mirror of
https://github.com/berkeleydb/libdb.git
synced 2024-11-16 17:16:25 +00:00
174 lines
7.3 KiB
HTML
174 lines
7.3 KiB
HTML
<?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>Transactional Cursors and Concurrent Applications</title>
|
||
<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
|
||
<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
|
||
<link rel="start" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" />
|
||
<link rel="up" href="txnconcurrency.html" title="Chapter 4. Concurrency" />
|
||
<link rel="prev" href="isolation.html" title="Isolation" />
|
||
<link rel="next" href="exclusivelock.html" title="Exclusive Database Handles" />
|
||
</head>
|
||
<body>
|
||
<div xmlns="" class="navheader">
|
||
<div class="libver">
|
||
<p>Library Version 11.2.5.3</p>
|
||
</div>
|
||
<table width="100%" summary="Navigation header">
|
||
<tr>
|
||
<th colspan="3" align="center">Transactional Cursors and Concurrent Applications</th>
|
||
</tr>
|
||
<tr>
|
||
<td width="20%" align="left"><a accesskey="p" href="isolation.html">Prev</a> </td>
|
||
<th width="60%" align="center">Chapter 4. Concurrency</th>
|
||
<td width="20%" align="right"> <a accesskey="n" href="exclusivelock.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="txn_ccursor"></a>Transactional Cursors and Concurrent Applications</h2>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<div class="toc">
|
||
<dl>
|
||
<dt>
|
||
<span class="sect2">
|
||
<a href="txn_ccursor.html#cursordirtyreads">Using Cursors with Uncommitted Data</a>
|
||
</span>
|
||
</dt>
|
||
</dl>
|
||
</div>
|
||
<p>
|
||
When you use transactional cursors with a concurrent application, remember that
|
||
in the event of a deadlock you must make sure that you close your cursor before you abort and retry your
|
||
transaction.
|
||
</p>
|
||
<p>
|
||
Also, remember that when you are using the default isolation level,
|
||
every time your cursor reads a record it locks
|
||
that record until the encompassing transaction is resolved. This
|
||
means that walking your database with a transactional cursor
|
||
increases the chance of lock contention.
|
||
</p>
|
||
<p>
|
||
For this reason, if you must routinely walk your database with a
|
||
transactional cursor, consider using a reduced isolation level
|
||
such as read committed.
|
||
</p>
|
||
<div class="sect2" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h3 class="title"><a id="cursordirtyreads"></a>Using Cursors with Uncommitted Data</h3>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
|
||
As described in <a class="xref" href="isolation.html#dirtyreads" title="Reading Uncommitted Data">Reading Uncommitted Data</a>
|
||
above, it is possible to relax your transaction's isolation
|
||
level such that it can read data modified but not yet committed
|
||
by another transaction. You can configure this when you create
|
||
your transaction handle, and when you do so then all cursors opened
|
||
inside that transaction will automatically use uncommitted reads.
|
||
</p>
|
||
<p>
|
||
You can also do this when you create a cursor handle from within
|
||
a serializable transaction. When you do this, only those
|
||
cursors configured for uncommitted reads uses uncommitted reads.
|
||
</p>
|
||
<p>
|
||
Either way, you must first configure your database
|
||
handle to support
|
||
uncommitted reads before you can configure your transactions or
|
||
your cursors to use them.
|
||
</p>
|
||
<p>
|
||
The following example shows how to configure an individual cursor handle
|
||
to read uncommitted data from within a serializable (full isolation) transaction.
|
||
For an example of
|
||
configuring a transaction to perform uncommitted reads in
|
||
general, see <a class="xref" href="isolation.html#dirtyreads" title="Reading Uncommitted Data">Reading Uncommitted Data</a>.
|
||
</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
|
||
|
||
u_int32_t db_flags = DB_CREATE | // Create the db if it does
|
||
// not exist
|
||
DB_AUTO_COMMIT | // Enable auto commit
|
||
DB_READ_UNCOMMITTED; // Enable uncommitted reads
|
||
|
||
Db *dbp = NULL;
|
||
const char *file_name = "mydb.db";
|
||
|
||
std::string envHome("/export1/testEnv");
|
||
DbEnv myEnv(0);
|
||
|
||
Dbc *cursorp = NULL;
|
||
|
||
try {
|
||
|
||
myEnv.open(envHome.c_str(), env_flags, 0);
|
||
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
|
||
|
||
DbTxn *txn = NULL;
|
||
myEnv.txn_begin(NULL, &txn, 0);
|
||
try {
|
||
// Get our cursor. Note that we pass the transaction
|
||
// handle here. Note also that we pass the
|
||
// DB_READ_UNCOMMITTED flag here so as to cause the
|
||
// cursor to perform uncommitted reads.
|
||
db.cursor(txn, &cursorp, DB_READ_UNCOMMITTED);
|
||
|
||
// From here, you perform your cursor reads and writes
|
||
// as normal, committing and aborting the transactions as
|
||
// is necessary, and testing for deadlock exceptions as
|
||
// normal (omitted for brevity).
|
||
|
||
... </pre>
|
||
</div>
|
||
</div>
|
||
<div class="navfooter">
|
||
<hr />
|
||
<table width="100%" summary="Navigation footer">
|
||
<tr>
|
||
<td width="40%" align="left"><a accesskey="p" href="isolation.html">Prev</a> </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="u" href="txnconcurrency.html">Up</a>
|
||
</td>
|
||
<td width="40%" align="right"> <a accesskey="n" href="exclusivelock.html">Next</a></td>
|
||
</tr>
|
||
<tr>
|
||
<td width="40%" align="left" valign="top">Isolation </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="h" href="index.html">Home</a>
|
||
</td>
|
||
<td width="40%" align="right" valign="top"> Exclusive Database Handles</td>
|
||
</tr>
|
||
</table>
|
||
</div>
|
||
</body>
|
||
</html>
|