'; zhtm += ''; zhtm += '

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Using Oracle8


Chapter 13

Selecting and Implementing a Backup Strategy


Selecting a Backup Strategy

Oracle offers very rich and comprehensive backup and recovery options to suit every application's demand. The key is to implement a suitable strategy to meet the desired service levels between end users and the database operations group. A backup strategy suitable for a database depends on several factors, based on the nature of the application and the availability requirements. These factors will also decide whether your database is running in ARCHIVE LOG mode or in NOARCHIVE LOG mode. Table 13.1 lists the available database options that depend on the database's ARCHIVELOG mode.

The backup option in NOARCHIVELOG mode
Full cold backup is the only backup option available when the database is in NOARCHIVELOG mode.

Table 13.1  Database mode and the available backup options
Backup Type:
ARCHIVELOG
NOARCHIVELOG
Full cold backup
Yes
Yes
Partial cold backup
Yes
No
Hot backup
Yes
No

Recovery Manager and OS-level backup rely on making a copy of the users' data on a backup media-most commonly tape and disk. These methods preserve the data in a safe, inactive form. The data stored by these methods isn't used unless there's a loss of the current data for some reason. You can also copy the data stored in an Oracle database by using any of the following methods:

Recovery Manager
Recovery Manager is Oracle's new backup and recovery tool. Chapter 15, "Using Recovery Manager for Backup and Recovery," gives the details.

In summary, all these methods offer unique advantages and need to be used depending on your business requirements. However, they can also be used to complement each other.

Consider Using Recovery Manager

Oracle8's Recovery Manager is an integrated backup and recovery tool. Consider the following salient features before deciding your backup strategy:

Media management libraries
The Media Management library contains the vendor-supplied software programs that supply the backup Media Manager. If you don't have any backup media installed on the machine, disks can be used to take back-ups.

If you aren't using Recovery Manager to perform backup and recovery, you can use OS-level physical backups for making copies of the data to protect against loss. This has so far been the most widely used method for backup operations. There are important considerations to make while implementing a backup strategy:

Recovery Manager Scripts

Recovery Manager is an easy-to-use tool available in command-line interface (CLI) and graphical user interface (GUI) mode. (The GUI mode of the Recovery Manager is available through Enterprise Manager on the Windows NT platform.) Oracle also provides comprehensive scripts to start with. The scripts case1.rcv and case4.rcv provide comprehensive information about using the Recovery Manager to perform backup. These scripts are located in the $ORACLE_HOME/rdbms/demo directory on a UNIX system and the \ORANT\rdbms\demo folder on a Windows NT system.

case1.rcv

This script contains code for the following backup, restore, and recovery operations using Recovery Manager:

Print the script case1.rcv while reading this part
case1.rcv is a long script that shows how to write Recovery Manager scripts. Keep a printout of it handy while reading this section.

Keep the following in mind when you perform these functions:

No single backup piece is greater than 2GB.
Puts a maximum of 20 archived logs in any one backup set.
No more than 200 buffers are allowed to be read per file per second, which limits the effect the backup file scan has on online users and batch jobs.
A channel can have a maximum of only 32 files open at any one time.

Includes, at the most, six files in one backup set.

Making backups on disk with Recovery Manager
To make backup sets to disk with Recovery Manager, change the channel allocation commands to be type disk and modify the format clause to include the full path name the backup is to be written to. Otherwise, the backup files will typically be written to the dbs directory in Oracle home.

case4.rcv

This script has the code for taking a consistent backup of a database using the Recovery Manager. It performs the backup operations with the following considerations:

Performing an Offline (Cold) Backup

Cold backup or consistent backup of a file is the backup taken when the file is offline (not being accessed by the database instance, for instance). Thus, a file's cold backup is its image copy as it existed when the file was taken offline. You can take a partial or full cold backup off the database. The following factors determine whether a full or a partial cold backup is taken:

Enabling ARCHIVELOG mode
By default, the database is created in the NOARCHIVELOG mode. Refer to Chapter 12, "Understanding Oracle8 Backup Options," for steps required to alter the database to ARCHIVELOG mode.

Taking an Offline (Cold) Backup of a Tablespace

To make a cold backup of a tablespace (other than system tablespace data files), take the corresponding tablespace offline by issuing the following command from Server Manager:

You can't take the SYSTEM tablespace offline
To make an offline tablespace backup, the database needs to be shut down. You can't perform a cold backup of the SYSTEM tablespace while the database is running because Oracle requires that it always be available.

SQL> alter tablespace tablespace name offline ;

Use any of the OS utilities tar, dd, and cpio on UNIX and Backup on Windows NT to copy the corresponding data files. When the backup is completed, use the following command to put the tablespace back online:

SQL> alter tablespace tablespace_name online ;

Listing 13.1 lists a sample script that performs an offline backup of a tablespace on a UNIX system. This script does the following:

Download this code
You can download the TS_BACK-UP_ OFF.SH script from the Web at http://www.mcp.com/ info. You'll be asked to enter an ISBN; enter 0789716534 and then click the Search button to go to the Book Info page for Using Oracle8.


Listing 13.1  TS_BACKUP_OFF.SH-Performing an offline backup of a tablespace

     01:     # Script to perform offline backup of a tablespace
     02:     # on a UNIX system
     03:     #
     04:     #  Takes two input arguments
     05:     #            1. Name of the tar file
     06:     #            2. Name of the tablespace to be backed up.
     07:     #
     08:     #
     09:     # Listing database files belonging to the tablespace
     10:     #
     11:     #
     12:     sqlplus -s sys/<password> > tsfile_name.dat <<!
     13:     set heading off;
     14:     set pagesize 0
     15:     set linesize 2048
     16:     set feedback off;
     17:     REM
     18:     REM  Listing datafiles
     19:     REM
     20:     select file_name
     21:     from DBA_DATA_FILES
     22:     where tablespace_name='$2';
     23:     exit;
     24:     !
     25:     #
     26:     # Offline the tablespace to be backed up
     27:     #
     28:     sqlplus -s sys/change_on_install <<!
     29:     alter tablespace $2 offline ;
     30:     exit;
     31:     !
     32:     #
     33:     #
     34:     # Creating the tar file and
     35:     # backing up the datafiles
     36:     #
     37:     #
     38:     First_file='Yes'
     39:     for i in `cat tsfile_name.dat`
     40:     do
     41:             echo "----Backing up the datafile $i"
     42:        if [ $First_file = Yes ]
     43:        then
     44:         tar -cvf  $1 $i
     45:         First_file='No'
     46:        else
     47:         tar -rvf $1 $i
     48:        fi
     49:     done
     50:     #
     51:     sqlplus -s sys/change_on_install <<!
     52:     REM
     53:     REM Bringing the tablespace back online
     54:     REM
     55:     REM
     56:     alter tablespace $2 online ;
     57:     !
     58:     # end of the script tablespace backup script

Contents of offline tablespaces aren't available to users
When a tablespace is made offline, user objects residing in it aren't available for use until it's put back online. Applications trying to use these objects will signal errors while the tablespace remains offline.

Taking a tablespace offline
You can't take offline a table-space that contains an active rollback segment. First take the rollback segment offline, then the tablespace.

Making a Full Cold Backup

To perform a full offline database backup, the database must be shut down. Listing 13.2 shows a script that performs a cold backup of a database on a UNIX system. This script performs the following sequence of operations:


Listing 13.2  COLD_BACKUP.SH-Performing a full cold backup of a database

     01:     # Script to perform cold backup of a database
     02:     # on a UNIX system
     03:     #
     04:     # Uses environment variable ORACLE_SID to decide
     05:     #   which database to backup
     06:     # The database should be up and running.
     07:     #
     08:     # Connects to the database using sys account,
     09:     # to list name of all the
     10:     # Control files,
     11:     # online redo log files and
     12:     # datafiles.
     13:     #
     14:     # It also backs up the default INIT.ORA
     15:     #  file from $ORACLE_HOME/dbs directory
     16:     #
     17:     # The script does not backup archived redo logs.
     18:     # If you are running in the ARCHIVELOG mode
     19:     # then please be sure to back them up.
     20:     #
     21:     # Modify this script to shutdown all the instances
     22:     # accessing the database in an Oracle Parallel Server
     23:     # environment. It creates the tar file with the
     24:     # following naming convent|ion
     25:     #       bkp<ORACLE_SID><mmddhh>.tar (month, data, hour)
     26:     # Creates tar file in the directory specified
     27:     # by BACKUP_DIR as defined in the
     28:     # following line
     29:     ##
     30:     BACKUP_DIR=/home/oracle/backup ; export BACKUP_DIR
     31:     BKPTIME=`date '+%m%d%H'`
     32:     BKPFILE=$BACKUP_DIR/bkp$ORACLE_SID$BKPTIME.tar
     33:     #
     34:     # Creating database file list in the file file_name.dat
     35:     #
     36:     sqlplus -s sys/change_on_install > file_name.dat <<!
     37:     set heading off;
     38:     set pagesize 0
     39:     set linesize 2000
     40:     set feedback off;
     41:     REM
     42:     REM  Listing datafiles
     43:     REM
     44:     select name from V\$DATAFILE ;
     45:     REM
     46:     REM  Listing control files
     47:     REM
     48:     select name from V\$CONTROLFILE;
     49:     REM
     50:     REM  Listing online redo log files
     51:     REM
     52:     select member from V\$LOGFILE;
     53:     exit;
     54:     !
     55:     #
     56:     # Shutting down the database
     57:     # Using Shutdown normal.
     58:     #
     59:     svrmgrl <<!
     60:     connect internal ;
     61:     shutdown;
     62:     exit;
     63:     !
     64:     #
     65:     #
     66:     # Backing up the initialization parameter file
     67:     #
     68:     tar cvf $BKPFILE $ORACLE_HOME/dbs/init$ORACLE_SID.ora
     69:     #Backing up the files using the tar command
     70:     # to the file BKPFILE as defined above
     71:     #
     72:     for i in `cat file_name.dat`
     73:     do
     74:     echo "Backing up the datafile $i"
     75:         tar -rvf $BKPFILE $i
     76:     done
     77:     #
     78:     # Starting the database
     79:     # after backup
     80:     #
     81:     svrmgrl <<!
     82:     connect internal ;
     83:     startup;
     84:     exit;
     85:     !
     86:     #
     87:     exit ;
     88:     # end of the script cold_backup.sh

Shut down all instances in an OPS environment
In order to perform a cold backup of a database in an Oracle Parallel Server environment, you must shut down all instances accessing the database before copying the data files at the OS level.

Shutdown immediate disconnects all active users
Shutdown immediate disconnects users and rolls back all current uncommitted transactions. The data in uncommitted transactions is lost. If you don't want to forcefully disconnect the users for performing backup, use shutdown normal.

Pay attention to the following points when performing a cold backup:

Don't use Shutdown abort before a cold backup
If you shut down the database with the ABORT option, you should restart the database in RESTRICT mode and use Shutdown normal before copying the database files.

Performing an Online (Hot) Backup

A hot or online backup refers to the backup of a database's data files taken while the database is still open. Use online backup when business requirements don't permit the database to be unavailable for an interval long enough to perform the offline data files backup. To perform online backup of a data file, you need to put the corresponding tablespace in backup mode by using the following command:

SQL>alter tablespace tablespace_name begin backup;

Copy the file at the OS level after issuing this command. When the copy operation is completed, the tablespace should be returned to normal mode with the following command:

SQL>alter tablespace tablespace_name end backup;

Oracle copies entire block images of the modified data blocks on the first change to the blocks; this is done while the tablespace is in the backup mode. This increases the size of the redo log generated while the tablespace is in the backup mode. Follow these guidelines to minimize its impact on the performance:

View V$BACKUP
Use the dynamic performance view V$BACKUP to determine the back-up status of the data files at any time. The ACTIVE value in this column indicates that the file is marked as now being backed up.

What Is a Block Split?

An Oracle data block consists of multiple OS disk blocks. The Oracle DBWR process writes modified blocks to the disk through OS write calls, which write to the disk in chunks that are multiples of OS blocks. The same Oracle block may be read and written to the storage media at the same time. This simultaneous read/write operation causes what's known as an "Oracle block split."

Figure 13.1 shows three Oracle blocks. Each Oracle block consists of four OS blocks-a total of 12 OS blocks. Assume that time t1, t2, and t3 are in increasing order, and consider the following situation:

Figure 13.1 : Oracle blocks splits/fractures as DBWR writes the block while it's being copied at the OS level.

Clearly, the backup file contains the first half of Oracle Block 2 before the update, and the second half after the update. Thus, an inconsistent copy of the Oracle block is present in the backup file. To handle this problem, Oracle writes the complete image of the modified blocks in its redo log while the tablespace is in backup mode. During the recovery process before applying the changes to a block, Oracle copies the block image from its redo log to the disk-making the block consistent-and applies the remaining redo for the block.

Hot Backup Script

You can use the HOT_BACKUP.SH script shown in Listing 13.3 to perform a hot backup of a database on a UNIX system. This script backs up all data files in online mode. It also appends a backup copy of the control file and all archived redo logs present in the archive destination directory. Tailor it to suit your environment. This script calls another script, bkp_ts.sh (shown in Listing 13.4), which issues the necessary begin and end backup commands and copies all the data files belonging to a tablespace.


Listing 13.3  HOT_BACKUP.SH-Performing a hot backup of a database

     01:     # Script to perform hot backup of a database
     02:     # on a UNIX system.
     03:     #
     04:     # Uses environment variable ORACLE_SID to decide
     05:     #   which database to backup
     06:     # The database should be up and running.
     07:     #
     08:     # Connects to the database using sys account,
     09:     # to list name of all the
     10:     # tablespace and the corresponding datafiles.
     11:     #
     12:     # It also backs up the default INIT.ORA
     13:     #  file from $ORACLE_HOME/dbs directory
     14:     #
     15:     # The script  performs a log switch at the end and
     16:     # performs backup of all the archive logs present in
     17:     # the directory. It creates the tar file with the
     18:     # following naming convention
     19:     #       bkp<ORACLE_SID><mmddhh>.tar (month, data, hour)
     20:     # Creates tar file in the directory specified
     21:     # by BACKUP_DIR as defined in the
     22:     # following line
     23:     #
     24:     BACKUP_DIR=/home/oracle/backup ; export BACKUP_DIR
     25:     BKPTIME=`date '+%m%d%H'`
     26:     BKPFILE=$BACKUP_DIR/bkp$ORACLE_SID$BKPTIME.tar
     27:     #
     28:     # Listing the tablspaces to backup
     29:     #
     30:     sqlplus -s sys/<password> > tablespace_name.dat |<<!
     31:     set heading off;
     32:     set pagesize 0
     33:     set linesize 2048
     34:     set feedback off;
     35:     REM
     36:     REM  Listing all the tablespaces
     37:     REM
     38:     select tablespace_name from DBA_TABLESPACES
           where tablespace_name='SYSTEM';
     39:     exit;
     40:     !
     41:     #
     42:     #
     43:     #
     44:     # Backing up the initialization parameter file
     45:     #
     46:     tar cvf $BKPFILE $ORACLE_HOME/dbs/init$ORACLE_SID.ora
     46:     #Backing up the files using the tar command
     47:     # to the file BKPFILE as defined above
     48:     #
     49:     for ts in `cat tablespace_name.dat`
     50:     do
     51:             echo "Beginning backup of the tablespace $ts"
     52:             bkp_ts.sh  $BKPFILE $ts
     53:     done
     54:     #
     55:     # Backing up the control file to a backup file
     56:     #
     57:     sqlplus -s sys/change_on_install <<!
     58:     set pagesize 0
     59:     set linesize 2048
     60:     set feedback off;
     61:     REM
     62:     REM Backing up control file
     63:     REM
     64:     alter database backup controlfile to '$PWD/control.bkp';
     65:     exit;
     66:     !
     67:     #
     68:     #Appending the control file to tar file
     69:     #
     70:     tar -rvf $BKPFILE control.bkp
     71:     rm -f control.bkp
     72:     #
     73:     # Performing a log switch to archiving all the
     74:     # logs
     75:     sqlplus -s sys/change_on_install <<!
     76:     set linesize 2048
     77:     set feedback off;
     78:     alter system switch logfile;
     79:     alter system archive log all;
     80:     exit;
     81:     !
     82:     #
     83:     # Backing up the archive redo logs.
     84:     # This script assumes that the archive log destination
     85:     # is set to its default value of $ORACLE_HOME/dbs
     86:     # This might have to be changed to suit
     87:     # your environment
     88:     #
     89:     FILES=`ls $ORACLE_HOME/dbs/arch*`; export FILES
     90:     tar -rvf $BKPFILE $FILES
     91:     rm -f $FILES
     92:     #
     93:     # end of the script hot_backup.sh

Log switch after a hot backup
Always back up the redo logs generated during a hot backup by forcing a log switch and then copying the generated archived redo logs.

Listing 13.4  BKP_TS.SH-Performing a hot backup of a tablespace

     01:     # Script to perform hot backup of a tablespace
     02:     #
     03:     #
     04:     #  Takes two input arguments
     05:     #            1. Name of the tar file
     06:     #            2. Name of the tablespace to be backed up.
     07:     #
     08:     #
     09:     # Listing database files belonging to the tablespace
     10:     #
     11:     #
     12:     sqlplus -s sys/<password> > tsfile_name.dat <<!
     13:     set heading off;
     14:     set pagesize 0
     15:     set linesize 2048
     16:     set feedback off;
     17:     REM
     18:     REM  Listing datafiles
     19:     REM
     20:     select file_name
     21:     from DBA_DATA_FILES
     22:     where tablespace_name='$2';
     23:     exit;
     24:     !
     25:     #
     26:     # Putting the tablespace in hot backup mode
     27:     #
     28:     sqlplus -s sys/change_on_install <<!
     29:     alter tablespace $2 begin backup ;
     30:     exit;
     31:     !
     32:     #
     33:     #
     34:     # Appending the files to the file
     35:     # BKPFILE as defined above
     36:     #
     37:     for i in `cat tsfile_name.dat`
     38:     do
     39:             echo "----Backing up the datafile $i"
     40:         tar rvf $1 $i
     41:     done
     42:     #
     43:     sqlplus -s sys/change_on_install <<!
     44:     set heading off;
     45:     set pagesize 0
     46:     set linesize 2048
     47:     set feedback off;
     48:     REM
     49:     REM Changing the tablespace from hot backup mode
     50:     REM to normal mode
     51:     REM
     52:     alter tablespace $2 end backup ;
     53:     !
     54:     # end of the script tablespace backup script

Creating a Standby Database

If a database is used for mission-critical applications, you want to avoid downtime at any cost. Modern hardware and software techniques enable you to increase the availability of systems approaching 100 percent. If for any reason the primary production database can't be made available to the end users, however, maintaining a standby may fulfill a business's availability requirements.

The standby database can't be queried
A standby database is constantly in recovery mode and not available for querying the data unless you activate it. Once a standby database is activated, it's no longer available as a standby unless you recreate it as another standby database.

A standby database is a copy of the primary database that can be brought online with minimum delay when needed. This feature is part of Oracle's disaster recovery procedures. With this feature, you can maintain a hot standby copy of the production database, preferably at another remote site, which can be activated on short notice. When it's activated, this database will serve as the production database. You might need to start from the beginning to create a standby database for this database.

Create a standby database

  1. Perform an online or offline backup of the production database using the proper procedures. (If your system is mission-critical, however, you would most likely perform an online backup.) It's good practice to perform an online backup while setting up the standby database for the first time; this will give invaluable experience in recreating the standby database whenever the standby database is put into production use.
  2. Create a control file for the standby database by using the following command:
SQL>alter database create standby
controlfile as control_file_name;
    The following is a sample command session executing this command. Notice that the filename is included in quotation marks. Because the full path name isn't given for the file, the standby control file is created in the default location-the $ORACLE_HOME/dbs directory.
SVRMGR> alter database create standby
     2> controlfile as 'control.std';
Statement processed.
The standby control file
Standby control file contents are different from those in the original control file. You should not use a backup copy of the original control file generated by the backup control file command instead of this file.

  1. Archive the production database's current online log files with this command:
SQL> alter system archive log current ;
    This command forces a log switch and then archives the current redo log file group. The database must be open to issue this command. This command may not be required if you've performed an offline backup of the database in Step 1, as data files are checkpointed (synchronized) before an offline backup. If the database is open, issuing this command is important because it ensures consistency among the data files in Step 1 and the control file in Step 2.

  1. Transfer all files generated in Steps 1 through 3 to the system where you want to build the standby database.

The standby should be built on a similar system
The standby database must be built on the same hardware platform as that of the primary database. It's recommended there be similar architecture and software on both machines.

    Create the initialization parameter file for the standby database. It's highly desirable and also recommended to have the standby database's parameters similar to the primary database parameters, because the standby database will be used as the primary database after the failover. Keeping most parameters the same will help you avoid any surprises during its operation.

Table 13.2 lists important initialization parameters related to the standby database configuration.

Table 13.2  Parameters related to standby database
Parameter Name:
Description:
COMPATIBLEThis parameter must be the same on the primary and standby databases.
DB_FILESThis parameter and the MAXDATAFILES clause of the CREATE DATABASE or CREATE CONTROLFILE commands specifies the maximum number of data files for the database. Keep it the same, as the number of data files allowed/needed will be identical at both places.
CONTROL_FILESThis parameter specifies the name of the control files used for the database. Both databases should point to different files. Their names can be the same, as they're located on different machines.
DB_FILE_NAME_CONVERTThis parameter is set only on the standby database. Use this parameter only if the directory paths to the data files are different at both sites.
LOG_FILE_NAME_CONVERTThis parameter is the same as DB_FILE_NAME_ CONVERT, except that it applies to the online redo log members.

Maintaining a Standby Database

After transferring all data from the primary database to the host machine for the standby database, you need to keep the standby database closely synchronized with the primary database.

Keep the standby synchronized with the primary database

  1. Start the standby database instance in the nomount state with the following command:
svrmgrl> startup nomount ;
Automate the archive log transfer to the standby database
Archived redo logs generated at the primary site need to be applied to the hot standby on a regular basis. In a production environment, it's desirable to automate the process of transferring and applying the archived redo log to the standby database. Use the operating system's or Oracle's job-scheduling features to implement this.

  1. Mount the standby database in the standby exclusive mode by using the following command:
svrmgrl> alter database mount standby database;

  1. Prepare the standby database for recovery to apply the archived redo logs received from the primary database:
svrmgrl> recover [from location] standby database until cancel;

  1. Oracle will prompt the next log to be applied. Apply all the logs that have been generated so far and then cancel the recovery. If the archived redo logs to be applied are being made available in the directory specified by LOG_ARCHIVE_DEST, the FROM location clause doesn't need to be specified. Because it's a recurring process, you might want to automate it by using Oracle job queues or operating system job-scheduling features.

Using the Unrecoverable Option on the Primary Database

Unrecoverable data loads and the no-logging feature associated with tables and indexes that don't generate redo logs aren't propagated to the standby database. If you activate the standby database without paying attention to them, the affected target objects will be marked as corrupted. You can drop the affected tablespace at the standby database after the activation, or recreate it by using a fresh backup of the concerned tablespace at the primary site and transferring it over to the standby database before activation.

Adding and Deleting Data Files

If you add a data file at the primary database, you should make a copy of the added data file and add it to the standby database before applying the redo log that was generated while the file was added. This adds the new file to the standby control file and will allow the standby database's recovery to proceed as normal. A dropped data file at the primary site is also removed from the standby database control file when the corresponding redo log is applied.

Adding, Removing, and Clearing Redo Log Files

The standby database's online redo log files
Online redo log files for a standby database are initialized when the standby database is activated. You might consider using the alter database clear logfile command to initialize the online redo logs in advance; this saves time while activating the standby database.

Adding and removing online redo logs doesn't have any impact on the configuration of the standby database's redo log con-figuration.

Performing an incomplete recovery at the primary database and opening it with the resetlogs option will invalidate the standby database, as the resetlogs option initializes all the online redo logs and resets the log sequence number at the primary site.

Activating a Standby Database

The standby database is mounted and is in recovery mode. Issue the following command to activate it and make it available to the users:

svrmgr> alter database activate standby database;
Standby databases become normal databases after activation
As soon as you activate the standby database, it no longer remains a standby database. It becomes a normal database totally independent of its primary database. If needed, use it to recreate another standby data-base.

Be careful to apply all the archived and unarchived redo logs from the primary database before activating the standby database. Once you activate the standby database, all the transactions from the primary database that are present in the unapplied redo logs are lost.

Using Replication as a Failover

Oracle offers the following solutions for implementing highly available systems:

For information on the Oracle Failsafe option, available on Windows NT cluster systems, refer to the Oracle Fail Safe Concepts and Administration Guide from Oracle Corporation.

SEE ALSO
An overview of Oracle Parallel Server options,

Oracle's Replication option enables you to maintain the same data in multiple databases. By using replication, you can design applications that are available to end users all the time. If one database fails for any reason-including the unavailability of its host site due to natural catastrophe-the sites having the replicated database can be used by the application. If you're maintaining data at more than two locations, the application can access the data as long as one replicated site is functioning. The nice thing about this scheme is that it can be completely transparent to end users. If the database instance they were working on goes down, the application can connect the users to another replicated site immediately without the users knowing that. Look at this option's important features with respect to a fail-over strategy:

SEE ALSO
Information on advanced replication options,

Building a Failover Database with Replication

To use replication as a means providing increased availability, you can choose from the following options:

Setting Up Read-Only Failover Database with Snapshots

Set up snapshots to provide read-only access to data with the master site

  1. To set up snapshot replication, you need to decide which tables need to be replicated. If the application doesn't require all the tables at the failover site, you can save resources by not configuring snapshots for tables that aren't needed.

Refer to Oracle8's Server Replication Manual for complete details
Detailed instructions on setting up snapshots and replication is beyond the scope of this book. An overview of the tasks to be performed is provided; please refer to the Oracle8 Server Replication manual for more details and command syntax.
  1. Create the snapshot schema at the snapshot site and then establish database links to the master site.
  2. Create a snapshot log at the master site to allow fast refresh of the snapshots. If you don't create a snapshot log, every refresh will be a complete refresh (which may not be acceptable in most cases).
  3. Create a snapshot at the snapshot site. During this process, all the data from the master site is propagated to the snapshot site.
  4. Create snapshot groups and assign snapshots to them. This is the process where you define the snapshot update interval.

Setting Up Failover Database with Multimaster Replication

The steps required to set up multimaster replication are very similar to setting up snapshots.

Set up multimaster replication

  1. Set up the replication environment at the master definition site. This site includes creating the replication owner and granting necessary privileges to it, defining a database link to be used for replication, and scheduling jobs to replicate changes from one site to another at regular intervals.
  2. Decide which tables need to be replicated and who owns them at the master definition site. Create the schema and the tables to be replicated at the master definition site.
  3. Repeat the functions from Step 1 on the remote (master) site.
  4. Repeat the functions from Step 2 on the remote (master) site.
  5. Create a master replication group at the master definition site.
  6. Add replication objects to the replication group and generate the replication support for these objects.
  7. Start the replication process by enabling changes in the replication group from QUIESCED mode to NORMAL mode.

After the replication is set up, Oracle maintains the data at all participating sites. No additional steps are required to maintain the replicated data, except to resolve error conditions that might result from what's called the CONFLICTS. Refer to the Oracle8 Server Replication Manual for more details.


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