database recovery

DATABASE RECOVERY

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: not only the database files but also related database

objects (repository, database indexes, source libraries, etc) 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

backup Differential backups: record changes made since the last

full/normal backup the differences since the last full backup.

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

Back-up Facilities

Source: http://www.qadit.com/blog/?p=283

Source: http://kb.acronis.com/content/1536

Journalizing Facilities Audit trail of transactions and database 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

PRODUCES AN AUDIT TRAIL

Database audit trail

FROM THE BACKUP AND LOGS, DATABASES CAN BE RESTORED IN CASE OF DAMAGE OR LOSS

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

This allows recovery manager to resume processing from short period, instead of repeating entire day

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 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


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


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 processingRecovery 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 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.


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BASIC RECOVERY TECHNIQUESROLLBACK


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.

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


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BASIC RECOVERY TECHNIQUESROLLFORWARD


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


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..

UPDATEINSERT

.

.COMMIT


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

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 (New) 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 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

THE END

database recovery

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