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>Transactional Cursors and Concurrent Applications</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="txnconcurrency.html" title="Chapter 4. Concurrency" /> 11 <link rel="prev" href="isolation.html" title="Isolation" /> 12 <link rel="next" href="readmodifywrite.html" title="Read/Modify/Write" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">Transactional Cursors and Concurrent Applications</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="isolation.html">Prev</a> </td> 22 <th width="60%" align="center">Chapter 4. Concurrency</th> 23 <td width="20%" align="right"> <a accesskey="n" href="readmodifywrite.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="txn_ccursor"></a>Transactional Cursors and Concurrent Applications</h2> 33 </div> 34 </div> 35 </div> 36 <div class="toc"> 37 <dl> 38 <dt> 39 <span class="sect2"> 40 <a href="txn_ccursor.html#cursordirtyreads">Using Cursors with Uncommitted Data</a> 41 </span> 42 </dt> 43 </dl> 44 </div> 45 <p> 46 When you use transactional cursors with a concurrent application, remember that 47 in the event of a deadlock you must make sure that you close your cursor before you abort and retry your 48 transaction. 49 </p> 50 <p> 51 Also, remember that when you are using the default isolation level, 52 every time your cursor reads a record it locks 53 that record until the encompassing transaction is resolved. This 54 means that walking your database with a transactional cursor 55 increases the chance of lock contention. 56 </p> 57 <p> 58 For this reason, if you must routinely walk your database with a 59 transactional cursor, consider using a reduced isolation level 60 such as read committed. 61 </p> 62 <div class="sect2" lang="en" xml:lang="en"> 63 <div class="titlepage"> 64 <div> 65 <div> 66 <h3 class="title"><a id="cursordirtyreads"></a>Using Cursors with Uncommitted Data</h3> 67 </div> 68 </div> 69 </div> 70 <p> 71 72 As described in <a class="xref" href="isolation.html#dirtyreads" title="Reading Uncommitted Data">Reading Uncommitted Data</a> 73 above, it is possible to relax your transaction's isolation 74 level such that it can read data modified but not yet committed 75 by another transaction. You can configure this when you create 76 your transaction handle, and when you do so then all cursors opened 77 inside that transaction will automatically use uncommitted reads. 78 </p> 79 <p> 80 You can also do this when you create a cursor handle from within 81 a serializable transaction. When you do this, only those 82 cursors configured for uncommitted reads uses uncommitted reads. 83 </p> 84 <p> 85 Either way, you must first configure your database 86 handle to support 87 uncommitted reads before you can configure your transactions or 88 your cursors to use them. 89 </p> 90 <p> 91 The following example shows how to configure an individual cursor handle 92 to read uncommitted data from within a serializable (full isolation) transaction. 93 For an example of 94 configuring a transaction to perform uncommitted reads in 95 general, see <a class="xref" href="isolation.html#dirtyreads" title="Reading Uncommitted Data">Reading Uncommitted Data</a>. 96 </p> 97 <pre class="programlisting">#include "db_cxx.h" 98 99... 100 101int main(void) 102{ 103 u_int32_t env_flags = DB_CREATE | // If the environment does not 104 // exist, create it. 105 DB_INIT_LOCK | // Initialize locking 106 DB_INIT_LOG | // Initialize logging 107 DB_INIT_MPOOL | // Initialize the cache 108 DB_INIT_TXN; // Initialize transactions 109 110 u_int32_t db_flags = DB_CREATE | // Create the db if it does 111 // not exist 112 DB_AUTO_COMMIT | // Enable auto commit 113 DB_READ_UNCOMMITTED; // Enable uncommitted reads 114 115 Db *dbp = NULL; 116 const char *file_name = "mydb.db"; 117 118 std::string envHome("/export1/testEnv"); 119 DbEnv myEnv(0); 120 121 Dbc *cursorp = NULL; 122 123 try { 124 125 myEnv.open(envHome.c_str(), env_flags, 0); 126 dbp = new Db(&myEnv, 0); 127 dbp->open(NULL, // Txn pointer 128 file_name, // File name 129 NULL, // Logical db name 130 DB_BTREE, // Database type (using btree) 131 db_flags, // Open flags 132 0); // File mode. Using defaults 133 134 DbTxn *txn = NULL; 135 myEnv.txn_begin(NULL, &txn, 0); 136 try { 137 // Get our cursor. Note that we pass the transaction 138 // handle here. Note also that we pass the 139 // DB_READ_UNCOMMITTED flag here so as to cause the 140 // cursor to perform uncommitted reads. 141 db.cursor(txn, &cursorp, DB_READ_UNCOMMITTED); 142 143 // From here, you perform your cursor reads and writes 144 // as normal, committing and aborting the transactions as 145 // is necessary, and testing for deadlock exceptions as 146 // normal (omitted for brevity). 147 148 ... </pre> 149 </div> 150 </div> 151 <div class="navfooter"> 152 <hr /> 153 <table width="100%" summary="Navigation footer"> 154 <tr> 155 <td width="40%" align="left"><a accesskey="p" href="isolation.html">Prev</a> </td> 156 <td width="20%" align="center"> 157 <a accesskey="u" href="txnconcurrency.html">Up</a> 158 </td> 159 <td width="40%" align="right"> <a accesskey="n" href="readmodifywrite.html">Next</a></td> 160 </tr> 161 <tr> 162 <td width="40%" align="left" valign="top">Isolation </td> 163 <td width="20%" align="center"> 164 <a accesskey="h" href="index.html">Home</a> 165 </td> 166 <td width="40%" align="right" valign="top"> Read/Modify/Write</td> 167 </tr> 168 </table> 169 </div> 170 </body> 171</html> 172