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

 A DBMS COPY utility that produces a backup copy (save) of the entire database or a subset of the database  Periodic backup (e.g. nightly, weekly)  Backups stored in secure, off-site location  Backup copy-used to restore the database  Cold backup –database is shut down during backup  Hot backup –selected portion is shut down and backed up at a given time  Incremental backups: record changes made since the last full backup

 Audit trail of transactions and database updates/changes  In the event of failure: consistent database state can be reestablished using the information in the journals together with the most recent complete backup  Two basic journals or logs:  Transaction log–record of essential data for each transaction processed against the database  Transaction code, action, time, terminal no/user ID, input data values, tables/records accessed & modified and the old & new field values.  Database change log–images of updated data  Before-image–copy of a record before modification  After-image–copy of a record after modification

 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

 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.

 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)

 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 Recovery and Restart Procedures

 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

 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

 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  Example: transfer 100 from account for cust A to cust B  Program reads the record for customer A and subtracts 100 from the acc balance  Program reads the record for customer B and adds 100 to the acc 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. Recovery and Restart Procedures

14 Recovery and Restart Procedures

 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.  Much faster than Restore/Rerun:  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

16 Recovery and Restart Procedures

 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

 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

 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  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

 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 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  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