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diff --git a/db/docs/ref/lock/twopl.html b/db/docs/ref/lock/twopl.html new file mode 100644 index 000000000..6cf112c09 --- /dev/null +++ b/db/docs/ref/lock/twopl.html @@ -0,0 +1,50 @@ +<!--$Id: twopl.so,v 10.7 2000/03/18 21:43:14 bostic Exp $--> +<!--Copyright 1997, 1998, 1999, 2000 by Sleepycat Software, Inc.--> +<!--All rights reserved.--> +<html> +<head> +<title>Berkeley DB Reference Guide: Locking with transactions: two-phase locking</title> +<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> +<meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,java,C,C++"> +</head> +<body bgcolor=white> + <a name="2"><!--meow--></a> +<table><tr valign=top> +<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Locking Subsystem</dl></h3></td> +<td width="1%"><a href="../../ref/lock/notxn.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/am_conv.html"><img src="../../images/next.gif" alt="Next"></a> +</td></tr></table> +<p> +<h1 align=center>Locking with transactions: two-phase locking</h1> +<p>Berkeley DB uses a locking protocol called two-phase locking. This is the +traditional protocol used in conjunction with lock-based transaction +systems. +<p>In a two-phase locking (2PL) system, transactions are broken up into two +distinct phases. During the first phase, the transaction only acquires +locks. During the second phase, the transaction only releases locks. +More formally, once a transaction releases a lock, it may not acquire any +additional locks. Practically, this translates into a system where locks +are acquired as they are needed throughout a transaction and retained +until the transaction ends, either by committing or aborting. In Berkeley DB, +locks are released during <a href="../../api_c/txn_abort.html">txn_abort</a> or <a href="../../api_c/txn_commit.html">txn_commit</a>. The +only exception to this protocol occurs when we use lock-coupling to +traverse a data structure. If the locks are held only for traversal +purposes, then the locks may be released before transaction commit or +abort. +<p>For applications, the implications of 2PL are that long-running +transactions will hold locks for a long time. When designing +applications, lock contention should be considered. In order to reduce +the probability of deadlock and achieve the best level of concurrency +possible, the following guidelines are helpful. +<p><ol> +<p><li>When accessing multiple databases, design all transactions so +that they access the files in the same order. +<p><li>If possible, access your most hotly contested resources last +(so that their locks are held for the shortest time possible). +<p><li>If possible, use nested transactions to protect the parts of +your transaction most likely to deadlock. +</ol> +<table><tr><td><br></td><td width="1%"><a href="../../ref/lock/notxn.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/am_conv.html"><img src="../../images/next.gif" alt="Next"></a> +</td></tr></table> +<p><font size=1><a href="http://www.sleepycat.com">Copyright Sleepycat Software</a></font> +</body> +</html> |