Why Backup Data?  Backing up data is vital for businesses Lost information can cause major crisis or lead to business failure  Common Problems System outage ○ Power and hardware failure Transaction failure ○ Users may inadvertently corrupt the database Media Failure ○ Disk drive becomes unusable Disaster ○ Database facility damaged by fire, flooding or other catastrophe

Database Recovery  Mechanism for restoring a database quickly and accurately after loss or damage  RESPONSIBILITY OF ?????  Recovery facilities: Backup Facilities Journalizing Facilities Checkpoint Facility Recovery Manager

Back-up Facilities  A DBMS COPY utility that produces a backup copy (save) of the entire database or a subset of the database  Backup: user data, catalogs and all control files (buffer pool files, table space file, database configuration file)  Periodic backup (e.g. nightly, weekly)  Backups stored in secure, off-site location  Backup copy-used to restore the database

Back-up Facilities  Cold backup / offline backup (DB2)– database is shut down during backup  Hot backup / online backup (DB2) – selected portion is shut down and backed up at a given time  Incremental backups: record changes made since the last backup  Differential backups: record changes made since the last full/normal backup  the differences since the last full backup.

Database backup syntax (DB2) BACKUP DATABASE [ONLINE] [TO ]  Offline backup example BACKUP DATABASE sample TO C:\backups  Online backup example BACKUP DATABASE sample ONLINE TO C:\backups

Database backups – File naming convention

Table space backup  Enable user to backup a subset of database  Multiple table spaces can be specified  Database must be using archival logging  Table space backup can run in both online and offline backup  Table space can be restored from either database backup or table space backup  Use keyword TABLESPACE to specify table spaces

Database Backup  In DB2, backup can now be automatically done and/or compressed  To configure automatic backup: Graphical user interface tool Command line interface Stored procedure

Back-up Facilities  Database downtime can be very expensive  The lost revenue needs to be balanced against the cost of additional technology, primarily disk storage, to achieve a desired level of availability  To achieve: some DBMS automatically make backup copies in real time.  Stored in on separate disk drives

Journalizing Facilities  Keep track of changes made to database objects and their data using audit trail / log  In the event of failure: consistent database state can be reestablished using the information in the journals together with the most recent complete backup  Can be stored in files or raw devices  Logging is always ON for regular tables in DB2 It's possible to mark some tables or columns as NOT LOGGED It's possible to declare and use USER temporary tables which may not be logged

Database logging

Types of logs  Types of logs – based on log file allocation: Primary logs are PRE-ALLOCATED Secondary logs are ALLOCATED as needed (costly)  For day to day operations, ensure that you stay within your primary log allocation  Types of logs – based on information stored in logs: Active logs: ○ Information has not been externalized (Not on the tablespace disk) Archive logs ○ All information externalized

Types of logging  Circular logging For non-production systems Logs that become archived, can be overwritten What if information externalized to the table space was wrong? (Human error). No logs to redo things!  Archival logging For Production systems No logs are deleted. ○ Some are stored online (with active logs), others offline in an external media

Checkpoint Facilities  A facility by which the DBMS periodically refuses to accept new transactions. The system is in a quiet state and the database and transaction logs are synchronized  All transactions in progress are completed and journal files are brought up-to-date  DBMS writes a special record (checkpoint record) to the log file: snapshot of the state of the database  Checkpoint record contains information necessary to restart the system  Any dirty data blocks (pages of memory that contain changes that have not yet been written out to disk) are written from memory to disk storage  Automatically or response to commands in user application programs

Recovery Manager  A module of the DBMS that restores the database to a correct condition when a failure occurs and then resumes processing user requests.  Type of restart used depends on the nature of failure.

Recovery and Restart Procedures  Disk Mirroring–switch between identical copies of databases  Restore/Rerun–reprocess transactions against the backup  Transaction Integrity–commit or abort all transaction changes  Backward Recovery (Rollback)–apply before images  Forward Recovery (Roll Forward)–apply after images (preferable to restore/rerun)

Disk Mirroring Recovery and Restart Procedures

Disk Mirroring  Database must be mirrored  switch to an existing copy of the database  2 copies of the database must be kept & updated simultaneously  Media failure occurs: processing switch to the duplicate copy  Allows fastest recovery  Hot-swappable: damaged disk can be rebuilt from mirrored disk with no disruption in service to user Recovery and Restart Procedures

Restore/Rerun  Involves reprocessing the day’s transactions (up to the point of failure) against the backup copy of the database  Database is shut down  The most recent copy of the database /file to be recovered is mounted  All transactions that have occurred since that copy (stored on the transaction log) are rerun Recovery and Restart Procedures

Restore/Rerun  Advantage:  Simplicity  DBMS does not need to create a database change journal & no special restart procedures required  Disadvantages:  Time to reprocess transactions may be prohibitive  Processing of new transactions delayed until recovery completed  Sequencing of transactions will often be different from when they were originally processed: may lead to different results.  Original Run: customer deposit may be posted before withdrawal  Rerun: Withdrawal transaction may be attempted first.  Last resort in database processing Recovery and Restart Procedures

DB2 Restore Utility  Restore utility is the complement of backup utility  Restores database or table space from a previously taken backup  TAKEN AT – specify the time stamp of the database backup image. Backup image timestamp is displayed after successful completion of a backup  Without prompting – overrides any warnings

Example DB2 Restore SAMPLE.0.DB2INST.NODE0000.CATN RESTORE DATABASE dbalias FROM TAKEN AT

Transaction Integrity  Integrity of transactions: DB is updated by processing transactions that results in changes to one or more DB records  When processing transactions, DBMS must ensure that the transactions follow four well-accepted properties – ACID  Atomic  Consistent  Isolated  Durable Recovery and Restart Procedures

Transaction Integrity  Atomic  Transaction cannot be subdivided  Once transaction is processed – changes are committed  Transaction fails - aborted  Consistent  Constraints that are true before the transaction must be true after  Isolated  Changes to DB not revealed to users until transaction is committed  Durable  Changes are permanent – once committed no failure can reverse the effect of the transaction

Transaction Integrity  To maintain transaction integrity – DBMS must provide facilities for the user or application program to define transaction boundaries – logical beginning and end of transaction. BEGIN TRANSACTION. UPDATE INSERT. COMMIT Recovery and Restart Procedures

Backward Recovery (Rollback)  DBMS backs out of or undo unwanted changes to the DB – before images captured  Reverse the changes made by transactions that have aborted or terminated abnormally Recovery and Restart Procedures

Backward Recovery (Rollback)  Example: transfer 100 from CUSOMER A account to CUSTOMER B account  Program reads the record for customer A and subtracts 100 from the account balance  Program reads the record for customer B and adds 100 to the account balance.  Program writes the updated record for A to the dbase.  In attempting to write the record for B, program encounters an error condition and cannot write the record.  An UNDO command – recovery manager to apply the before image for record A to restore acc balance to its original value.

Forward Recovery (Roll Forward)  A technique that starts with an earlier copy of the database. After images are applied to the database and the database is quickly moved forward to a later state.  The Rollforward is redoing the changes made by a transaction that is after the committed transaction and to over-write the changed value once again to ensure the consistency.  The time consuming logic of reprocessing each transaction does not have to be repeated  Only the most recent after-images need to be applied. DB record may have series of after image – most recent (good) after image is required for rollback Recovery and Restart Procedures

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Difference in summary:  Rollback :- Undoing the changes made by a transaction before it commits or to cancel any changes to a database made during the current transaction  RollForward :- Re-doing the changes made by a transaction after it commits or to overwrite the changed value again to ensure consistency

Database Failure Responses  Aborted transactions  Preferred recovery: rollback  Alternative: Rollforward to state just prior to abort  Incorrect data  Preferred recovery: rollback  Alternative 1: rerun transactions not including inaccurate data updates  Alternative 2: compensating transactions – human intervention

Database Failure Responses  System failure (database intact)  Preferred recovery: switch to duplicate database  Alternative 1: rollback  Alternative 2: restart from checkpoint  Database destruction  Preferred recovery: switch to duplicate database  Alternative 1: rollforward  Alternative 2: reprocess transactions

Disaster Recovery

 Contingency plans to cater for disasters – destroy/damage data center  Natural disasters  Planning for DR  Develop a detailed DR plan  Schedule regular test of plan  Choose multi-disciplinary team to carry out plan  Fast backup data center – off site location  Send back up copies to backup data center

Contingency Plan  Contingency plan is established to deal with unusual events that are not part of the normal daily routine  Contingency plans detail the response necessary to deal with the types of event that may occur

Contingency Plan  A contingency plan should include :  who the key personnel are and how they can be contacted  if the key personnel are unavailable, a list of alternative personnel and how they can be contacted  who decides that a contingency exists and how that is decided  the technical requirements of transferring operations elsewhere  the operational requirements of transferring operations elsewhere  any outside contacts who may help  whether any insurance exists to cover the situation