DbEnv::txn_begin
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#include <db_cxx.h>
int
DbEnv::txn_begin(DbTxn *parent, DbTxn **tid, u_int32_t flags);
Description: DbEnv::txn_begin
The DbEnv::txn_begin method creates a new transaction in the environment
and copies a pointer to a DbTxn that uniquely identifies it into
the memory to which tid refers. Calling the DbTxn::abort,
DbTxn::commit or DbTxn::discard methods will discard the returned
handle.
Note: Transactions may only span threads if they do so serially;
that is, each transaction must be active in only a single thread
of control at a time. This restriction holds for parents of nested
transactions as well; not two children may be concurrently active in
more than one thread of control at any one time.
Note: Cursors may not span transactions; that is, each cursor must be
opened and closed within a single transaction.
Note: A parent transaction may not issue any Berkeley DB operations -- except for
DbEnv::txn_begin, DbTxn::abort and DbTxn::commit -- while it has
active child transactions (child transactions that have not yet been
committed or aborted).
The DbEnv::txn_begin method
either returns a non-zero error value
or throws an exception that encapsulates a non-zero error value on
failure, and returns 0 on success.
Parameters
- flags
- The flags parameter must be set to 0 or by bitwise inclusively OR'ing together one
or more of the following values:
- DB_READ_COMMITTED
- This transaction will have degree 2 isolation. This provides for cursor
stability but not repeatable reads. Data items which have been
previously read by this transaction may be deleted or modified by other
transactions before this transaction completes.
- DB_READ_UNCOMMITTED
- This transaction will have degree 1 isolation. Read operations
performed by the transaction may read modified but not yet committed
data. Silently ignored if the DB_READ_UNCOMMITTED flag was not
specified when the underlying database was opened.
- DB_TXN_NOSYNC
- Do not synchronously flush the log when this transaction commits or
prepares. This means the transaction will exhibit the ACI (atomicity,
consistency, and isolation) properties, but not D (durability); that is,
database integrity will be maintained but it is possible that this
transaction may be undone during recovery.
This behavior may be set for a Berkeley DB environment using the
DbEnv::set_flags method. Any value specified to this method
overrides that setting.
- DB_TXN_NOWAIT
- If a lock is unavailable for any Berkeley DB operation performed in the context
of this transaction, cause the operation to
return DB_LOCK_DEADLOCK or throw a DbDeadlockException
immediately instead of blocking on the lock.
- DB_TXN_SYNC
- Synchronously flush the log when this transaction commits or prepares.
This means the transaction will exhibit all of the ACID (atomicity,
consistency, isolation, and durability) properties.
This behavior is the default for Berkeley DB environments unless the
DB_TXN_NOSYNC flag was specified to the
DbEnv::set_flags method. Any value specified to this method
overrides that setting.
- parent
- If the parent parameter is non-NULL, the new transaction will
be a nested transaction, with the transaction indicated by
parent as its parent. Transactions may be nested to any level.
In the presence of distributed transactions and two-phase commit, only
the parental transaction, that is a transaction without a parent
specified, should be passed as an parameter to DbTxn::prepare.
Errors
If the maximum number of concurrent transactions has been reached, the DbEnv::txn_begin method will fail and
either return ENOMEM or
throw a DbMemoryException.
Class
DbEnv, DbTxn
See Also
Transaction Subsystem and Related Methods
Copyright (c) 1996-2005 Sleepycat Software, Inc. - All rights reserved.