2011-09-13 17:44:24 +00:00
|
|
|
|
<?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" />
|
2012-11-14 21:35:20 +00:00
|
|
|
|
<link rel="next" href="exclusivelock.html" title="Exclusive Database Handles" />
|
2011-09-13 17:44:24 +00:00
|
|
|
|
</head>
|
|
|
|
|
|
<body>
|
|
|
|
|
|
<div xmlns="" class="navheader">
|
|
|
|
|
|
<div class="libver">
|
2012-11-14 21:35:20 +00:00
|
|
|
|
<p>Library Version 11.2.5.3</p>
|
2011-09-13 17:44:24 +00:00
|
|
|
|
</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>
|
2012-11-14 21:35:20 +00:00
|
|
|
|
<td width="20%" align="right"> <a accesskey="n" href="exclusivelock.html">Next</a></td>
|
2011-09-13 17:44:24 +00:00
|
|
|
|
</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>
|
2012-11-14 21:35:20 +00:00
|
|
|
|
<td width="40%" align="right"> <a accesskey="n" href="exclusivelock.html">Next</a></td>
|
2011-09-13 17:44:24 +00:00
|
|
|
|
</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>
|
2012-11-14 21:35:20 +00:00
|
|
|
|
<td width="40%" align="right" valign="top"> Exclusive Database Handles</td>
|
2011-09-13 17:44:24 +00:00
|
|
|
|
</tr>
|
|
|
|
|
|
</table>
|
|
|
|
|
|
</div>
|
|
|
|
|
|
</body>
|
|
|
|
|
|
</html>
|
