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 V1.73.2" /> 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. 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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> 36 <p> 37 If you are retrieving 38 39 40 <span>a record from the database or a class from the store</span> 41 for the purpose of modifying or deleting it, you should declare 42 a read-modify-write cycle at the time that you read the 43 <span>record.</span> 44 45 Doing so causes DB to obtain write locks (instead of a read 46 locks) at the time of the read. This helps to prevent deadlocks by 47 preventing another transaction from acquiring a read lock on the same 48 record while the read-modify-write cycle is in progress. 49 </p> 50 <p> 51 Note that declaring a read-modify-write cycle may actually increase the amount of blocking that your 52 application sees, because readers immediately obtain write locks and write locks cannot be shared. For this 53 reason, you should use read-modify-write cycles only if you are seeing a large amount of deadlocking 54 occurring in your application. 55 </p> 56 <p> 57 In order to declare a read/modify/write cycle when you perform a 58 read operation, 59 60 61 62 63 64 <span> 65 specify 66 <code class="classname">com.sleepycat.db.LockMode.RMW</code> 67 68 to the database, cursor, 69 <code class="classname">PrimaryIndex</code>, or 70 <code class="classname">SecondaryIndex</code> get method. 71 </span> 72 </p> 73 <p> 74 For example: 75 </p> 76 <pre class="programlisting">// Begin the deadlock retry loop as is normal. 77while (retry_count < MAX_DEADLOCK_RETRIES) { 78 try { 79 txn = myEnv.beginTransaction(null, null); 80 81 ... 82 // key and data are DatabaseEntry objects. 83 // Their usage is omitted for brevity. 84 ... 85 86 // Read the data. Declare the read/modify/write cycle here 87 myDatabase.get(txn, key, data, LockMode.RMW); 88 89 90 // Put the data. Note that you do not have to provide any 91 // additional flags here due to the read/modify/write 92 // cycle. Simply put the data and perform your deadlock 93 // detection as normal. 94 myDatabase.put(txn, key, data); 95 txn.commit(); 96 return 0; 97 } catch (DeadlockException de) { 98 // Deadlock detection and exception handling omitted 99 // for brevity 100 ... </pre> 101 <p> 102 Or, with the DPL: 103</p> 104 <pre class="programlisting">// Begin the deadlock retry loop as is normal 105 while (retry_count < MAX_DEADLOCK_RETRIES) { 106 try { 107 txn = myEnv.beginTransaction(null, null); 108 109 ... 110 // 'store' is an EntityStore and 'Inventory' is an entity class 111 // Their usage and implementation is omitted for brevity. 112 ... 113 114 // Read the data, using the PrimaryIndex for the entity object 115 PrimaryIndex<String,Inventory> pi = 116 store.getPrimaryIndex(String.class, Inventory.class); 117 Inventory iv = pi.get(txn, "somekey", LockMode.RMW); 118 119 // Do something to the retreived object 120 121 122 // Put the object. Note that you do not have to provide any 123 // additional flags here due to the read/modify/write 124 // cycle. Simply put the data and perform your deadlock 125 // detection as normal. 126 127 pi.put(txn, iv); 128 txn.commit(); 129 return 0; 130 131 } catch (DeadlockException de) { 132 // Deadlock detection and exception handling omitted 133 // for brevity 134 ... </pre> 135 </div> 136 <div class="navfooter"> 137 <hr /> 138 <table width="100%" summary="Navigation footer"> 139 <tr> 140 <td width="40%" align="left"><a accesskey="p" href="txn_ccursor.html">Prev</a> </td> 141 <td width="20%" align="center"> 142 <a accesskey="u" href="txnconcurrency.html">Up</a> 143 </td> 144 <td width="40%" align="right"> <a accesskey="n" href="txnnowait.html">Next</a></td> 145 </tr> 146 <tr> 147 <td width="40%" align="left" valign="top">Transactional Cursors and Concurrent Applications </td> 148 <td width="20%" align="center"> 149 <a accesskey="h" href="index.html">Home</a> 150 </td> 151 <td width="40%" align="right" valign="top"> No Wait on Blocks</td> 152 </tr> 153 </table> 154 </div> 155 </body> 156</html> 157