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<html>
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<title>Berkeley DB Reference Guide: Standard lock modes</title>
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<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
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<meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,java,C,C++">
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<a name="2"><!--meow--></a>
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<table><tr valign=top>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Locking Subsystem</dl></h3></td>
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<td width="1%"><a href="../../ref/lock/page.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../../ref/toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../../ref/lock/notxn.html"><img src="../../images/next.gif" alt="Next"></a>
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</td></tr></table>
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<p>
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<h1 align=center>Standard lock modes</h1>
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<p>The Berkeley DB locking protocol is described by a conflict matrix. A conflict
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matrix is an n x n array where n is the number of different lock modes
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supported, and the (i, j)th entry of the array indicates whether a lock of
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mode i conflicts with a lock of mode j.
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<p>The Berkeley DB include files declare two commonly used conflict arrays:
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<p><dl compact>
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<p><dt>const u_int8_t db_rw_conflicts[ ];<dd>This is a conflict matrix for a simple scheme using shared and exclusive
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lock modes.
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<p><dt>const u_int8_t db_riw_conflicts[ ];<dd>This is a conflict matrix that involves various intent lock modes (e.g.,
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intent shared) that are used for multigranularity locking.
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</dl>
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<p>The number of modes associated with each matrix are DB_LOCK_RW_N and
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DB_LOCK_RIW_N, respectively.
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<p>In addition, the Berkeley DB include file defines the type <b>db_lockmode_t</b>,
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which is the type of the lock modes used with the standard tables above:
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<p><dl compact>
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<p><dt>DB_LOCK_NG<dd>not granted (always 0)
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<p><dt>DB_LOCK_READ<dd>read (shared)
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<p><dt>DB_LOCK_WRITE<dd>write (exclusive)
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</dl>
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<p>As an example, consider the basic multiple-reader/single writer conflict
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matrix described by <b>db_rw_conflicts</b>. In the following
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example (and in the appropriate file), a 1 represents a conflict (i.e.,
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do not grant the lock if the indicated lock is held) and a 0 indicates
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that it is OK to grant the lock.
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<p>The rows indicate the lock that is held and the columns indicate the lock
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that is requested.
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<p><blockquote><pre> Notheld Read Write
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Notheld 0 0 0
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Read* 0 0 1
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Write** 0 1 1
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</pre></blockquote>
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<p><dl compact>
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<p><dt>*<dd>In this case, suppose that there is a read lock held on an object. A new
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request for a read lock would be granted, but a request for a write lock
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would not.
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<p><dt>**<dd>In this case, suppose that there is a write lock held on an object. A
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new request for either a read or write lock would be denied.
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</dl>
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<table><tr><td><br></td><td width="1%"><a href="../../ref/lock/page.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../../ref/toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../../ref/lock/notxn.html"><img src="../../images/next.gif" alt="Next"></a>
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</td></tr></table>
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<p><font size=1><a href="http://www.sleepycat.com">Copyright Sleepycat Software</a></font>
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