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174 lines
7.3 KiB
174 lines
7.3 KiB
<?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>Transactional Cursors and Concurrent Applications</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="txnconcurrency.html" title="Chapter 4. Concurrency" />
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<link rel="prev" href="isolation.html" title="Isolation" />
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<link rel="next" href="exclusivelock.html" title="Exclusive Database Handles" />
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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 12.1.6.2</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">Transactional Cursors and Concurrent Applications</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="isolation.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 4. Concurrency</th>
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<td width="20%" align="right"> <a accesskey="n" href="exclusivelock.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="txn_ccursor"></a>Transactional Cursors and Concurrent Applications</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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<dl>
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<dt>
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<span class="sect2">
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<a href="txn_ccursor.html#cursordirtyreads">Using Cursors with Uncommitted Data</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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When you use transactional cursors with a concurrent application, remember that
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in the event of a deadlock you must make sure that you close your cursor before you abort and retry your
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transaction.
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</p>
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<p>
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Also, remember that when you are using the default isolation level,
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every time your cursor reads a record it locks
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that record until the encompassing transaction is resolved. This
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means that walking your database with a transactional cursor
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increases the chance of lock contention.
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</p>
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<p>
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For this reason, if you must routinely walk your database with a
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transactional cursor, consider using a reduced isolation level
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such as read committed.
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</p>
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<div class="sect2" 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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<h3 class="title"><a id="cursordirtyreads"></a>Using Cursors with Uncommitted Data</h3>
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</div>
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</div>
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</div>
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<p>
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As described in <a class="xref" href="isolation.html#dirtyreads" title="Reading Uncommitted Data">Reading Uncommitted Data</a>
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above, it is possible to relax your transaction's isolation
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level such that it can read data modified but not yet committed
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by another transaction. You can configure this when you create
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your transaction handle, and when you do so then all cursors opened
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inside that transaction will automatically use uncommitted reads.
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</p>
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<p>
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You can also do this when you create a cursor handle from within
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a serializable transaction. When you do this, only those
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cursors configured for uncommitted reads uses uncommitted reads.
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</p>
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<p>
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Either way, you must first configure your database
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handle to support
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uncommitted reads before you can configure your transactions or
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your cursors to use them.
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</p>
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<p>
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The following example shows how to configure an individual cursor handle
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to read uncommitted data from within a serializable (full isolation) transaction.
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For an example of
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configuring a transaction to perform uncommitted reads in
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general, see <a class="xref" href="isolation.html#dirtyreads" title="Reading Uncommitted Data">Reading Uncommitted Data</a>.
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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 | // Create the db if it does
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// not exist
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DB_AUTO_COMMIT | // Enable auto commit
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DB_READ_UNCOMMITTED; // Enable uncommitted reads
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Db *dbp = NULL;
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const char *file_name = "mydb.db";
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std::string envHome("/export1/testEnv");
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DbEnv myEnv(0);
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Dbc *cursorp = NULL;
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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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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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DbTxn *txn = NULL;
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myEnv.txn_begin(NULL, &txn, 0);
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try {
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// Get our cursor. Note that we pass the transaction
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// handle here. Note also that we pass the
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// DB_READ_UNCOMMITTED flag here so as to cause the
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// cursor to perform uncommitted reads.
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db.cursor(txn, &cursorp, DB_READ_UNCOMMITTED);
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// From here, you perform your cursor reads and writes
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// as normal, committing and aborting the transactions as
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// is necessary, and testing for deadlock exceptions as
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// normal (omitted for brevity).
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... </pre>
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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="isolation.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="txnconcurrency.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="exclusivelock.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">Isolation </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"> Exclusive Database Handles</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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