File: readmodifywrite.html

package info (click to toggle)
db5.3 5.3.28%2Bdfsg1-0.5
  • links: PTS, VCS
  • area: main
  • in suites: buster
  • size: 158,360 kB
  • sloc: ansic: 448,411; java: 111,824; tcl: 80,544; sh: 44,326; cs: 33,697; cpp: 21,604; perl: 14,557; xml: 10,799; makefile: 4,077; yacc: 1,003; awk: 965; sql: 801; erlang: 342; python: 216; php: 24; asm: 14
file content (159 lines) | stat: -rw-r--r-- 6,177 bytes parent folder | download | duplicates (8)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <title>Read/Modify/Write</title>
    <link rel="stylesheet" href="gettingStarted.css" type="text/css" />
    <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
    <link rel="start" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" />
    <link rel="up" href="txnconcurrency.html" title="Chapter 4. Concurrency" />
    <link rel="prev" href="exclusivelock.html" title="Exclusive Database Handles" />
    <link rel="next" href="txnnowait.html" title="No Wait on Blocks" />
  </head>
  <body>
    <div xmlns="" class="navheader">
      <div class="libver">
        <p>Library Version 11.2.5.3</p>
      </div>
      <table width="100%" summary="Navigation header">
        <tr>
          <th colspan="3" align="center">Read/Modify/Write</th>
        </tr>
        <tr>
          <td width="20%" align="left"><a accesskey="p" href="exclusivelock.html">Prev</a> </td>
          <th width="60%" align="center">Chapter 4. Concurrency</th>
          <td width="20%" align="right"> <a accesskey="n" href="txnnowait.html">Next</a></td>
        </tr>
      </table>
      <hr />
    </div>
    <div class="sect1" lang="en" xml:lang="en">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both"><a id="readmodifywrite"></a>Read/Modify/Write</h2>
          </div>
        </div>
      </div>
      <p>
            If you are retrieving 
                
                
                <span>a record from the database or a class from the store</span>
            for the purpose of modifying or deleting it, you should declare 
            a read-modify-write cycle at the time that you read the 
                    <span>record.</span>
                    
            Doing so causes DB to obtain write locks (instead of a read
            locks) at the time of the read. This helps to prevent deadlocks by
            preventing another transaction from acquiring a read lock on the same
            record while the read-modify-write cycle is in progress.
        </p>
      <p>
            Note that declaring a read-modify-write cycle may actually increase the amount of blocking that your
            application sees, because readers immediately obtain write locks and write locks cannot be shared. For this
            reason, you should use read-modify-write cycles only if you are seeing a large amount of deadlocking
            occurring in your application.
        </p>
      <p>
            In order to declare a read/modify/write cycle when you perform a
            read operation, 

            

            

            <span>
                specify 
                    <code class="classname">com.sleepycat.db.LockMode.RMW</code>
                    
                    to the database, cursor,
                    <code class="classname">PrimaryIndex</code>, or
                    <code class="classname">SecondaryIndex</code> get method.
            </span>
        </p>
      <p>
            For example:
        </p>
      <pre class="programlisting">// Begin the deadlock retry loop as is normal.
while (retry_count &lt; MAX_DEADLOCK_RETRIES) {
    try {
        txn = myEnv.beginTransaction(null, null);

        ...
        // key and data are DatabaseEntry objects.
        // Their usage is omitted for brevity.
        ...

        // Read the data. Declare the read/modify/write cycle here
        myDatabase.get(txn, key, data, LockMode.RMW);


        // Put the data. Note that you do not have to provide any 
        // additional flags here due to the read/modify/write 
        // cycle. Simply put the data and perform your deadlock 
        // detection as normal.
        myDatabase.put(txn, key, data);
        txn.commit();
        return 0;
    } catch (DeadlockException de) {
        // Deadlock detection and exception handling omitted
        // for brevity
        ... </pre>
      <p>
        Or, with the DPL:
</p>
      <pre class="programlisting">// Begin the deadlock retry loop as is normal
        while (retry_count &lt; MAX_DEADLOCK_RETRIES) {
    try {
        txn = myEnv.beginTransaction(null, null);

        ...
        // 'store' is an EntityStore and 'Inventory' is an entity class
        // Their usage and implementation is omitted for brevity.
        ...

        // Read the data, using the PrimaryIndex for the entity object
        PrimaryIndex&lt;String,Inventory&gt; pi = 
                store.getPrimaryIndex(String.class, Inventory.class);
        Inventory iv = pi.get(txn, "somekey", LockMode.RMW);

        // Do something to the retreived object


        // Put the object. Note that you do not have to provide any 
        // additional flags here due to the read/modify/write 
        // cycle. Simply put the data and perform your deadlock 
        // detection as normal.

        pi.put(txn, iv);
        txn.commit();
        return 0;

    } catch (DeadlockException de) {
        // Deadlock detection and exception handling omitted
        // for brevity
        ... </pre>
    </div>
    <div class="navfooter">
      <hr />
      <table width="100%" summary="Navigation footer">
        <tr>
          <td width="40%" align="left"><a accesskey="p" href="exclusivelock.html">Prev</a> </td>
          <td width="20%" align="center">
            <a accesskey="u" href="txnconcurrency.html">Up</a>
          </td>
          <td width="40%" align="right"> <a accesskey="n" href="txnnowait.html">Next</a></td>
        </tr>
        <tr>
          <td width="40%" align="left" valign="top">Exclusive Database Handles </td>
          <td width="20%" align="center">
            <a accesskey="h" href="index.html">Home</a>
          </td>
          <td width="40%" align="right" valign="top"> No Wait on Blocks</td>
        </tr>
      </table>
    </div>
  </body>
</html>