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. 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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 40 41 <span>a record from the database or a class from the store</span> 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 62 63 64 65 <span> 66 specify 67 <tt class="classname">com.sleepycat.db.LockMode.RMW</tt> 68 69 to the database, cursor, 70 <tt class="classname">PrimaryIndex</tt>, or 71 <tt class="classname">SecondaryIndex</tt> get method. 72 </span> 73 </p> 74 <p> 75 For example: 76 </p> 77 <pre class="programlisting">// Begin the deadlock retry loop as is normal. 78while (retry_count < MAX_DEADLOCK_RETRIES) { 79 try { 80 txn = myEnv.beginTransaction(null, null); 81 82 ... 83 // key and data are DatabaseEntry objects. 84 // Their usage is omitted for brevity. 85 ... 86 87 // Read the data. Declare the read/modify/write cycle here 88 myDatabase.get(txn, key, data, LockMode.RMW); 89 90 91 // Put the data. Note that you do not have to provide any 92 // additional flags here due to the read/modify/write 93 // cycle. Simply put the data and perform your deadlock 94 // detection as normal. 95 myDatabase.put(txn, key, data); 96 txn.commit(); 97 return 0; 98 } catch (DeadlockException de) { 99 // Deadlock detection and exception handling omitted 100 // for brevity 101 ... </pre> 102 <p> 103 Or, with the DPL: 104</p> 105 <pre class="programlisting">// Begin the deadlock retry loop as is normal 106 while (retry_count < MAX_DEADLOCK_RETRIES) { 107 try { 108 txn = myEnv.beginTransaction(null, null); 109 110 ... 111 // 'store' is an EntityStore and 'Inventory' is an entity class 112 // Their usage and implementation is omitted for brevity. 113 ... 114 115 // Read the data, using the PrimaryIndex for the entity object 116 PrimaryIndex<String,Inventory> pi = 117 store.getPrimaryIndex(String.class, Inventory.class); 118 Inventory iv = pi.get(txn, "somekey", LockMode.RMW); 119 120 // Do something to the retreived object 121 122 123 // Put the object. Note that you do not have to provide any 124 // additional flags here due to the read/modify/write 125 // cycle. Simply put the data and perform your deadlock 126 // detection as normal. 127 128 pi.put(txn, iv); 129 txn.commit(); 130 return 0; 131 132 } catch (DeadlockException de) { 133 // Deadlock detection and exception handling omitted 134 // for brevity 135 ... </pre> 136 </div> 137 <div class="navfooter"> 138 <hr /> 139 <table width="100%" summary="Navigation footer"> 140 <tr> 141 <td width="40%" align="left"><a accesskey="p" href="txn_ccursor.html">Prev</a> </td> 142 <td width="20%" align="center"> 143 <a accesskey="u" href="txnconcurrency.html">Up</a> 144 </td> 145 <td width="40%" align="right"> <a accesskey="n" href="txnnowait.html">Next</a></td> 146 </tr> 147 <tr> 148 <td width="40%" align="left" valign="top">Transactional Cursors and Concurrent Applications </td> 149 <td width="20%" align="center"> 150 <a accesskey="h" href="index.html">Home</a> 151 </td> 152 <td width="40%" align="right" valign="top"> No Wait on Blocks</td> 153 </tr> 154 </table> 155 </div> 156 </body> 157</html> 158