2011-09-13 17:44:24 +00:00
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<title>Nested transactions</title>
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="transapp.html" title="Chapter 11. Berkeley DB Transactional Data Store Applications" />
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<link rel="prev" href="transapp_cursor.html" title="Transactional cursors" />
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<link rel="next" href="transapp_admin.html" title="Environment infrastructure" />
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<div class="libver">
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2012-11-14 21:35:20 +00:00
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<p>Library Version 11.2.5.3</p>
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2011-09-13 17:44:24 +00:00
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Nested transactions</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="transapp_cursor.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 11.
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Berkeley DB Transactional Data Store Applications
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="transapp_admin.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="transapp_nested"></a>Nested transactions</h2>
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</div>
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</div>
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</div>
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<p>Berkeley DB provides support for nested transactions. Nested transactions
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allow an application to decompose a large or long-running transaction
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into smaller units that may be independently aborted.</p>
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<p>Normally, when beginning a transaction, the application will pass a NULL
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value for the parent argument to <a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a>. If, however, the
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parent argument is a <a href="../api_reference/C/txn.html" class="olink">TXN</a> handle, the newly created transaction
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will be treated as a nested transaction within the parent. Transactions
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may nest arbitrarily deeply. For the purposes of this discussion,
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transactions created with a parent identifier will be called
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<span class="emphasis"><em>child transactions</em></span>.</p>
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<p>Once a transaction becomes a parent, as long as any of its child
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transactions are unresolved (that is, they have neither committed nor
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aborted), the parent may not issue any Berkeley DB calls except to begin more
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child transactions, or to commit or abort. For example, it may not
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issue any access method or cursor calls. After all of a parent's
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children have committed or aborted, the parent may again request
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operations on its own behalf.</p>
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<p>The semantics of nested transactions are as follows. When a child
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transaction is begun, it inherits all the locks of its parent. This
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means that the child will never block waiting on a lock held by its
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parent. Further, locks held by two children of the same parent will
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also conflict. To make this concrete, consider the following set of
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transactions and lock acquisitions.</p>
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<p>Transaction T1 is the parent transaction. It acquires a write lock on
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item A and then begins two child transactions: C1 and C2. C1 also wants
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to acquire a write lock on A; this succeeds. If C2 attempts to acquire
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a write lock on A, it will block until C1 releases the lock, at which
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point it will succeed. Now, let's say that C1 acquires a write lock on
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B. If C2 now attempts to obtain a lock on B, it will block. However,
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let's now assume that C1 commits. Its locks are anti-inherited, which
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means they are given to T1, so T1 will now hold a lock on B. At this
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point, C2 would be unblocked and would then acquire a lock on B.</p>
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<p>Child transactions are entirely subservient to their parent transaction.
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They may abort, undoing their operations regardless of the eventual fate
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of the parent. However, even if a child transaction commits, if its
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parent transaction is eventually aborted, the child's changes are undone
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and the child's transaction is effectively aborted. Any child
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transactions that are not yet resolved when the parent commits or aborts
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are resolved based on the parent's resolution -- committing if the
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parent commits and aborting if the parent aborts. Any child
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transactions that are not yet resolved when the parent prepares are also
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prepared.</p>
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</div>
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="transapp_cursor.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="transapp.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="transapp_admin.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Transactional cursors </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Environment infrastructure</td>
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</tr>
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