1<!--$Id: txn_begin.so,v 10.80 2006/12/13 18:09:09 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: DB_ENV->txn_begin</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<table width="100%"><tr valign=top> 12<td> 13<b>DB_ENV->txn_begin</b> 14</td> 15<td align=right> 16<a href="../api_c/api_core.html"><img src="../images/api.gif" alt="API"></a> 17<a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a></td> 18</tr></table> 19<hr size=1 noshade> 20<tt> 21<b><pre> 22#include <db.h> 23<p> 24int 25DB_ENV->txn_begin(DB_ENV *env, 26 DB_TXN *parent, DB_TXN **tid, u_int32_t flags); 27</pre></b> 28<hr size=1 noshade> 29<b>Description: DB_ENV->txn_begin</b> 30<p>The DB_ENV->txn_begin method creates a new transaction in the environment 31and copies a pointer to a <a href="../api_c/txn_class.html">DB_TXN</a> that uniquely identifies it into 32the memory to which <b>tid</b> refers. Calling the <a href="../api_c/txn_abort.html">DB_TXN->abort</a>, 33<a href="../api_c/txn_commit.html">DB_TXN->commit</a> or <a href="../api_c/txn_discard.html">DB_TXN->discard</a> methods will discard the returned 34handle.</p> 35<p><b>Note: Transactions may only span threads if they do so serially; 36that is, each transaction must be active in only a single thread 37of control at a time. This restriction holds for parents of nested 38transactions as well; no two children may be concurrently active in 39more than one thread of control at any one time.</b></p> 40<p><b>Note: Cursors may not span transactions; that is, each cursor must be 41opened and closed within a single transaction.</b></p> 42<p><b>Note: A parent transaction may not issue any Berkeley DB operations -- except for 43DB_ENV->txn_begin, <a href="../api_c/txn_abort.html">DB_TXN->abort</a> and <a href="../api_c/txn_commit.html">DB_TXN->commit</a> -- while it has 44active child transactions (child transactions that have not yet been 45committed or aborted).</b></p> 46<p>The DB_ENV->txn_begin method 47returns a non-zero error value on failure 48and 0 on success. 49</p> 50<b>Parameters</b> <br> 51 <b>flags</b><ul compact><li>The <b>flags</b> parameter must be set to 0 or by bitwise inclusively <b>OR</b>'ing together one 52or more of the following values: 53<br> 54<b><a name="DB_READ_COMMITTED">DB_READ_COMMITTED</a></b><ul compact><li>This transaction will have degree 2 isolation. This provides for cursor 55stability but not repeatable reads. Data items which have been 56previously read by this transaction may be deleted or modified by other 57transactions before this transaction completes.</ul> 58<b><a name="DB_READ_UNCOMMITTED">DB_READ_UNCOMMITTED</a></b><ul compact><li>This transaction will have degree 1 isolation. Read operations 59performed by the transaction may read modified but not yet committed 60data. Silently ignored if the DB_READ_UNCOMMITTED flag was not 61specified when the underlying database was opened.</ul> 62<b><a name="DB_TXN_NOSYNC">DB_TXN_NOSYNC</a></b><ul compact><li>Do not synchronously flush the log when this transaction commits or 63prepares. This means the transaction will exhibit the ACI (atomicity, 64consistency, and isolation) properties, but not D (durability); that is, 65database integrity will be maintained but it is possible that this 66transaction may be undone during recovery. 67<p>This behavior may be set for a Berkeley DB environment using the 68<a href="../api_c/env_set_flags.html">DB_ENV->set_flags</a> method. Any value specified to this method overrides 69that setting.</p></ul> 70<b><a name="DB_TXN_NOWAIT">DB_TXN_NOWAIT</a></b><ul compact><li>If a lock is unavailable for any Berkeley DB operation performed in the context 71of this transaction, cause the operation to 72return <a href="../ref/program/errorret.html#DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a> (or <a href="../ref/program/errorret.html#DB_LOCK_NOTGRANTED">DB_LOCK_NOTGRANTED</a> if the 73database environment has been configured using the 74<a href="../api_c/env_set_flags.html#DB_TIME_NOTGRANTED">DB_TIME_NOTGRANTED</a> flag). 75<p>This behavior may be set for a Berkeley DB environment using the 76<a href="../api_c/env_set_flags.html">DB_ENV->set_flags</a> method. Any value specified to this method overrides 77that setting.</p></ul> 78<b><a name="DB_TXN_SNAPSHOT">DB_TXN_SNAPSHOT</a></b><ul compact><li>This transaction will execute with <a href="../ref/transapp/read.html">snapshot isolation</a>. For databases with the <a href="../api_c/db_open.html#DB_MULTIVERSION">DB_MULTIVERSION</a> 79flag set, data values will be read as they are when the transaction 80begins, without taking read locks. Silently ignored for operations on 81databases with <a href="../api_c/db_open.html#DB_MULTIVERSION">DB_MULTIVERSION</a> not set on the underlying 82database (read locks are acquired). 83<p>The error <a name="DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a> will be returned from update 84operations if a snapshot transaction attempts to update data 85which was modified after the snapshot transaction read it.</p></ul> 86<b><a name="DB_TXN_SYNC">DB_TXN_SYNC</a></b><ul compact><li>Synchronously flush the log when this transaction commits or prepares. 87This means the transaction will exhibit all of the ACID (atomicity, 88consistency, isolation, and durability) properties. 89<p>This behavior is the default for Berkeley DB environments unless the 90DB_TXN_NOSYNC flag was specified to the 91<a href="../api_c/env_set_flags.html">DB_ENV->set_flags</a> method. Any value specified to this method overrides 92that setting.</p></ul> 93<b><a name="DB_TXN_WAIT">DB_TXN_WAIT</a></b><ul compact><li>If a lock is unavailable for any Berkeley DB operation performed in the context 94of this transaction, wait for the lock. 95<p>This behavior is the default for Berkeley DB environments unless the 96DB_TXN_NOWAIT flag was specified to the 97<a href="../api_c/env_set_flags.html">DB_ENV->set_flags</a> method. Any value specified to this method overrides 98that setting.</p></ul> 99<b><a name="DB_TXN_WRITE_NOSYNC">DB_TXN_WRITE_NOSYNC</a></b><ul compact><li>Write, but do not synchronously flush, the log when this transaction 100commits. This means the transaction will exhibit the ACI (atomicity, 101consistency, and isolation) properties, but not D (durability); that is, 102database integrity will be maintained, but if the system fails, it is 103possible some number of the most recently committed transactions may be 104undone during recovery. The number of transactions at risk is governed 105by how often the system flushes dirty buffers to disk and how often the 106log is flushed or checkpointed. 107<p>This behavior may be set for a Berkeley DB environment using the 108<a href="../api_c/env_set_flags.html">DB_ENV->set_flags</a> method. Any value specified to this method overrides 109that setting.</p></ul> 110<br></ul> 111 <b>parent</b><ul compact><li>If the <b>parent</b> parameter is non-NULL, the new transaction will 112be a nested transaction, with the transaction indicated by 113<b>parent</b> as its parent. Transactions may be nested to any level. 114In the presence of distributed transactions and two-phase commit, only 115the parental transaction, that is a transaction without a <b>parent</b> 116specified, should be passed as an parameter to <a href="../api_c/txn_prepare.html">DB_TXN->prepare</a>.</ul> 117<br> 118<br><b>Errors</b> 119<p>The DB_ENV->txn_begin method 120may fail and return one of the following non-zero errors:</p> 121<br> 122<b>ENOMEM</b><ul compact><li>The maximum number of concurrent transactions has been reached.</ul> 123<br> 124<hr size=1 noshade> 125<br><b>Class</b> 126<a href="../api_c/env_class.html">DB_ENV</a>, <a href="../api_c/txn_class.html">DB_TXN</a> 127<br><b>See Also</b> 128<a href="../api_c/txn_list.html">Transaction Subsystem and Related Methods</a> 129</tt> 130<table width="100%"><tr><td><br></td><td align=right> 131<a href="../api_c/api_core.html"><img src="../images/api.gif" alt="API"></a><a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a> 132</td></tr></table> 133<p><font size=1>Copyright (c) 1996,2008 Oracle. All rights reserved.</font> 134</body> 135</html> 136
