mirror of
https://github.com/berkeleydb/je.git
synced 2024-11-20 11:16:25 +00:00
261 lines
11 KiB
HTML
261 lines
11 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>JE Lock Management</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="blocking_deadlocks.html" title="Locks, Blocks, and Deadlocks" />
|
|||
|
|
<link rel="next" href="isolation.html" title="Isolation" />
|
|||
|
|
</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">JE Lock Management</th>
|
|||
|
|
</tr>
|
|||
|
|
<tr>
|
|||
|
|
<td width="20%" align="left"><a accesskey="p" href="blocking_deadlocks.html">Prev</a> </td>
|
|||
|
|
<th width="60%" align="center">Chapter 4. Concurrency</th>
|
|||
|
|
<td width="20%" align="right"> <a accesskey="n" href="isolation.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="jelock"></a>JE Lock Management</h2>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
<div class="toc">
|
|||
|
|
<dl>
|
|||
|
|
<dt>
|
|||
|
|
<span class="sect2">
|
|||
|
|
<a href="jelock.html#jelocktimeout">Managing JE Lock Timeouts</a>
|
|||
|
|
</span>
|
|||
|
|
</dt>
|
|||
|
|
<dt>
|
|||
|
|
<span class="sect2">
|
|||
|
|
<a href="jelock.html#jedeadlock">Managing Deadlocks and other Lock Conflicts</a>
|
|||
|
|
</span>
|
|||
|
|
</dt>
|
|||
|
|
</dl>
|
|||
|
|
</div>
|
|||
|
|
<p>
|
|||
|
|
To manage locks in JE, you must do two things:
|
|||
|
|
</p>
|
|||
|
|
<div class="orderedlist">
|
|||
|
|
<ol type="1">
|
|||
|
|
<li>
|
|||
|
|
<p>
|
|||
|
|
Manage lock timeouts.
|
|||
|
|
</p>
|
|||
|
|
</li>
|
|||
|
|
<li>
|
|||
|
|
<p>
|
|||
|
|
Detect and respond to lock conflicts. Conceptually,
|
|||
|
|
these are deadlocks. But from a coding point of view
|
|||
|
|
there is no difference between what you do if a lock
|
|||
|
|
times out, and what you do if you encounter a deadlock.
|
|||
|
|
In fact, in JE, you cannot tell the difference
|
|||
|
|
based on the exceptions that are thrown.
|
|||
|
|
</p>
|
|||
|
|
</li>
|
|||
|
|
</ol>
|
|||
|
|
</div>
|
|||
|
|
<div class="sect2" lang="en" xml:lang="en">
|
|||
|
|
<div class="titlepage">
|
|||
|
|
<div>
|
|||
|
|
<div>
|
|||
|
|
<h3 class="title"><a id="jelocktimeout"></a>Managing JE Lock Timeouts</h3>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
<p>
|
|||
|
|
Like transaction timeouts (see <a class="xref" href="maxtxns.html" title="Configuring the Transaction Subsystem">Configuring the Transaction Subsystem</a>),
|
|||
|
|
JE allows you to identify the longest period of time that it is allowed to hold a lock.
|
|||
|
|
This value plays an important part in performing deadlock detection, because the only way JE can
|
|||
|
|
identify a deadlock is if a lock is held past its timeout value.
|
|||
|
|
</p>
|
|||
|
|
<p>
|
|||
|
|
However, unlike transaction timeouts, lock timeouts are on a true timer. Transaction
|
|||
|
|
timeouts are only identified when JE is has a reason to examine its lock table; that is,
|
|||
|
|
when it is attempting to acquire a lock. If no such activity is
|
|||
|
|
occurring in your application, a transaction can exist for a long time past its expiration timeout.
|
|||
|
|
Conversely, lock timeouts are managed by a timer maintained by the JVM. Once this timer has expired,
|
|||
|
|
your application will be notified of the event (see the next section on deadlock detection for more
|
|||
|
|
information).
|
|||
|
|
</p>
|
|||
|
|
<p>
|
|||
|
|
You can set the lock timeout on a transaction by transaction basis, or for the entire environment. To
|
|||
|
|
set it on a transaction basis, use <code class="methodname">Transaction.setLockTimeout()</code>.
|
|||
|
|
To set it for your entire environment, use <code class="methodname">EnvironmentConfig.setLockTimeout()</code>
|
|||
|
|
or use the <code class="literal">je.lock.timeout</code> parameter in the <code class="filename">je.properties</code> file.
|
|||
|
|
</p>
|
|||
|
|
<p>
|
|||
|
|
The value that you specify for the lock timeout is in microseconds. <code class="literal">500000</code> is used by
|
|||
|
|
default.
|
|||
|
|
</p>
|
|||
|
|
<p>
|
|||
|
|
Note that changing this value can have an affect on your application's performance. If you set it too
|
|||
|
|
low, locks may expire and be considered deadlocked even though the thread is in fact making
|
|||
|
|
forward progress. This will cause your application to abort and retry transactions unnecessarily, which
|
|||
|
|
can ultimately harm application throughput.
|
|||
|
|
If you set it too high, threads may deadlock for too long before your application receives notification
|
|||
|
|
and is able to take corrective action. Again, this can harm application throughput.
|
|||
|
|
</p>
|
|||
|
|
<p>
|
|||
|
|
Note that for applications in which you will have extremely long-lived locks, you
|
|||
|
|
may want to set this value to <code class="literal">0</code>. Doing so disables lock timeouts
|
|||
|
|
entirely. Be aware that disabling lock timeouts can be dangerous because then your
|
|||
|
|
application will never be notified of deadlocks. So, alternatively, you might want
|
|||
|
|
to set this value to a very large timeout (such as ten minutes) if your application
|
|||
|
|
is using extremely long-lived locks.
|
|||
|
|
</p>
|
|||
|
|
</div>
|
|||
|
|
<div class="sect2" lang="en" xml:lang="en">
|
|||
|
|
<div class="titlepage">
|
|||
|
|
<div>
|
|||
|
|
<div>
|
|||
|
|
<h3 class="title"><a id="jedeadlock"></a>Managing Deadlocks and other Lock Conflicts</h3>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
<p>
|
|||
|
|
A deadlock is the result of a lock conflict that cannot be
|
|||
|
|
resolved by the underlying JE code before the lock
|
|||
|
|
times out. Generically, we consider this situation a
|
|||
|
|
<span class="emphasis"><em>lock conflict</em></span> because there is no way
|
|||
|
|
to tell if the lock timed out because of a true deadlock,
|
|||
|
|
or if it timed out because a long-running operation simply
|
|||
|
|
held the lock for too long a period of time.
|
|||
|
|
</p>
|
|||
|
|
<p>
|
|||
|
|
When a lock conflict occurs in JE, the thread of control holding
|
|||
|
|
that lock is notified of the event using a
|
|||
|
|
<code class="classname">LockConflictException</code> exception. Note
|
|||
|
|
that this exception is actual a common base class for several
|
|||
|
|
exception classes that might be able to give you more of a
|
|||
|
|
hint as to what the actual problem is. However, the
|
|||
|
|
response that you make for any of these exceptions is
|
|||
|
|
probably going to be the same, so the best thing to do is
|
|||
|
|
simply catch and manage <code class="classname">LockConflictException</code>.
|
|||
|
|
</p>
|
|||
|
|
<p>
|
|||
|
|
When a <code class="classname">LockConflictException</code> is
|
|||
|
|
thrown, the thread must:
|
|||
|
|
</p>
|
|||
|
|
<div class="orderedlist">
|
|||
|
|
<ol type="1">
|
|||
|
|
<li>
|
|||
|
|
<p>
|
|||
|
|
Cease all read and write operations.
|
|||
|
|
</p>
|
|||
|
|
</li>
|
|||
|
|
<li>
|
|||
|
|
<p>
|
|||
|
|
Close all open cursors.
|
|||
|
|
</p>
|
|||
|
|
</li>
|
|||
|
|
<li>
|
|||
|
|
<p>
|
|||
|
|
Abort the transaction.
|
|||
|
|
</p>
|
|||
|
|
</li>
|
|||
|
|
<li>
|
|||
|
|
<p>
|
|||
|
|
Optionally retry the operation. If your application
|
|||
|
|
retries operations that are aborted due to a lock
|
|||
|
|
conflict, the new attempt must be made using a new transaction.
|
|||
|
|
</p>
|
|||
|
|
</li>
|
|||
|
|
</ol>
|
|||
|
|
</div>
|
|||
|
|
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
|
|||
|
|
<h3 class="title">Note</h3>
|
|||
|
|
<p>
|
|||
|
|
If a thread has encountered a lock conflict, it may not make any
|
|||
|
|
additional database calls using the transaction handle that has
|
|||
|
|
experienced the lock conflict.
|
|||
|
|
</p>
|
|||
|
|
</div>
|
|||
|
|
<p>
|
|||
|
|
For example:
|
|||
|
|
</p>
|
|||
|
|
<pre class="programlisting">// retry_count is a counter used to identify how many times
|
|||
|
|
// we've retried this operation. To avoid the potential for
|
|||
|
|
// endless looping, we won't retry more than MAX_DEADLOCK_RETRIES
|
|||
|
|
// times.
|
|||
|
|
|
|||
|
|
// txn is a transaction handle.
|
|||
|
|
// key and data are DatabaseEntry handles. Their usage is not shown here.
|
|||
|
|
while (retry_count < MAX_DEADLOCK_RETRIES) {
|
|||
|
|
try {
|
|||
|
|
txn = myEnv.beginTransaction(null, null);
|
|||
|
|
myDatabase.put(txn, key, data);
|
|||
|
|
txn.commit();
|
|||
|
|
return 0;
|
|||
|
|
} catch (LockConflictException le) {
|
|||
|
|
try {
|
|||
|
|
// Abort the transaction and increment the
|
|||
|
|
// retry counter
|
|||
|
|
if (txn != null) {
|
|||
|
|
txn.abort();
|
|||
|
|
}
|
|||
|
|
retry_count++;
|
|||
|
|
if (retry_count >= MAX_DEADLOCK_RETRIES) {
|
|||
|
|
System.err.println("Exceeded retry limit. Giving up.");
|
|||
|
|
return -1;
|
|||
|
|
}
|
|||
|
|
} catch (DatabaseException ae) {
|
|||
|
|
System.err.println("txn abort failed: " + ae.toString());
|
|||
|
|
return -1;
|
|||
|
|
}
|
|||
|
|
} catch (DatabaseException e) {
|
|||
|
|
// If we catch a generic DatabaseException instead of
|
|||
|
|
// a LockConflictException, we simply abort and give
|
|||
|
|
// up -- we don't retry the operation.
|
|||
|
|
try {
|
|||
|
|
// Abort the transaction.
|
|||
|
|
if (txn != null) {
|
|||
|
|
txn.abort();
|
|||
|
|
}
|
|||
|
|
} catch (DatabaseException ae) {
|
|||
|
|
System.err.println("txn abort failed: " + ae.toString());
|
|||
|
|
}
|
|||
|
|
return -1;
|
|||
|
|
}
|
|||
|
|
} </pre>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
<div class="navfooter">
|
|||
|
|
<hr />
|
|||
|
|
<table width="100%" summary="Navigation footer">
|
|||
|
|
<tr>
|
|||
|
|
<td width="40%" align="left"><a accesskey="p" href="blocking_deadlocks.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="isolation.html">Next</a></td>
|
|||
|
|
</tr>
|
|||
|
|
<tr>
|
|||
|
|
<td width="40%" align="left" valign="top">Locks, Blocks, and Deadlocks </td>
|
|||
|
|
<td width="20%" align="center">
|
|||
|
|
<a accesskey="h" href="index.html">Home</a>
|
|||
|
|
</td>
|
|||
|
|
<td width="40%" align="right" valign="top"> Isolation</td>
|
|||
|
|
</tr>
|
|||
|
|
</table>
|
|||
|
|
</div>
|
|||
|
|
</body>
|
|||
|
|
</html>
|