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6<title>Berkeley DB Reference Guide: Configuring transactions</title>
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13<td><b><dl><dt>Berkeley DB Reference Guide:<dd>Transaction Subsystem</dl></b></td>
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16<p align=center><b>Configuring transactions</b></p>
17<p>The application may change the number of simultaneous outstanding
18transactions supported by the Berkeley DB environment by calling the
19<a href="/api_c/env_set_tx_max.html">DB_ENV-&gt;set_tx_max</a> method.  This will also set the size of the
20underlying transaction subsystem's region.  When the number of
21outstanding transactions is reached, additional calls to
22<a href="/api_c/txn_begin.html">DB_ENV-&gt;txn_begin</a> will fail until some active transactions complete.</p>
23<p>The application can limit how long a transaction runs or blocks on
24contested resources.
25The <a href="/api_c/env_set_timeout.html">DB_ENV-&gt;set_timeout</a> method specifies the length of the timeout.
26This value is checked whenever deadlock detection is performed or
27when the transaction is about to block on a lock that cannot be
28immediately granted.
29Because timeouts are only checked at these times, the accuracy of the
30timeout depends on how often deadlock detection is performed or how
31frequently the transaction blocks.</p>
32<p>There is an additional parameter used in configuring transactions; the
33<a href="/api_c/env_set_flags.html#DB_TXN_NOSYNC">DB_TXN_NOSYNC</a> flag.  Setting the <a href="/api_c/env_set_flags.html#DB_TXN_NOSYNC">DB_TXN_NOSYNC</a> flag to
34<a href="/api_c/env_set_flags.html">DB_ENV-&gt;set_flags</a> when opening a transaction region changes the
35behavior of transactions to not write or synchronously flush the log
36during transaction commit.</p>
37<p>This change may significantly increase application transactional
38throughput.  However, it means that although transactions will continue
39to exhibit the ACI (atomicity, consistency, and isolation) properties,
40they will not have D (durability).  Database integrity will be
41maintained, but it is possible that some number of the most recently
42committed transactions may be undone during recovery instead of being
43redone.</p>
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