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388 lines
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HTML
388 lines
15 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>Chapter 3. Transaction Basics</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="index.html" title="Getting Started with Berkeley DB Transaction Processing" />
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<link rel="prev" href="envopen.html" title="Opening a Transactional Environment and Database" />
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<link rel="next" href="nodurabletxn.html" title="Non-Durable 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">Chapter 3. Transaction Basics</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="envopen.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="nodurabletxn.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="chapter" 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"><a id="usingtxns"></a>Chapter 3. Transaction Basics</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<p>
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<b>Table of Contents</b>
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</p>
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<dl>
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<dt>
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<span class="sect1">
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<a href="usingtxns.html#commitresults">Committing a Transaction</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="nodurabletxn.html">Non-Durable Transactions</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="abortresults.html">Aborting a Transaction</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="autocommit.html">Auto Commit</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="nestedtxn.html">Nested Transactions</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="txncursor.html">Transactional Cursors</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="txnindices.html">Secondary Indices with Transaction Applications</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="maxtxns.html">Configuring the Transaction Subsystem</a>
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</span>
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</dt>
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</dl>
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</div>
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<p>
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Once you have enabled transactions for your environment and your databases,
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you can use them to protect your database operations. You do this by
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acquiring a transaction handle and then using that handle for any
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database operation that you want to participate in that transaction.
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</p>
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<p>
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You obtain a transaction handle using the
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<span><code class="methodname">DbEnv::txn_begin()</code> method.</span>
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</p>
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<p>
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Once you have completed all of the operations that you want to include
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in the transaction, you must commit the transaction using the
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<span><code class="methodname">DbTxn::commit()</code> method.</span>
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</p>
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<p>
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If, for any reason, you want to abandon the transaction, you abort
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it using
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<span><code class="methodname">DbTxn::abort()</code>.</span>
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</p>
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<p>
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Any transaction handle that has been committed or aborted can no longer
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be used by your application.
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</p>
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<p>
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Finally, you must make sure that all transaction handles are either
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committed or aborted before closing your databases and environment.
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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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If you only want to transaction protect a single database write operation, you can use auto commit to
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perform the transaction administration. When you use auto commit, you do not need an explicit transaction
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handle. See <a class="xref" href="autocommit.html" title="Auto Commit">Auto Commit</a> for more information.
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</p>
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</div>
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<p>
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For example, the following example opens a transactional-enabled environment and
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database, obtains a transaction handle, and then performs a write
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operation under its protection. In the event of any failure in the
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write operation, the transaction is aborted and the database is left in a
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state as if no operations had ever been attempted in the first place.
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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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DbTxn *txn = NULL;
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myEnv.txn_begin(NULL, &txn, 0);
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try {
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db->put(txn, &key, &data, 0);
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txn->commit(0);
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} catch (DbException &e) {
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std::cerr << "Error in transaction: "
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<< e.what() << std::endl;
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txn->abort();
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}
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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 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="commitresults"></a>Committing a Transaction</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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In order to fully understand what is happening when you commit
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a transaction, you must first understand a little about what
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DB is doing with
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<span>
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the logging subsystem.
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</span>
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Logging causes all database write operations to be identified in
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<span>logs, and by default these
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logs are backed by files on disk. These logs are used to restore your databases
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</span>
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in the event of a system or application failure, so by performing
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logging, DB ensures the integrity of your data.
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</p>
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<p>
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Moreover, DB performs <span class="emphasis"><em>write-ahead</em></span>
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logging. This means that information is written to the logs
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<span class="emphasis"><em>before</em></span> the actual database
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is changed.
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This means that all write activity performed under the
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protection of the transaction is noted in the log before
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the transaction is committed. Be aware, however, that database
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maintains logs in-memory. If you are backing your logs on
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disk, the log information will eventually be written to the log
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files, but while the transaction is on-going the log data may be
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held only in memory.
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</p>
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<p>
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When you commit a transaction, the following occurs:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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A commit record is written to the log. This
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indicates that the modifications made by the
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transaction are now permanent. By default, this write is performed synchronously to disk so the
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commit record arrives in the log files before any other actions are taken.
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</p>
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</li>
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<li>
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<p>
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Any log information held in memory is (by default)
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synchronously written to disk. Note that this requirement can be
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relaxed, depending on the type of commit you perform.
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See <a class="xref" href="nodurabletxn.html" title="Non-Durable Transactions">Non-Durable Transactions</a> for
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more information.
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<span>Also, if you are
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maintaining your logs entirely in-memory, then this
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step will of course not be taken. To configure your
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logging system for in-memory usage, see
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<a class="xref" href="logconfig.html#inmemorylogging" title="Configuring In-Memory Logging">Configuring In-Memory Logging</a>.
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</span>
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</p>
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</li>
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<li>
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<p>
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All locks held by the transaction are released. This means
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that read operations performed by other transactions or
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threads of control can now see the modifications without
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resorting to uncommitted reads (see <a class="xref" href="isolation.html#dirtyreads" title="Reading Uncommitted Data">Reading Uncommitted Data</a> for more information).
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</p>
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</li>
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</ul>
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</div>
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<p>
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To commit a transaction, you simply call
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<span><code class="methodname">DbTxn::commit()</code>.</span>
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</p>
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<p>
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Notice that committing a transaction does not necessarily cause data
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modified in your memory cache to be written to the files
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backing your databases on disk. Dirtied database pages are written
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for a number of reasons, but a transactional
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commit is not one of them. The following are the things that can cause a dirtied
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database page to be written to the backing database file:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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Checkpoints.
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</p>
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<p>
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Checkpoints cause all dirtied pages currently existing
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in the cache to be written to disk, and a checkpoint
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record is then written to the logs. You can run checkpoints
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explicitly. For more information on checkpoints,
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see <a class="xref" href="filemanagement.html#checkpoints" title="Checkpoints">Checkpoints</a>.
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</p>
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</li>
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<li>
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<p>
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Cache is full.
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</p>
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<p>
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If the in-memory cache fills up, then dirtied pages
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might be written to disk in order to free up space for other
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pages that your application needs to use. Note that if
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dirtied pages are written to the database files, then
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any log records that describe how those pages were
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dirtied are written to disk before the database
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pages are written.
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</p>
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</li>
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|
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</ul>
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</div>
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<p>
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Be aware that because your transaction commit caused database
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modifications recorded in your logs to be forced to disk, your modifications
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|
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are by default "persistent" in that they can be recovered in the event of
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|
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an application or system failure. However, recovery time is
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gated by how much data has been modified since the last
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checkpoint, so for applications that perform a lot of writes,
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you may want to run a checkpoint with some frequency.
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</p>
|
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|
|
<p>
|
|||
|
|
Note that once you have committed a transaction, the transaction
|
|||
|
|
handle that you used for the transaction is no longer valid. To
|
|||
|
|
perform database activities under the control of a new
|
|||
|
|
transaction, you must obtain a fresh transaction handle.
|
|||
|
|
</p>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
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|
<div class="navfooter">
|
|||
|
|
<hr />
|
|||
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<table width="100%" summary="Navigation footer">
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|||
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<tr>
|
|||
|
|
<td width="40%" align="left"><a accesskey="p" href="envopen.html">Prev</a> </td>
|
|||
|
|
<td width="20%" align="center"> </td>
|
|||
|
|
<td width="40%" align="right"> <a accesskey="n" href="nodurabletxn.html">Next</a></td>
|
|||
|
|
</tr>
|
|||
|
|
<tr>
|
|||
|
|
<td width="40%" align="left" valign="top">Opening a Transactional Environment and
|
|||
|
|
<span>Database</span>
|
|||
|
|
|
|||
|
|
|
|||
|
|
</td>
|
|||
|
|
<td width="20%" align="center">
|
|||
|
|
<a accesskey="h" href="index.html">Home</a>
|
|||
|
|
</td>
|
|||
|
|
<td width="40%" align="right" valign="top"> Non-Durable Transactions</td>
|
|||
|
|
</tr>
|
|||
|
|
</table>
|
|||
|
|
</div>
|
|||
|
|
</body>
|
|||
|
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</html>
|