DbEnv::txn_begin()
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## DbEnv::txn_begin()

    #include <db_cxx.h>

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

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; no 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 DB_LOCK_NOTGRANTED if the database environment has been configured using the DB_TIME_NOTGRANTED flag).

  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_SNAPSHOT`

  This transaction will execute with snapshot isolation. For databases with the DB_MULTIVERSION flag set, data values will be read as they are when the transaction begins, without taking read locks. Silently ignored for operations on databases with DB_MULTIVERSION not set on the underlying database (read locks are acquired).

  The error `DB_LOCK_DEADLOCK` will be returned from update operations if a snapshot transaction attempts to update data which was modified after the snapshot transaction read it.

- `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.

- `DB_TXN_WAIT`

  If a lock is unavailable for any Berkeley DB operation performed in the context of this transaction, wait for the lock.

  This behavior is the default for Berkeley DB environments unless the `DB_TXN_NOWAIT` flag was specified to the DbEnv::set_flags() method. Any value specified to this method overrides that setting.

- `DB_TXN_WRITE_NOSYNC`

  Write, but do not synchronously flush, the log when this transaction commits. 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 if the system fails, it is possible some number of the most recently committed transactions may be undone during recovery. The number of transactions at risk is governed by how often the system flushes dirty buffers to disk and how often the log is flushed or checkpointed.

  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.

#### 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

The `DbEnv::txn_begin()` method may fail and throw a DbException exception, encapsulating one of the following non-zero errors, or return one of the following non-zero errors:

#### DbMemoryException or ENOMEM

The maximum number of concurrent transactions has been reached.

DbMemoryException is thrown if your Berkeley DB API is configured to throw exceptions. Otherwise, `ENOMEM` is returned.

### Class

DbEnv, DbTxn

### See Also

Transaction Subsystem and Related Methods

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