DbEnv::set_app_dispatch()
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## DbEnv::set_app_dispatch()

    #include <db_cxx.h>

    int
    DbEnv::set_app_dispatch(int (*tx_recover)(DbEnv *dbenv,
        Dbt *log_rec, DbLsn *lsn, db_recops op));

Declare a function to be called during transaction abort and recovery to process application-specific log records.

The `DbEnv::set_app_dispatch()` method configures operations performed using the specified DbEnv handle, not all operations performed on the underlying database environment.

The `DbEnv::set_app_dispatch()` method may not be called after the DbEnv::open() method is called. If the database environment already exists when DbEnv::open() is called, the information specified to `DbEnv::set_app_dispatch()` must be consistent with the existing environment or corruption can occur.

The `DbEnv::set_app_dispatch()` method returns a non-zero error value on failure and 0 on success.

### Parameters

#### tx_recover

The tx_recover parameter is the application's abort and recovery function. The function takes four parameters:

- `dbenv`

  The dbenv parameter is the enclosing database environment handle.

- `log_rec`

  The log_rec parameter is a log record.

- `lsn`

  The lsn parameter is a log sequence number.

- `op`

  The op parameter is one of the following values:

  - `DB_TXN_BACKWARD_ROLL`

    The log is being read backward to determine which transactions have been committed and to abort those operations that were not; undo the operation described by the log record.

  - `DB_TXN_FORWARD_ROLL`

    The log is being played forward; redo the operation described by the log record.

  - `DB_TXN_ABORT`

    The log is being read backward during a transaction abort; undo the operation described by the log record.

  - `DB_TXN_APPLY`

    The log is being applied on a replica site; redo the operation described by the log record.

  - `DB_TXN_PRINT`

    The log is being printed for debugging purposes; print the contents of this log record in the desired format.

  The DB_TXN_FORWARD_ROLL and DB_TXN_APPLY operations frequently imply the same actions, redoing changes that appear in the log record, although if a recovery function is to be used on a replication client where reads may be taking place concurrently with the processing of incoming messages, DB_TXN_APPLY operations should also perform appropriate locking. The macro DB_REDO(op) checks that the operation is one of DB_TXN_FORWARD_ROLL or DB_TXN_APPLY, and should be used in the recovery code to refer to the conditions under which operations should be redone. Similarly, the macro DB_UNDO(op) checks if the operation is one of DB_TXN_BACKWARD_ROLL or DB_TXN_ABORT.

The function must return 0 on success and either errno or a value outside of the Berkeley DB error name space on failure.

### Errors

The `DbEnv::set_app_dispatch()` 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:

#### EINVAL

If the method was called after DbEnv::open() was called; or if an invalid flag value or parameter was specified.

### Class

DbEnv, DbTxn

### See Also

Transaction Subsystem and Related Methods

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