1<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml"> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 6 <title>Chapter��4.��Concurrency</title> 7 <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> 9 <link rel="start" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" /> 10 <link rel="up" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" /> 11 <link rel="prev" href="maxtxns.html" title="Configuring the Transaction Subsystem" /> 12 <link rel="next" href="blocking_deadlocks.html" title="Locks, Blocks, and Deadlocks" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">Chapter��4.��Concurrency</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="maxtxns.html">Prev</a>��</td> 22 <th width="60%" align="center">��</th> 23 <td width="20%" align="right">��<a accesskey="n" href="blocking_deadlocks.html">Next</a></td> 24 </tr> 25 </table> 26 <hr /> 27 </div> 28 <div class="chapter" lang="en" xml:lang="en"> 29 <div class="titlepage"> 30 <div> 31 <div> 32 <h2 class="title"><a id="txnconcurrency"></a>Chapter��4.��Concurrency</h2> 33 </div> 34 </div> 35 </div> 36 <div class="toc"> 37 <p> 38 <b>Table of Contents</b> 39 </p> 40 <dl> 41 <dt> 42 <span class="sect1"> 43 <a href="txnconcurrency.html#concurrenthandles">Which DB Handles are Free-Threaded</a> 44 </span> 45 </dt> 46 <dt> 47 <span class="sect1"> 48 <a href="blocking_deadlocks.html">Locks, Blocks, and Deadlocks</a> 49 </span> 50 </dt> 51 <dd> 52 <dl> 53 <dt> 54 <span class="sect2"> 55 <a href="blocking_deadlocks.html#locks">Locks</a> 56 </span> 57 </dt> 58 <dt> 59 <span class="sect2"> 60 <a href="blocking_deadlocks.html#blocks">Blocks</a> 61 </span> 62 </dt> 63 <dt> 64 <span class="sect2"> 65 <a href="blocking_deadlocks.html#deadlocks">Deadlocks</a> 66 </span> 67 </dt> 68 </dl> 69 </dd> 70 <dt> 71 <span class="sect1"> 72 <a href="lockingsubsystem.html">The Locking Subsystem</a> 73 </span> 74 </dt> 75 <dd> 76 <dl> 77 <dt> 78 <span class="sect2"> 79 <a href="lockingsubsystem.html#configuringlock">Configuring the Locking Subsystem</a> 80 </span> 81 </dt> 82 <dt> 83 <span class="sect2"> 84 <a href="lockingsubsystem.html#configdeadlkdetect">Configuring Deadlock Detection</a> 85 </span> 86 </dt> 87 <dt> 88 <span class="sect2"> 89 <a href="lockingsubsystem.html#deadlockresolve">Resolving Deadlocks</a> 90 </span> 91 </dt> 92 </dl> 93 </dd> 94 <dt> 95 <span class="sect1"> 96 <a href="isolation.html">Isolation</a> 97 </span> 98 </dt> 99 <dd> 100 <dl> 101 <dt> 102 <span class="sect2"> 103 <a href="isolation.html#degreesofisolation">Supported Degrees of Isolation</a> 104 </span> 105 </dt> 106 <dt> 107 <span class="sect2"> 108 <a href="isolation.html#dirtyreads">Reading Uncommitted Data</a> 109 </span> 110 </dt> 111 <dt> 112 <span class="sect2"> 113 <a href="isolation.html#readcommitted">Committed Reads</a> 114 </span> 115 </dt> 116 <dt> 117 <span class="sect2"> 118 <a href="isolation.html#snapshot_isolation">Using Snapshot Isolation</a> 119 </span> 120 </dt> 121 </dl> 122 </dd> 123 <dt> 124 <span class="sect1"> 125 <a href="txn_ccursor.html">Transactional Cursors and Concurrent Applications</a> 126 </span> 127 </dt> 128 <dd> 129 <dl> 130 <dt> 131 <span class="sect2"> 132 <a href="txn_ccursor.html#cursordirtyreads">Using Cursors with Uncommitted Data</a> 133 </span> 134 </dt> 135 </dl> 136 </dd> 137 <dt> 138 <span class="sect1"> 139 <a href="readmodifywrite.html">Read/Modify/Write</a> 140 </span> 141 </dt> 142 <dt> 143 <span class="sect1"> 144 <a href="txnnowait.html">No Wait on Blocks</a> 145 </span> 146 </dt> 147 <dt> 148 <span class="sect1"> 149 <a href="reversesplit.html">Reverse BTree Splits</a> 150 </span> 151 </dt> 152 </dl> 153 </div> 154 <p> 155 156 DB offers a great deal of support for multi-threaded 157 <span> 158 and multi-process 159 </span> 160 applications even when transactions are not in use. Many of DB's 161 handles are 162 thread-safe<span>, or 163 can be made thread-safe by providing the appropriate flag at handle creation time,</span> 164 and DB provides a 165 flexible locking subsystem for managing databases in a concurrent 166 application. Further, DB provides a robust mechanism for 167 detecting and responding to deadlocks. All of these concepts are 168 explored in this chapter. 169 </p> 170 <p> 171 Before continuing, it is useful to define a few terms that will appear 172 throughout this chapter: 173 </p> 174 <div class="itemizedlist"> 175 <ul type="disc"> 176 <li> 177 <p> 178 <span class="emphasis"><em>Thread of control</em></span> 179 </p> 180 <p> 181 Refers to a thread that is performing work in your application. 182 Typically, in this book that thread will be performing DB 183 operations. 184 </p> 185 <p> 186 Note that this term can also be taken to mean a separate process 187 that is performing work ��� DB supports multi-process 188 operations on your databases. 189 </p> 190 <p> 191 Also, DB is agnostic with regard to the type or style of 192 threads in use in your application. So if you are using 193 multiple threads 194 (as opposed to multiple processes) to perform concurrent 195 database access, you are free to use whatever thread package is 196 best for your platform and application. That said, this manual 197 will use pthreads for its threading examples because those have 198 the best chance of being supported across a large range of 199 platforms. 200 </p> 201 </li> 202 <li> 203 <p> 204 <span class="emphasis"><em>Locking</em></span> 205 </p> 206 <p> 207 When a thread of control obtains 208 access to a shared resource, it is said to be 209 <span class="emphasis"><em>locking</em></span> that resource. Note that 210 DB supports both exclusive and non-exclusive locks. See 211 <a class="xref" href="blocking_deadlocks.html#locks" title="Locks">Locks</a> for more information. 212 </p> 213 </li> 214 <li> 215 <p> 216 <span class="emphasis"><em>Free-threaded</em></span> 217 </p> 218 <p> 219 Data structures and objects are free-threaded if they can be 220 shared across threads of control without any explicit locking on 221 the part of the application. Some books, libraries, and 222 programming languages may use the term 223 <span class="emphasis"><em>thread-safe</em></span> for data structures or objects 224 that have this characteristic. The two terms mean the 225 same thing. 226 </p> 227 <p> 228 For a description of free-threaded DB objects, see 229 <a class="xref" href="txnconcurrency.html#concurrenthandles" title="Which DB Handles are Free-Threaded">Which DB Handles are Free-Threaded</a>. 230 </p> 231 </li> 232 <li> 233 <p> 234 <span class="emphasis"><em>Blocked</em></span> 235 </p> 236 <p> 237 When a thread cannot obtain a lock because some other 238 thread already holds a lock on that object, the lock 239 attempt is said to be <span class="emphasis"><em>blocked</em></span>. See 240 <a class="xref" href="blocking_deadlocks.html#blocks" title="Blocks">Blocks</a> for more information. 241 </p> 242 </li> 243 <li> 244 <p> 245 <span class="emphasis"><em>Deadlock</em></span> 246 </p> 247 <p> 248 Occurs when two or more threads of control attempt to access conflicting resource in such a way as none 249 of the threads can any longer make further progress. 250 </p> 251 <p> 252 For example, if Thread A is blocked waiting for a resource held by Thread 253 B, while at the same time Thread B is blocked waiting for a 254 resource held by Thread A, then neither thread can make any 255 forward progress. In this situation, Thread A and Thread B 256 are said to be <span class="emphasis"><em>deadlocked.</em></span> 257 </p> 258 <p> 259 For more information, see <a class="xref" href="blocking_deadlocks.html#deadlocks" title="Deadlocks">Deadlocks</a>. 260 </p> 261 </li> 262 </ul> 263 </div> 264 <div class="sect1" lang="en" xml:lang="en"> 265 <div class="titlepage"> 266 <div> 267 <div> 268 <h2 class="title" style="clear: both"><a id="concurrenthandles"></a>Which DB Handles are Free-Threaded</h2> 269 </div> 270 </div> 271 </div> 272 <p> 273 The following describes to what extent and under what conditions 274 individual handles are free-threaded. 275 </p> 276 <div class="itemizedlist"> 277 <ul type="disc"> 278 <li> 279 <p> 280 <code class="classname">DB_ENV</code> 281 282 283 284 </p> 285 <p> 286 Free-threaded so long as 287 <span> 288 the <code class="literal">DB_THREAD</code> flag is provided to 289 the environment <code class="methodname">open()</code> 290 method. 291 </span> 292 293 </p> 294 </li> 295 <li> 296 <p> 297 <code class="classname">DB</code> 298 299 300 301 </p> 302 <p> 303 Free-threaded so long as 304 <span> 305 the <code class="literal">DB_THREAD</code> flag is provided to 306 the database <code class="methodname">open()</code> 307 method, or if the database is opened using a 308 free-threaded environment handle. 309 </span> 310 311 </p> 312 </li> 313 <li> 314 <p> 315 <code class="classname">DBC</code> 316 317 318 319 </p> 320 <p> 321 Cursors are not free-threaded. However, they 322 can be used by multiple threads of control so 323 long as the application serializes access to the handle. 324 </p> 325 </li> 326 <li> 327 <p> 328 <code class="classname">DB_TXN</code> 329 330 331 332 </p> 333 <p> 334 Access must be serialized by the application across threads of control. 335 </p> 336 </li> 337 </ul> 338 </div> 339 </div> 340 </div> 341 <div class="navfooter"> 342 <hr /> 343 <table width="100%" summary="Navigation footer"> 344 <tr> 345 <td width="40%" align="left"><a accesskey="p" href="maxtxns.html">Prev</a>��</td> 346 <td width="20%" align="center">��</td> 347 <td width="40%" align="right">��<a accesskey="n" href="blocking_deadlocks.html">Next</a></td> 348 </tr> 349 <tr> 350 <td width="40%" align="left" valign="top">Configuring the Transaction Subsystem��</td> 351 <td width="20%" align="center"> 352 <a accesskey="h" href="index.html">Home</a> 353 </td> 354 <td width="40%" align="right" valign="top">��Locks, Blocks, and Deadlocks</td> 355 </tr> 356 </table> 357 </div> 358 </body> 359</html> 360