# Roll-forward recovery

Derby supports roll-forward recovery to restore a damaged database to the most recent state before a failure occurred.

Derby restores a database from full backup and replays all the transactions after the backup. All the log files after a backup are required to replay the transactions after the backup. By default, the database keeps only logs that are required for crash-recovery. For roll-forward recovery to be successful, all log files must be archived after a backup. Log files can be archived using the backup function calls that enable log archiving.

In roll-forward recovery the log archival mode ensures that all old log files are available. The log files are available only from the time that the log archival mode is enabled.

Derby uses the following information to restore the database:

- The backup copy of the database
- The set of archived logs
- The current online active log

You cannot use roll-forward recovery to restore individual tables. Roll-forward recovery recovers the entire database.

To restore a database by using roll-forward recovery, you must already have a backup copy of the database, all the archived logs since the backup was created, and the active log files. All the log files should be in the database log directory.

There are two types of log files in Derby: active logs and online archived logs.

Active logs
  Active logs are used during crash recovery to prevent a failure that might leave a database in an inconsistent state. Roll-forward recovery can also use the active logs to recover to the end of the log files. Active logs are located in the database log path directory.
Online archived logs
  Log files that are stored for roll-forward recovery use when they are no longer needed for crash recovery. Online archived logs are also kept in the database log path directory.

Enabling log archival mode

Online archive logs are available only if the database is enabled for log archival mode. You can use the following system procedure to enable the database for log archival mode:

    SYSCS_UTIL.SYSCS_BACKUP_DATABASE_AND_ENABLE_LOG_ARCHIVE_MODE
    (IN BACKUPDIR VARCHAR(32672), IN SMALLINT DELETE_ARCHIVED_LOG_FILES)

The input parameters for the calls in the previous example specify the location where the backup should be stored and specify whether or not the database should keep online archived logs for the backup. Existing online archived log files that were created before this backup will be deleted if the input parameter value for the deleteOnlineArchivedLogFiles parameter is non-zero. The log files are deleted only after a successful backup.
Note: Make sure to store the backup database in a safe place when you choose the log file removal option.

The SYSCS_UTIL.SYSCS_BACKUP_DATABASE_AND_ENABLE_LOG_ARCHIVE_MODE procedure will issue an error if there are any unlogged operations in the same transaction as backup procedure.

If any unlogged operations are in progress in other transactions in the system when the backup starts, this procedure will block until those transactions are complete before performing the backup. Derby automatically converts unlogged operations to logged mode if they are started while the backup is in progress (except operations that maintain application jar files in the database). Procedures to install, replace, and remove jar files in a database are blocked while the backup is in progress.

If you do not want backup to block until unlogged operations in other transactions are complete, use the SYSCS_UTIL.SYSCS_BACKUP_DATABASE_AND_ENABLE_LOG_ARCHIVE_MODE_NOWAIT procedure. This procedure issues an error immediately at the start of the backup if there are any transactions in progress with unlogged operations, instead of waiting for those transactions to complete.

Disabling log archival mode:

After you enable log archival mode, the database will always have the log archival mode enabled even if it is subsequently booted or backed up. The only way to disable the log archive mode is to run the following procedure:

    SYSCS_UTIL.SYSCS_DISABLE_LOG_ARCHIVE_MODE(IN SMALLINT DELETE_ARCHIVED_LOG_FILES)

This system procedure disables the log archive mode and deletes any existing online archived log files if the input parameter DELETE_ARCHIVED_LOG_FILES is non-zero.

Performing roll-forward recovery:

By using the full backup copy, archived logs, and active logs, you can restore a database to its most recent state by performing roll-forward recovery. You perform a roll-forward recovery by specifying a connection URL attribute rollForwardRecoveryFrom=<BackupPath> at boot time. This brings the database to its most recent state by using full backup copy, archived logs, and active logs. All the log files should be in the database log path directory.

Backing up a database:

In the following example, a database named wombat is backed up to the d:/backup directory with log archive mode enabled:

    connect 'jdbc:derby:wombat;create=true';

    create table t1(a int not null primary key);
    ------------------DML/DDL Operations
    CALL SYSCS_UTIL.SYSCS_BACKUP_DATABASE_AND_ENABLE_LOG_ARCHIVE_MODE
    ('d:/backup', 0);
    insert into t1 values(19);
    create table t2(a int);
    -----------------DML/DDL Operations
    -----------------Database Crashed (Media Corruption on data disks)

Restoring a database using roll-forward recovery:

In the following example, the database is restored using roll-forward recovery after a media failure:

    connect 'jdbc:derby:wombat;rollForwardRecoveryFrom=d:/backup/wombat';
    select * from t1;
    ---------------DML/DDL Operations

The following attribute can be specified in the JDBC boot time connection URL:

rollForwardRecoveryFrom=<Path>

For more information, see the rollForwardRecoveryFrom=<Path> section in the Derby Reference Manual.

After a database is restored from full backup, transactions from the online archived logs and active logs are replayed.

Parent topic: Backing up and restoring databases
Related concepts
Backing up a database
Related tasks
Restoring a database from a backup copy
Creating a database from a backup copy