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32            <h2 class="title" style="clear: both"><a id="readmodifywrite"></a>Read/Modify/Write</h2>
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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">// Begin the deadlock retry loop as is normal.
78while (retry_count &lt; MAX_DEADLOCK_RETRIES) {
79    try {
80        envp-&gt;txn_begin(NULL, txn, 0);
81
82        ...
83        // key and data are DBTs. Their usage is omitted for brevity.
84        ...
85
86        // Read the data. Declare the read/modify/write cycle here
87        dbp-&gt;get(txn, &amp;key, &amp;data, DB_RMW);
88
89        ...
90        // Modify the data as is required. (not shown here)
91        ...
92
93        // Put the data. Note that you do not have to provide any 
94        // additional flags here due to the read/modify/write 
95        // cycle. Simply put the data and perform your deadlock 
96        // detection as normal.
97        dbp-&gt;put(txn, &amp;key, &amp;data, 0);
98        txn-&gt;commit(0);
99        return (EXIT_SUCCESS);
100    } catch (DbDeadlockException &amp;de) {
101        // Deadlock detection and exception handling omitted
102        // for brevity
103        ...</pre>
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