db_cxx.h"
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...
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int main(void)
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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_LOCK | // Initialize locking
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DB_INIT_LOG | // Initialize logging
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DB_INIT_MPOOL | // Initialize the cache
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DB_INIT_TXN; // Initialize transactions
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u_int32_t db_flags = DB_CREATE | DB_AUTO_COMMIT;
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Db *dbp = NULL;
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const char *file_name = "mydb.db";
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const char *keystr ="thekey";
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const char *datastr = "thedata";
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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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dbp = new Db(&myEnv, 0);
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// Open the database. Note that we are using auto commit for
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// the open, so the database is able to support transactions.
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dbp->open(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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Dbt key, data;
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key.set_data(keystr);
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key.set_size((strlen(keystr) + 1) * sizeof(char));
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key.set_data(datastr);
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key.set_size((strlen(datastr) + 1) * sizeof(char));
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// Perform the write. Because the database was opened to support
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// auto commit, this write is performed using auto commit.
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db->put(NULL, &key, &data, 0);
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} catch(DbException &e) {
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std::cerr << "Error opening database and environment: "
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<< file_name << ", "
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<< envHome << std::endl;
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std::cerr << e.what() << std::endl;
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}
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try {
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if (dbp != NULL)
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dbp->close(0);
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myEnv.close(0);
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} catch(DbException &e) {
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std::cerr << "Error closing database and environment: "
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<< file_name << ", "
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<< envHome << std::endl;
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std::cerr << e.what() << std::endl;
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return (EXIT_FAILURE);
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}
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return (EXIT_SUCCESS);
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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="abortresults.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="usingtxns.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="nestedtxn.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">Aborting a Transaction </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"> Nested Transactions</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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