Chapter22 database security in dbms.pptx

Fundamentals of Database Systems
Seventh Edition
Chapter 22
Database Recovery
Techniques
Copyright © 2016, 2011, 2007 Pearson Education, Inc. All Rights Reserved

Introduction
• Recovery algorithms
• Recovery concepts
– Write-ahead logging
– In-place versus shadow updates
– Rollback
– Deferred update
– Immediate update
• Certain recovery techniques best used with specific
concurrency control methods

22.1 Recovery Concepts (1 of 9)
• Recovery process restores database to most recent
consistent state before time of failure
• Information kept in system log
• Typical recovery strategies
– Restore backed-up copy of database
▪Best in cases of extensive damage
– Identify any changes that may cause inconsistency
▪Best in cases of noncatastrophic failure
▪Some operations may require redo

Recovery Concepts (2 of 9)
• Deferred update techniques
– Do not physically update the database until after
transaction commits
– Undo is not needed; redo may be needed
• Immediate update techniques
– Database may be updated by some operations of a
transaction before it reaches commit point
– Operations also recorded in log
– Recovery still possible

• Undo and redo operations required to be idempotent
– Executing operations multiple times equivalent to
executing just once
– Entire recovery process should be idempotent
• Caching (buffering) of disk blocks
– DBMS cache: a collection of in-memory buffers
– Cache directory keeps track of which database items
are in the buffers

• Cache buffers replaced (flushed) to make space for new
items
• Dirty bit associated with each buffer in the cache
– Indicates whether the buffer has been modified
• Contents written back to disk before flush if dirty bit
equals one
• Pin-unpin bit
– Page is pinned if it cannot be written back to disk yet

• Main strategies
– In-place updating
▪Writes the buffer to the same original disk location
▪Overwrites old values of any changed data items
– Shadowing
▪Writes an updated buffer at a different disk
location, to maintain multiple versions of data items
▪Not typically used in practice
• Before-image: old value of data item
• After-image: new value of data item

• Write-ahead logging
– Ensure the before-image (BFIM) is recorded
– Appropriate log entry flushed to disk
– Necessary for UNDO operation if needed
• UNDO-type log entries
• REDO-type log entries

• Steal/no-steal and force/no-force
– Specify rules that govern when a page from the
database cache can be written to disk
• No-steal approach
– Cache buffer page updated by a transaction cannot
be written to disk before the transaction commits
• Steal approach
– Recovery protocol allows writing an updated buffer
before the transaction commits

• Force approach
– All pages updated by a transaction are immediately
written to disk before the transaction commits
– Otherwise, no-force
• Typical database systems employ a steal/no-force
strategy
– Avoids need for very large buffer space
– Reduces disk I/O operations for heavily updated
pages

• Write-ahead logging protocol for recovery algorithm
requiring both UNDO and REDO
– BFIM of an item cannot be overwritten by its after
image until all UNDO-type log entries have been
force-written to disk
– Commit operation of a transaction cannot be
completed until all REDO-type and UNDO-type log
records for that transaction have been force-written to
disk

Checkpoints in the System Log and Fuzzy
Checkpointing (1 of 2)
• Taking a checkpoint
– Suspend execution of all transactions temporarily
– Force-write all main memory buffers that have been
modified to disk
– Write a checkpoint record to the log, and force-write
the log to the disk
– Resume executing transactions
• DBMS recovery manager decides on checkpoint interval

Checkpoints in the System Log and Fuzzy
Checkpointing (2 of 2)
• Fuzzy checkpointing
– System can resume transaction processing after a
begin_checkpoint record is written to the log
– Previous checkpoint record maintained until
end_checkpoint record is written

Transaction Rollback
• Transaction failure after update but before commit
– Necessary to roll back the transaction
– Old data values restored using undo-type log entries
• Cascading rollback
– If transaction T is rolled back, any transaction S that
has read value of item written by T must also be rolled
back
– Almost all recovery mechanisms designed to avoid
this

Figure 22.1 Illustrating Cascading Rollback (A
Process That Never Occurs in Strict or
Cascadeless Schedules)
(a) The read and write operations of three transactions (b)
System log at point of crash (c) Operations before the crash

Transactions That Do Not Affect the
Database
• Example actions: generating and printing messages and
reports
• If transaction fails before completion, may not want user
to get these reports
– Reports should be generated only after transaction
reaches commit point
• Commands that generate reports issued as batch jobs
executed only after transaction reaches commit point
– Batch jobs canceled if transaction fails

22.2 NO-UNDO/REDO Recovery Based on
Deferred Update (1 of 3)
• Deferred update concept
– Postpone updates to the database on disk until the
transaction completes successfully and reaches its
commit point
– Redo-type log entries are needed
– Undo-type log entries not necessary
– Can only be used for short transactions and
transactions that change few items
▪Buffer space an issue with longer transactions

NO-UNDO/REDO Recovery Based on
• Deferred update protocol
– Transaction cannot change the database on disk until
it reaches its commit point
▪All buffers changed by the transaction must be
pinned until the transaction commits (no-steal
policy)
– Transaction does not reach its commit point until all
its REDO-type log entries are recorded in log and log
buffer is force-written to disk

NO-UNDO/REDO Recovery Based on
Figure 22.2 An example of a recovery timeline to illustrate the effect of
checkpointing

22.3 Recovery Techniques Based on
Immediate Update
• Database can be updated immediately
– No need to wait for transaction to reach commit point
– Not a requirement that every update be immediate
• UNDO-type log entries must be stored
• Recovery algorithms
– UNDO/NO-REDO (steal/force strategy)
– UNDO/REDO (steal/no-force strategy)

Figure 22.3 An Example of Recovery Using
Deferred Update with Concurrent Transactions
(a) The READ and WRITE operations of four transactions (b) System
log at the point of crash

22.4 Shadow Paging (1 of 3)
• No log required in a single-user environment
– Log may be needed in a multiuser environment for the
concurrency control method
• Shadow paging considers disk to be made of n fixed-size
disk pages
– Directory with n entries is constructed
– When transaction begins executing, directory copied
into shadow directory to save while current directory
is being used
– Shadow directory is never modified

Shadow Paging (2 of 3)
• New copy of the modified page created and stored
elsewhere
– Current directory modified to point to new disk block
– Shadow directory still points to old disk block
• Failure recovery
– Discard current directory
– Free modified database pages
– NO-UNDO/NO-REDO technique

Shadow Paging (3 of 3)
Figure 22.4 An example of shadow paging
5
The directory is similar to the page table maintained by the operating system
for each process.

22.5 The ARIES Recovery Algorithm (1 of 3)
• Used in many IBM relational database products
• Uses a steal/no-force approach for writing
• Concepts
– Write-ahead logging
– Repeating history during redo
▪Retrace all database system actions prior to crash
to reconstruct database state when crash occurred
– Logging changes during undo
▪Prevents ARIES from repeating completed undo
operations if failure occurs during recovery

The ARIES Recovery Algorithm (2 of 3)
• Analysis step
– Identifies dirty (updated) pages in the buffer and set of
transactions active at the time of crash
– Determines appropriate start point in the log for the
REDO operation
• REDO
– Reapplies updates from the log to the database
– Only necessary REDO operations are applied

The ARIES Recovery Algorithm (3 of 3)
• UNDO
– Log is scanned backward
– Operations of transactions that were active at the
time of the crash are undone in reverse order
• Every log record has associated log sequence number
(LSN)
– Indicates address of log record on disk
– Corresponds to a specific change of some
transaction

ARIES Recovery Example
Figure 22.5 An example of recovery in ARIES (a) The log at point of crash (b)
The Transaction and Dirty Page Tables at time of checkpoint (c) The
Transaction and Dirty Page Tables after the analysis phase

22.6 Recovery in Multidatabase Systems (1 of 2)
• Two-level recovery mechanism
• Global recovery manager (coordinator) needed to
maintain recovery information
• Coordinator follows two-phase commit protocol
– Phase 1: Prepare for commit message
▪Ready to commit or cannot commit signal returned
– Phase 2: Issue commit signal
• Either all participating databases commit the effect of the
transaction or none of them do

Recovery in Multidatabase Systems (2 of 2)
• Always possible to recover to a state where either the
transaction is committed or it is rolled back
• Failure during phase 1 requires rollback
• Failure during phase 2 means successful transaction can
recover and commit

22.7 Database Backup and Recovery from
Catastrophic Failures (1 of 2)
• Database backup
– Entire database and log periodically copied onto
inexpensive storage medium
– Latest backup copy can be reloaded from disk in case
of catastrophic failure
• Backups often moved to physically separate locations
– Subterranean storage vaults

Database Backup and Recovery from
Catastrophic Failures (2 of 2)
• Backup system log at more frequent intervals and copy to
magnetic tape
– System log smaller than database
▪Can be backed up more frequently
– Benefit: users do not lose all transactions since last
database backup

22.8 Summary
• Main goal of recovery
– Ensure atomicity property of a transaction
• Caching
• In-place updating versus shadowing
• Before and after images of data items
• UNDO and REDO operations
• Deferred versus immediate update
• Shadow paging
• Catastrophic failure recovery

Copyright

Chapter22 database security in dbms.pptx

More Related Content

Similar to Chapter22 database security in dbms.pptx (20)

More from ubaidullah75790 (20)

Recently uploaded (20)

Chapter22 database security in dbms.pptx

Editor's Notes