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The DbTxn Handle
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<h2 class="title"><a id="txn"></a>Chapter 13.
The DbTxn Handle
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<pre class="programlisting">#include <db_cxx.h>
class DbTxn {
public:
DB_TXN *DbTxn::get_DB_TXN();
const DB_TXN *DbTxn::get_const_DB_TXN() const;
static DbTxn *DbTxn::get_DbTxn(DB_TXN *txn);
static const DbTxn *DbTxn::get_const_DbTxn(const DB_TXN *txn);
...
}; </pre>
<p>
The <code class="classname">DbTxn</code> object is the handle for a transaction. Methods of
the <code class="classname">DbTxn</code> handle are used to configure, abort and commit the
transaction. <code class="classname">DbTxn</code> handles are provided to
<a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> methods in order
to transactionally protect those database operations.
</p>
<p>
<code class="classname">DbTxn</code> handles are not free-threaded; transactions handles
may be used by multiple threads, but only serially, that is, the application must
serialize access to the <code class="classname">DbTxn</code> handle. Once the
<a class="xref" href="txnabort.html" title="DbTxn::abort()">DbTxn::abort()</a> or
<a class="xref" href="txncommit.html" title="DbTxn::commit()">DbTxn::commit()</a> methods are called, the handle
may not be accessed again, regardless of the method's return. In addition, parent
transactions may not issue any Berkeley DB operations while they have active child
transactions (child transactions that have not yet been committed or aborted) except for
<a class="xref" href="txnbegin.html" title="DbEnv::txn_begin()">DbEnv::txn_begin()</a>,
<a class="xref" href="txnabort.html" title="DbTxn::abort()">DbTxn::abort()</a> and
<a class="xref" href="txncommit.html" title="DbTxn::commit()">DbTxn::commit()</a>.
</p>
<p>
Each <code class="classname">DbTxn</code> object has an associated
<code class="literal">DB_TXN</code> struct, which is used by the underlying
implementation of Berkeley DB and its
<span>C++ language</span>
API. The <code class="methodname">DbTxn::get_DB_TXN()</code> method
returns a pointer to this struct. Given a const <code class="literal">DbTxn</code> object,
<code class="methodname">txnMget_const_DB_TXN()</code>
returns a const pointer to the same struct.
</p>
<p>
Given a <code class="literal">DB_TXN</code> struct, the <code class="methodname">DbTxn::get_DbTxn()</code>
method returns the corresponding <code class="classname">DbTxn</code> object, if there is
one. If the <code class="literal">DB_TXN</code> object was not associated with a
<code class="classname">DbTxn</code> (that is, it was not returned from a call to
<code class="methodname">DbTxn::get_DB_TXN()</code>), then the result of
<code class="methodname">DbTxn::get_DbTxn</code> is undefined. Given a const
<code class="literal">DB_TXN</code> struct, <code class="methodname">DbTxn::get_const_DbTxn()</code>
returns the associated const <code class="literal">DbTxn</code> object, if there is one.
</p>
<p>
These methods may be useful for Berkeley DB applications including both C and C++
language software. It should not be necessary to use these calls in a purely C++
application.
</p>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
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<div>
<h2 class="title" style="clear: both"><a id="txnlist"></a>Transaction Subsystem and Related Methods</h2>
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<thead>
<tr>
<th>Transaction Subsystem and Related Methods</th>
<th>Description</th>
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<tr>
<td>
<a class="xref" href="txnrecover.html" title="DbEnv::txn_recover()">DbEnv::txn_recover()</a>
</td>
<td>Distributed transaction recovery</td>
</tr>
<tr>
<td>
<a class="xref" href="txncheckpoint.html" title="DbEnv::txn_checkpoint()">DbEnv::txn_checkpoint()</a>
</td>
<td>Checkpoint the transaction subsystem</td>
</tr>
<tr>
<td>
<a class="xref" href="txnstat.html" title="DbEnv::txn_stat()">DbEnv::txn_stat()</a>
</td>
<td>Return transaction subsystem statistics</td>
</tr>
<tr>
<td>
<a class="xref" href="txnstat_print.html" title="DbEnv::txn_stat_print()">DbEnv::txn_stat_print()</a>
</td>
<td>Print transaction subsystem statistics</td>
</tr>
<tr>
<td>
<a class="xref" href="txnset_timeout.html" title="DbTxn::set_timeout()">DbTxn::set_timeout()</a>
</td>
<td>Set transaction timeout</td>
</tr>
<tr>
<td colspan="2">
<span class="bold">
<strong>Transaction Subsystem Configuration</strong>
</span>
</td>
</tr>
<tr>
<td><a class="xref" href="envset_timeout.html" title="DbEnv::set_timeout()">DbEnv::set_timeout()</a>, <a class="xref" href="envget_timeout.html" title="DbEnv::get_timeout()">DbEnv::get_timeout()</a></td>
<td>Set/get lock and transaction timeout</td>
</tr>
<tr>
<td>
<a class="xref" href="dbget_transactional.html" title="Db::get_transactional()">Db::get_transactional()</a>
</td>
<td>Does the Db have transaction support</td>
</tr>
<tr>
<td>
<a class="xref" href="envcdsgroup_begin.html" title="DbEnv::cdsgroup_begin()">DbEnv::cdsgroup_begin()</a>
</td>
<td>Get a locker ID in Berkeley DB Concurrent Data Store</td>
</tr>
<tr>
<td><a class="xref" href="envset_tx_max.html" title="DbEnv::set_tx_max()">DbEnv::set_tx_max()</a>, <a class="xref" href="envget_tx_max.html" title="DbEnv::get_tx_max()">DbEnv::get_tx_max()</a></td>
<td>Set/get maximum number of transactions</td>
</tr>
<tr>
<td><a class="xref" href="envset_tx_timestamp.html" title="DbEnv::set_tx_timestamp()">DbEnv::set_tx_timestamp()</a>, <a class="xref" href="envget_tx_timestamp.html" title="DbEnv::get_tx_timestamp()">DbEnv::get_tx_timestamp()</a></td>
<td>Set/get recovery timestamp</td>
</tr>
<tr>
<td colspan="2">
<span class="bold">
<strong>Transaction Operations</strong>
</span>
</td>
</tr>
<tr>
<td>
<a class="xref" href="txnbegin.html" title="DbEnv::txn_begin()">DbEnv::txn_begin()</a>
</td>
<td>Begin a transaction</td>
</tr>
<tr>
<td>
<a class="xref" href="txnabort.html" title="DbTxn::abort()">DbTxn::abort()</a>
</td>
<td>Abort a transaction</td>
</tr>
<tr>
<td>
<a class="xref" href="txncommit.html" title="DbTxn::commit()">DbTxn::commit()</a>
</td>
<td>Commit a transaction</td>
</tr>
<tr>
<td>
<a class="xref" href="txndiscard.html" title="DbTxn::discard()">DbTxn::discard()</a>
</td>
<td>Discard a prepared but not resolved transaction handle</td>
</tr>
<tr>
<td>
<a class="xref" href="txnid.html" title="DbTxn::id()">DbTxn::id()</a>
</td>
<td>Return a transaction's ID</td>
</tr>
<tr>
<td>
<a class="xref" href="txnprepare.html" title="DbTxn::prepare()">DbTxn::prepare()</a>
</td>
<td>Prepare a transaction for commit</td>
</tr>
<tr>
<td><a class="xref" href="txnset_name.html" title="DbTxn::set_name()">DbTxn::set_name()</a>, <a class="xref" href="txnget_name.html" title="DbTxn::get_name()">DbTxn::get_name()</a></td>
<td>Associate a string with a transaction</td>
</tr>
<tr>
<td><a class="xref" href="txnset_priority.html" title="DbTxn::set_priority()">DbTxn::set_priority()</a>, <a class="xref" href="txnget_priority.html" title="DbTxn::get_priority()">DbTxn::get_priority()</a></td>
<td>Set/get transaction's priority</td>
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