Edit File: txnconcurrency.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>Chapter 4. Concurrency</title> <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> <meta name="generator" content="DocBook XSL Stylesheets V1.62.4" /> <link rel="home" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" /> <link rel="up" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" /> <link rel="previous" href="maxtxns.html" title="Configuring the Transaction Subsystem" /> <link rel="next" href="blocking_deadlocks.html" title="Locks, Blocks, and Deadlocks" /> </head> <body> <div class="navheader"> <table width="100%" summary="Navigation header"> <tr> <th colspan="3" align="center">Chapter 4. Concurrency</th> </tr> <tr> <td width="20%" align="left"><a accesskey="p" href="maxtxns.html">Prev</a> </td> <th width="60%" align="center"> </th> <td width="20%" align="right"> <a accesskey="n" href="blocking_deadlocks.html">Next</a></td> </tr> </table> <hr /> </div> <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> </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> </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 deadlocks. 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 href="blocking_deadlocks.html#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 href="txnconcurrency.html#concurrenthandles">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 href="blocking_deadlocks.html#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 may 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 href="blocking_deadlocks.html#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></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> <tt class="classname">Environment</tt> </p> <p> Free-threaded so long as <span> <tt class="methodname">EnvironmentConfig.setThreaded()</tt> is set to <tt class="literal">true</tt>. </span> </p> </li> <li> <p> <tt class="classname">Database</tt> </p> <p> Free-threaded so long as <span> the database is opened in a free-threaded environment. </span> </p> </li> <li> <p> <tt class="classname">SecondaryDatabase</tt> </p> <p> Same conditions apply as for <tt class="classname">Database</tt> handles. </p> </li> <li> <p> <tt class="classname">Cursor</tt> </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> <tt class="classname">SecondaryCursor</tt> </p> <p> Same conditions apply as for <tt class="classname">Cursor</tt> handles. </p> </li> <li> <p> <tt class="classname">Transaction</tt> </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 classes found in the DPL (<tt class="literal">com.sleepycat.persist.*</tt>) are free-threaded. </p> <p> All classes found in the bind APIs (<tt class="literal">com.sleepycat.bind.*</tt>) are free-threaded. </p> </div> </div> </div> <div class="navfooter"> <hr /> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"><a accesskey="p" href="maxtxns.html">Prev</a> </td> <td width="20%" align="center"> <a accesskey="u" href="index.html">Up</a> </td> <td width="40%" align="right"> <a accesskey="n" href="blocking_deadlocks.html">Next</a></td> </tr> <tr> <td width="40%" align="left" valign="top">Configuring the Transaction Subsystem </td> <td width="20%" align="center"> <a accesskey="h" href="index.html">Home</a> </td> <td width="40%" align="right" valign="top"> Locks, Blocks, and Deadlocks</td> </tr> </table> </div> </body> </html>
Back to File Manager