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diff --git a/bdb/docs/ref/lock/intro.html b/bdb/docs/ref/lock/intro.html new file mode 100644 index 00000000000..b5c85af05b0 --- /dev/null +++ b/bdb/docs/ref/lock/intro.html @@ -0,0 +1,89 @@ +<!--$Id: intro.so,v 10.16 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: Berkeley DB and 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/program/runtime.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/page.html"><img src="../../images/next.gif" alt="Next"></a> +</td></tr></table> +<p> +<h1 align=center>Berkeley DB and locking</h1> +<p>The lock subsystem provides interprocess and intraprocess concurrency +control mechanisms. While the locking system is used extensively by the +Berkeley DB access methods and transaction system, it may also be used as a +stand-alone subsystem to provide concurrency control to any set of +designated resources. +<p>The lock subsystem is created, initialized, and opened by calls to +<a href="../../api_c/env_open.html">DBENV->open</a> with the <a href="../../api_c/env_open.html#DB_INIT_LOCK">DB_INIT_LOCK</a> or <a href="../../api_c/env_open.html#DB_INIT_CDB">DB_INIT_CDB</a> +flags specified. +<p>The <a href="../../api_c/lock_detect.html">lock_detect</a> function provides the programmatic interface to +the Berkeley DB deadlock detector. Whenever two threads of control issue lock +requests that are not carefully ordered or that require upgrading locks +(obtaining write locks on objects that are already read-locked), the +possibility for deadlock arises. A deadlock occurs when two or more +threads of control are blocked, waiting for actions that another one of +these blocked threads must take. For example, assume that threads one +and two have each obtained read locks on object A. Now suppose that both +threads wish to obtain write locks on object A. Neither thread can be +granted its writelock (because of the other thread's readlock). Both +threads block and will never unblock because the event for which they are +waiting can never happen. +<p>The deadlock detector examines all the locks held in the environment and +identifies situations where no thread can make forward progress. It then +selects one of the participants in the deadlock (according to the argument +that was specified to <a href="../../api_c/env_set_lk_detect.html">DBENV->set_lk_detect</a>) and forces it to return +the value DB_LOCK_DEADLOCK, which indicates that a deadlock occurred. +The thread receiving such an error should abort its current transaction, +or simply release all its locks if it is not running in a transaction, +and retry the operation. +<p>The <a href="../../api_c/lock_vec.html">lock_vec</a> interface is used to acquire and release locks. +<p>Two additional interfaces, <a href="../../api_c/lock_get.html">lock_get</a> and <a href="../../api_c/lock_put.html">lock_put</a>, are +provided. These interfaces are simpler front-ends to the <a href="../../api_c/lock_vec.html">lock_vec</a> +functionality, where <a href="../../api_c/lock_get.html">lock_get</a> acquires a lock, and +<a href="../../api_c/lock_put.html">lock_put</a> releases a lock that was acquired using <a href="../../api_c/lock_get.html">lock_get</a> +or <a href="../../api_c/lock_vec.html">lock_vec</a>. +<p>It is up to the application to specify lockers and objects appropriately. +When used with the Berkeley DB access methods, these lockers and objects are +handled completely internally, but an application using the lock manager +directly must either use the same conventions as the access methods or +define its own convention to which it adheres. If the application is +using the access methods with locking at the same time that it is calling +the lock manager directly, the application must follow a convention that +is compatible with the access methods' use of the locking subsystem. See +<a href="../../ref/lock/am_conv.html">Access method locking conventions</a> +for more information. +<p>The <a href="../../api_c/lock_id.html">lock_id</a> function returns a unique ID which may safely be used +as the locker parameter to the <a href="../../api_c/lock_vec.html">lock_vec</a> interface. The access +methods use <a href="../../api_c/lock_id.html">lock_id</a> to generate unique lockers for the cursors +associated with a database. +<p>The <a href="../../api_c/lock_vec.html">lock_vec</a> function performs any number of lock operations +atomically. It also provides the ability to release all locks held by a +particular locker and release all the locks on a particular object. +Performing multiple lock operations atomically is useful in performing +Btree traversals where you want to acquire a lock on a child page and once +acquired, immediately release the lock on its parent (this is +traditionally referred to as "lock-coupling"). Using <a href="../../api_c/lock_vec.html">lock_vec</a> +instead of separate calls to <a href="../../api_c/lock_put.html">lock_put</a> and <a href="../../api_c/lock_get.html">lock_get</a> reduces +the synchronization overhead between multiple threads or processes. +<p>The three interfaces, <a href="../../api_c/lock_get.html">lock_get</a>, <a href="../../api_c/lock_put.html">lock_put</a> and <a href="../../api_c/lock_vec.html">lock_vec</a>, +are fully compatible, and may be used interchangeably. +<p>All locks explicitly requested by an application should be released via +calls to <a href="../../api_c/lock_put.html">lock_put</a> or <a href="../../api_c/lock_vec.html">lock_vec</a>. +<p>The <a href="../../api_c/lock_stat.html">lock_stat</a> function returns information about the status of +the lock subsystem. It is the programmatic interface used by the +<a href="../../utility/db_stat.html">db_stat</a> utility. +<p>The locking subsystem is closed by the call to <a href="../../api_c/env_close.html">DBENV->close</a>. +<p>Finally, the entire locking subsystem may be discarded using the +<a href="../../api_c/env_remove.html">DBENV->remove</a> interface. +<table><tr><td><br></td><td width="1%"><a href="../../ref/program/runtime.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/page.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> |