# DbEnv::txn_begin

`

###

    #include <db_cxx.h>

    int
    DbEnv::txn_begin(DbTxn *parent, DbTxn **tid, u_int32_t flags);

# Description

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.

If the parent argument 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 argument to DbTxn::prepare.

The flags value must be set to 0 or by bitwise inclusively OR'ing together one or more of the following values:

DB_DIRTY_READ
  All read operations performed by the transaction will read modified but not yet committed data. Silently ignored if the DB_DIRTY_READ 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 interface. Any value specified in this interface overrides that setting.

DB_TXN_NOWAIT
  If a lock is unavailable for any Berkeley DB operation performed in the context of this transaction, return immediately instead of blocking on the lock. The error return in the case will be DB_LOCK_NOTGRANTED.

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 interface. Any value specified in this interface overrides that setting.

Note: A transaction may not span threads; that is, each transaction must begin and end in the same thread, and each transaction may be used only by a single thread.

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.

# Errors

The DbEnv::txn_begin method may fail and throw an exception or return a non-zero error for the following conditions:

ENOMEM
  The maximum number of concurrent transactions has been reached.

The DbEnv::txn_begin method may fail and throw an exception or return a non-zero error for errors specified for other Berkeley DB and C library or system methods. If a catastrophic error has occurred, the DbEnv::txn_begin method may fail and either return DB_RUNRECOVERY or throw an exception encapsulating DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail in the same way.

# Class

DbEnv, DbTxn

# See Also

DbEnv::set_tx_max, DbEnv::set_tx_recover, DbEnv::set_tx_timestamp, DbTxn::abort, DbEnv::txn_begin, DbEnv::txn_checkpoint, DbTxn::commit, DbTxn::discard, DbTxn::id, DbTxn::prepare, DbEnv::txn_recover, and DbEnv::txn_stat.`

Copyright Sleepycat Software