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>Read/Modify/Write</title> 7 <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.62.4" /> 9 <link rel="home" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" /> 10 <link rel="up" href="txnconcurrency.html" title="Chapter��4.��Concurrency" /> 11 <link rel="previous" href="txn_ccursor.html" title="Transactional Cursors and Concurrent Applications" /> 12 <link rel="next" href="txnnowait.html" title="No Wait on Blocks" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">Read/Modify/Write</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="txn_ccursor.html">Prev</a>��</td> 22 <th width="60%" align="center">Chapter��4.��Concurrency</th> 23 <td width="20%" align="right">��<a accesskey="n" href="txnnowait.html">Next</a></td> 24 </tr> 25 </table> 26 <hr /> 27 </div> 28 <div class="sect1" lang="en" xml:lang="en"> 29 <div class="titlepage"> 30 <div> 31 <div> 32 <h2 class="title" style="clear: both"><a id="readmodifywrite"></a>Read/Modify/Write</h2> 33 </div> 34 </div> 35 <div></div> 36 </div> 37 <p> 38 If you are retrieving 39 <span>a record from the database</span> 40 41 42 for the purpose of modifying or deleting it, you should declare 43 a read-modify-write cycle at the time that you read the 44 <span>record.</span> 45 46 Doing so causes DB to obtain write locks (instead of a read 47 locks) at the time of the read. This helps to prevent deadlocks by 48 preventing another transaction from acquiring a read lock on the same 49 record while the read-modify-write cycle is in progress. 50 </p> 51 <p> 52 Note that declaring a read-modify-write cycle may actually increase the amount of blocking that your 53 application sees, because readers immediately obtain write locks and write locks cannot be shared. For this 54 reason, you should use read-modify-write cycles only if you are seeing a large amount of deadlocking 55 occurring in your application. 56 </p> 57 <p> 58 In order to declare a read/modify/write cycle when you perform a 59 read operation, 60 61 <span> 62 pass the <tt class="literal">DB_RMW</tt> flag 63 <span> 64 to the database or cursor get method. 65 </span> 66 67 68 </span> 69 70 71 72 73 </p> 74 <p> 75 For example: 76 </p> 77 <pre class="programlisting">retry: 78 /* Get the transaction */ 79 ret = envp->txn_begin(envp, NULL, &txn, 0); 80 if (ret != 0) { 81 envp->err(envp, ret, "txn_begin failed"); 82 return (EXIT_FAILURE); 83 } 84 ... 85 /* key and data are Dbts. Their usage is omitted for brevity. */ 86 ... 87 /* Get the data. Declare the read/modify/write cycle here */ 88 ret = dbp->get(dbp, txn, &key, &data, DB_RMW); 89 90 ... 91 /* Modify the key and data as is required (not shown here) */ 92 ... 93 94 /* Put the data. Note that you do not have to provide any additional 95 * flags here due to the read/modify/write cycle. Simply put the 96 * data and perform your deadlock detection as normal. 97 */ 98 ret = dbp->put(dbp, txn, &key, &data, 0); 99 switch (ret) { 100 /* Deadlock detection omitted for brevity */ 101 .... </pre> 102 </div> 103 <div class="navfooter"> 104 <hr /> 105 <table width="100%" summary="Navigation footer"> 106 <tr> 107 <td width="40%" align="left"><a accesskey="p" href="txn_ccursor.html">Prev</a>��</td> 108 <td width="20%" align="center"> 109 <a accesskey="u" href="txnconcurrency.html">Up</a> 110 </td> 111 <td width="40%" align="right">��<a accesskey="n" href="txnnowait.html">Next</a></td> 112 </tr> 113 <tr> 114 <td width="40%" align="left" valign="top">Transactional Cursors and Concurrent Applications��</td> 115 <td width="20%" align="center"> 116 <a accesskey="h" href="index.html">Home</a> 117 </td> 118 <td width="40%" align="right" valign="top">��No Wait on Blocks</td> 119 </tr> 120 </table> 121 </div> 122 </body> 123</html> 124