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<th colspan="3" align="center">Chapter 4. Concurrency</th>
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<div class="chapter" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="txnconcurrency"></a>Chapter 4. Concurrency</h2>
</div>
</div>
</div>
<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="txnconcurrency.html#concurrenthandles">Which DB Handles are Free-Threaded</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="blocking_deadlocks.html">Locks, Blocks, and Deadlocks</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="blocking_deadlocks.html#locks">Locks</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="blocking_deadlocks.html#blocks">Blocks</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="blocking_deadlocks.html#deadlocks">Deadlocks</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="lockingsubsystem.html">The Locking Subsystem</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="lockingsubsystem.html#configuringlock">Configuring the Locking Subsystem</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="lockingsubsystem.html#configdeadlkdetect">Configuring Deadlock Detection</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="lockingsubsystem.html#deadlockresolve">Resolving Deadlocks</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="lockingsubsystem.html#setpriority">Setting Transaction Priorities</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="isolation.html">Isolation</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="isolation.html#degreesofisolation">Supported Degrees of Isolation</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="isolation.html#dirtyreads">Reading Uncommitted Data</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="isolation.html#readcommitted">Committed Reads</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="isolation.html#snapshot_isolation">Using Snapshot Isolation</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="txn_ccursor.html">Transactional Cursors and Concurrent Applications</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="txn_ccursor.html#cursordirtyreads">Using Cursors with Uncommitted Data</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="readmodifywrite.html">Read/Modify/Write</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="txnnowait.html">No Wait on Blocks</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="reversesplit.html">Reverse BTree Splits</a>
</span>
</dt>
</dl>
</div>
<p>
DB offers a great deal of support for multi-threaded
<span>
and multi-process
</span>
applications even when transactions are not in use. Many of DB's
handles are
thread-safe<span>, or
can be made thread-safe by providing the appropriate flag at handle creation time,</span>
and DB provides a
flexible locking subsystem for managing databases in a concurrent
application. Further, DB provides a robust mechanism for
detecting and responding to
<span>deadlocks</span>.
All of these concepts are
explored in this chapter.
</p>
<p>
Before continuing, it is useful to define a few terms that will appear
throughout this chapter:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<span class="emphasis"><em>Thread of control</em></span>
</p>
<p>
Refers to a thread that is performing work in your application.
Typically, in this book that thread will be performing DB
operations.
</p>
<p>
Note that this term can also be taken to mean a separate process
that is performing work — DB supports multi-process
operations on your databases.
</p>
</li>
<li>
<p>
<span class="emphasis"><em>Locking</em></span>
</p>
<p>
When a thread of control obtains
access to a shared resource, it is said to be
<span class="emphasis"><em>locking</em></span> that resource. Note that
DB supports both exclusive and non-exclusive locks. See
<a class="xref" href="blocking_deadlocks.html#locks" title="Locks">Locks</a> for more information.
</p>
</li>
<li>
<p>
<span class="emphasis"><em>Free-threaded</em></span>
</p>
<p>
Data structures and objects are free-threaded if they can be
shared across threads of control without any explicit locking on
the part of the application. Some books, libraries, and
programming languages may use the term
<span class="emphasis"><em>thread-safe</em></span> for data structures or objects
that have this characteristic. The two terms mean the
same thing.
</p>
<p>
For a description of free-threaded DB objects, see
<a class="xref" href="txnconcurrency.html#concurrenthandles" title="Which DB Handles are Free-Threaded">Which DB Handles are Free-Threaded</a>.
</p>
</li>
<li>
<p>
<span class="emphasis"><em>Blocked</em></span>
</p>
<p>
When a thread cannot obtain a lock because some other
thread already holds a lock on that object, the lock
attempt is said to be <span class="emphasis"><em>blocked</em></span>. See
<a class="xref" href="blocking_deadlocks.html#blocks" title="Blocks">Blocks</a> for more information.
</p>
</li>
<li>
<p>
<span class="emphasis"><em>Deadlock</em></span>
</p>
<p>
Occurs when two or more threads of control attempt to access conflicting resource in such a way as none
of the threads can any longer make further progress.
</p>
<p>
For example, if Thread A is blocked waiting for a resource held by Thread
B, while at the same time Thread B is blocked waiting for a
resource held by Thread A, then neither thread can make any
forward progress. In this situation, Thread A and Thread B
are said to be <span class="emphasis"><em>deadlocked.</em></span>
</p>
<p>
For more information, see <a class="xref" href="blocking_deadlocks.html#deadlocks" title="Deadlocks">Deadlocks</a>.
</p>
</li>
</ul>
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="concurrenthandles"></a>Which DB Handles are Free-Threaded</h2>
</div>
</div>
</div>
<p>
The following describes to what extent and under what conditions
individual handles are free-threaded.
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="classname">Environment</code>
<span>
and the DPL
<code class="classname">EntityStore</code>
</span>
</p>
<p>
Free-threaded so long as
<span>
<code class="methodname">EnvironmentConfig.setThreaded()</code>
is set to <code class="literal">true</code>.
</span>
</p>
</li>
<li>
<p>
<code class="classname">Database</code>
<span>
and the DPL
<code class="classname">PrimaryIndex</code>
</span>
</p>
<p>
Free-threaded so long as
<span>
the database
<span>
or DPL <code class="classname">PrimaryIndex</code>
</span>
is opened in a free-threaded environment.
</span>
</p>
</li>
<li>
<p>
<code class="classname">SecondaryDatabase</code>
and DPL <code class="classname">SecondaryIndex</code>
</p>
<p>
Same conditions apply as for <code class="classname">Database</code>
and <code class="classname">PrimaryIndex</code> handles.
</p>
</li>
<li>
<p>
<code class="classname">Cursor</code>
<span>
and the DPL
<code class="classname">EntityCursor</code>
</span>
</p>
<p>
Cursors are not free-threaded. However, they
can be used by multiple threads of control so
long as the application serializes access to the handle.
</p>
</li>
<li>
<p>
<code class="classname">SecondaryCursor</code>
</p>
<p>
Same conditions apply as for <code class="classname">Cursor</code>
handles.
</p>
</li>
<li>
<p>
<code class="classname">Transaction</code>
</p>
<p>
Access must be serialized by the application across threads of control.
</p>
</li>
</ul>
</div>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
All other classes found in the DPL
(<code class="literal">com.sleepycat.persist.*</code>) and not
mentioned above are free-threaded.
</p>
<p>
All classes found in the bind APIs (<code class="literal">com.sleepycat.bind.*</code>) are free-threaded.
</p>
</div>
</div>
</div>
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<td width="40%" align="left" valign="top">Configuring the Transaction Subsystem </td>
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