mirror of
https://github.com/berkeleydb/je.git
synced 2024-11-15 01:46:24 +00:00
159 lines
6.2 KiB
HTML
159 lines
6.2 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>Read/Modify/Write</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, Java Edition Transaction Processing" />
|
||
<link rel="up" href="txnconcurrency.html" title="Chapter 4. Concurrency" />
|
||
<link rel="prev" href="txn_ccursor.html" title="Transactional Cursors and Concurrent Applications" />
|
||
<link rel="next" href="jebackuprestore.html" title="Chapter 5. Backing up and Restoring Berkeley DB, Java Edition Applications" />
|
||
</head>
|
||
<body>
|
||
<div xmlns="" class="navheader">
|
||
<div class="libver">
|
||
<p>Library Version 12.2.7.5</p>
|
||
</div>
|
||
<table width="100%" summary="Navigation header">
|
||
<tr>
|
||
<th colspan="3" align="center">Read/Modify/Write</th>
|
||
</tr>
|
||
<tr>
|
||
<td width="20%" align="left"><a accesskey="p" href="txn_ccursor.html">Prev</a> </td>
|
||
<th width="60%" align="center">Chapter 4. Concurrency</th>
|
||
<td width="20%" align="right"> <a accesskey="n" href="jebackuprestore.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="readmodifywrite"></a>Read/Modify/Write</h2>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
If you are retrieving
|
||
|
||
|
||
<span>a record from the database or a class from the store</span>
|
||
for the purpose of modifying or deleting it, you should declare
|
||
a read-modify-write cycle at the time that you read the
|
||
<span>record.</span>
|
||
|
||
Doing so causes JE to obtain write locks (instead of a read
|
||
locks) at the time of the read. This helps to prevent deadlocks by
|
||
preventing another transaction from acquiring a read lock on the same
|
||
record while the read-modify-write cycle is in progress.
|
||
</p>
|
||
<p>
|
||
Note that declaring a read-modify-write cycle may actually increase the amount of blocking that your
|
||
application sees, because readers immediately obtain write locks and write locks cannot be shared. For this
|
||
reason, you should use read-modify-write cycles only if you are seeing a large amount of deadlocking
|
||
occurring in your application.
|
||
</p>
|
||
<p>
|
||
In order to declare a read/modify/write cycle when you perform a
|
||
read operation,
|
||
|
||
|
||
|
||
|
||
|
||
<span>
|
||
specify
|
||
|
||
<code class="classname">com.sleepycat.je.LockMode.RMW</code>
|
||
to the database, cursor,
|
||
<code class="classname">PrimaryIndex</code>, or
|
||
<code class="classname">SecondaryIndex</code> get method.
|
||
</span>
|
||
</p>
|
||
<p>
|
||
For example:
|
||
</p>
|
||
<pre class="programlisting">// Begin the deadlock retry loop as is normal.
|
||
while (retry_count < MAX_DEADLOCK_RETRIES) {
|
||
try {
|
||
txn = myEnv.beginTransaction(null, null);
|
||
|
||
...
|
||
// key and data are DatabaseEntry objects.
|
||
// Their usage is omitted for brevity.
|
||
...
|
||
|
||
// Read the data. Declare the read/modify/write cycle here
|
||
myDatabase.get(txn, key, data, LockMode.RMW);
|
||
|
||
|
||
// Put the data. Note that you do not have to provide any
|
||
// additional flags here due to the read/modify/write
|
||
// cycle. Simply put the data and perform your deadlock
|
||
// detection as normal.
|
||
myDatabase.put(txn, key, data);
|
||
txn.commit();
|
||
return 0;
|
||
} catch (DeadlockException de) {
|
||
// Deadlock detection and exception handling omitted
|
||
// for brevity
|
||
... </pre>
|
||
<p>
|
||
Or, with the DPL:
|
||
</p>
|
||
<pre class="programlisting">// Begin the deadlock retry loop as is normal
|
||
while (retry_count < MAX_DEADLOCK_RETRIES) {
|
||
try {
|
||
txn = myEnv.beginTransaction(null, null);
|
||
|
||
...
|
||
// 'store' is an EntityStore and 'Inventory' is an entity class
|
||
// Their usage and implementation is omitted for brevity.
|
||
...
|
||
|
||
// Read the data, using the PrimaryIndex for the entity object
|
||
PrimaryIndex<String,Inventory> pi =
|
||
store.getPrimaryIndex(String.class, Inventory.class);
|
||
Inventory iv = pi.get(txn, "somekey", LockMode.RMW);
|
||
|
||
// Do something to the retreived object
|
||
|
||
|
||
// Put the object. Note that you do not have to provide any
|
||
// additional flags here due to the read/modify/write
|
||
// cycle. Simply put the data and perform your deadlock
|
||
// detection as normal.
|
||
|
||
pi.put(txn, iv);
|
||
txn.commit();
|
||
return 0;
|
||
|
||
} catch (DeadlockException de) {
|
||
// Deadlock detection and exception handling omitted
|
||
// for brevity
|
||
... </pre>
|
||
</div>
|
||
<div class="navfooter">
|
||
<hr />
|
||
<table width="100%" summary="Navigation footer">
|
||
<tr>
|
||
<td width="40%" align="left"><a accesskey="p" href="txn_ccursor.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="jebackuprestore.html">Next</a></td>
|
||
</tr>
|
||
<tr>
|
||
<td width="40%" align="left" valign="top">Transactional Cursors and Concurrent Applications </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="h" href="index.html">Home</a>
|
||
</td>
|
||
<td width="40%" align="right" valign="top"> Chapter 5. Backing up and Restoring Berkeley DB, Java Edition Applications</td>
|
||
</tr>
|
||
</table>
|
||
</div>
|
||
</body>
|
||
</html>
|