1<!--$Id: nested.so,v 10.23 2003/01/24 18:01:58 bostic Exp $--> 2<!--Copyright (c) 1997,2008 Oracle. All rights reserved.--> 3<!--See the file LICENSE for redistribution information.--> 4<html> 5<head> 6<title>Berkeley DB Reference Guide: Nested transactions</title> 7<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> 8<meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> 9</head> 10<body bgcolor=white> 11<a name="2"><!--meow--></a> 12<table width="100%"><tr valign=top> 13<td><b><dl><dt>Berkeley DB Reference Guide:<dd>Berkeley DB Transactional Data Store Applications</dl></b></td> 14<td align=right><a href="../transapp/cursor.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../transapp/admin.html"><img src="../../images/next.gif" alt="Next"></a> 15</td></tr></table> 16<p align=center><b>Nested transactions</b></p> 17<p>Berkeley DB provides support for nested transactions. Nested transactions 18allow an application to decompose a large or long-running transaction 19into smaller units that may be independently aborted.</p> 20<p>Normally, when beginning a transaction, the application will pass a NULL 21value for the parent argument to <a href="../../api_c/txn_begin.html">DB_ENV->txn_begin</a>. If, however, the 22parent argument is a <a href="../../api_c/txn_class.html">DB_TXN</a> handle, the newly created transaction 23will be treated as a nested transaction within the parent. Transactions 24may nest arbitrarily deeply. For the purposes of this discussion, 25transactions created with a parent identifier will be called 26<i>child transactions</i>.</p> 27<p>Once a transaction becomes a parent, as long as any of its child 28transactions are unresolved (that is, they have neither committed nor 29aborted), the parent may not issue any Berkeley DB calls except to begin more 30child transactions, or to commit or abort. For example, it may not 31issue any access method or cursor calls. After all of a parent's 32children have committed or aborted, the parent may again request 33operations on its own behalf.</p> 34<p>The semantics of nested transactions are as follows. When a child 35transaction is begun, it inherits all the locks of its parent. This 36means that the child will never block waiting on a lock held by its 37parent. Further, locks held by two children of the same parent will 38also conflict. To make this concrete, consider the following set of 39transactions and lock acquisitions.</p> 40<p>Transaction T1 is the parent transaction. It acquires a write lock on 41item A and then begins two child transactions: C1 and C2. C1 also wants 42to acquire a write lock on A; this succeeds. If C2 attempts to acquire 43a write lock on A, it will block until C1 releases the lock, at which 44point it will succeed. Now, let's say that C1 acquires a write lock on 45B. If C2 now attempts to obtain a lock on B, it will block. However, 46let's now assume that C1 commits. Its locks are anti-inherited, which 47means they are given to T1, so T1 will now hold a lock on B. At this 48point, C2 would be unblocked and would then acquire a lock on B.</p> 49<p>Child transactions are entirely subservient to their parent transaction. 50They may abort, undoing their operations regardless of the eventual fate 51of the parent. However, even if a child transaction commits, if its 52parent transaction is eventually aborted, the child's changes are undone 53and the child's transaction is effectively aborted. Any child 54transactions that are not yet resolved when the parent commits or aborts 55are resolved based on the parent's resolution -- committing if the 56parent commits and aborting if the parent aborts. Any child 57transactions that are not yet resolved when the parent prepares are also 58prepared.</p> 59<table width="100%"><tr><td><br></td><td align=right><a href="../transapp/cursor.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../transapp/admin.html"><img src="../../images/next.gif" alt="Next"></a> 60</td></tr></table> 61<p><font size=1>Copyright (c) 1996,2008 Oracle. All rights reserved.</font> 62</body> 63</html> 64
