What is Database Backup? The 3 Important Recovery Techniques from transaction failures

What is Database Backup?
• Copying and archiving of computer data so it may be used to restore the original after a
data loss event.
• Purpose is to recover data after it is lost from corruption or deletion.
• Second purpose is to recover data from an earlier time.
What is Database recovery techniques
Recovery techniques are used to ensure database consistency and transaction automaticity and
durability despite failures.
Why recovery is needed? (What causes a Transaction to fail?)
There are several reasons that could cause a transaction to fail in the middle of execution
1. A computer failure (system crash): A hardware or software error occurs in the
computer system during transaction execution. If the hardware crashes, the contents of
the computer’s internal memory may be lost.
2. A transaction or system error : Some operation in the transaction may cause it to fail,
such as integer overflow or division by zero. Transaction failure may also occur because
of a logical programming error. In addition, the user may interrupt the transaction during
its execution.
3. Local errors or exception conditions detected by the transaction.
4. Disk failure: Some disk blocks may lose their data because of a disk read/write head
crash. This may happen during a read or a write operation of the transaction.
5. Physical problems and catastrophes: This refers to an endless list of problems that
includes power or air-conditioning failure, fire, theft, sabotage, overwriting disks or tapes
by mistake, and mounting of a wrong tape by the operator.
There are many different approaches to recover a database:
Manual reprocessing & a variety of automated recovery techniques.
1. Manual Reprocessing
The database is periodically backed up (a database save) and all transactions applied since the
last save are recorded. If the system crashes, the latest database save is restored and all of the
transactions are re-applied (by users) to bring the database back up to the point just before the
crash.
Limitations:
• Time required to reapply transactions
• Transactions might have other (physical) potential failures
• Reapplying concurrent transactions is not straight forward.
2. Automated Recovery
There are several types of automated recovery techniques including: Deferred update,
Immediate update, Shadow paging, etc.
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A transaction can be in one of the following states:
• Active - when the transaction just begins
• Partially Committed - after the last operation has completed (but before the commit
point is reached)
• Failed - Normal operation is prevented (e.g., in the event of a system crash)
• Aborted - Transaction is rolled back. That is, all of its effects are undone
• Committed - Transaction completes all operations and moved the database to the next
consistent state
The System Log
To be able to recover from failures that affect transactions, the system maintains a log to keep
track of all transactions that affect the values of database items. Each transaction writes the
following information to the log:
• Start(T) - the fact that transaction T has started
• Write(T, X, old_value, new_value) - the fact that transaction T has written to item X
with the new_value. old_value is also maintained.
• Read(T,X)-the fact that transaction T has read data item X either:
Commit(T)-transaction T committed or Abort(T) - transaction T was aborted
Recovery techniques use the following operations:
UNDO (Backward Recovery): Similar to rollback except that it applies to a single operation
rather than to a whole transaction.
REDO (Forward Recovery): This specifies that certain transaction operations must be redone
to ensure that all the operations of a committed transaction have been applied successfully to the
database.
The 3 Important Recovery Techniques from transaction failures:
1. Deferred Update Recovery: Also called NO-UNDO, REDO Technique
This technique defers or postpones any actual updates to the database until the transaction
reaches it commit point.
• During transaction execution, the updates are written to the log file.
• After the transaction reaches it commit point, the log file is force-written to disk, then
the updates are recorded in the database.
• If the transaction fails before reaching its commit point, there is no need to UNDO
any operations because the transaction has not affected the database on disk in any
way.
• REDO may be necessary.
A typical deferred update protocol uses the following rules:
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1. A transaction cannot change the database on disk until it reaches its commit point.
2. A transaction does not reach its commit point until all its update operations are
recorded in the log file and the log file is force-written to disk.
3. Because the database is never updated on disk until after the transactions commits,
there is never the need to UNDO any operations. That’s why a recovery techniques based
on deferred update are therefore known as NO UNDO, REDO techniques.
Example:
T1: Ra Rd Wd C
T2: Rb Wb Rd Wd C
T3: Ra Wa Rc Wc C
T4: Rb Wb Ra Wa C
Log file:
Start(T1)
Write(T1, d, old, new)
Commit(T1)
Start(T4)
Write(T4, b, old, new)
Write(T4, a, old, new)
Commit(T4)
Start(T2)
Write(T2, b, old, new)
Start(T3)
Write(T3, a, old, new)
s y s t e m c r a s h
Since T1 and T4 committed, their changes were written to disk. However, T2 and T3 did not
commit, hence their changes were not written to disk. To recover, we simply ignore those
transactions that did not commit.
Advantages:
• Recovery is made easier.
• Any transaction that reached the commit point (from the log) has its writes applied to the
database.
• All other transactions are ignored.
• Cascading rollback does not occur because no other transaction sees the work of another
until it is committed.
Disadvantages:
Concurrency is limited: Must employ Strict 2PL which limits concurrency.
2. Immediate Update Recovery: also called (UNDO, NO-REDO)
• The database is physically updated before the transaction reaches to its commit point.
• UNDO may be necessary when a transaction fails.
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• REDO is not needed.
A typical deferred update protocol uses the following rules:
1. In immediate update techniques, although the database can be updated immediately,
update operations must be recorded in the log (on disk) before being applied to the
database.
2. There is never a need to REDO any operations of committed transactions (this is
called the UNDO/NO-REDO recovery algorithm).
3. The effect of all active transactions at the time of failure must be undone.
Advantages:
Immediate update allows higher concurrency because transactions write continuously to the
database rather than waiting until the commit point.
Disadvantages:
Step 2 above can lead to cascading rollbacks - time consuming and may be problematic.
3. Shadow paging
• Is a technique for providing automaticity and durability properties related to transaction
control.
• This technique does not require the use of a log in a single-user environment. In a
multiuser environment, a log may be needed for the concurrency control method.
• Shadow paging considers the database to be made up of a number of fixed size disk pages
(or disk blocks)—say, n—for recovery purposes. A directory(page table) with n entries
is constructed, where the ith
entry points to the ith
database page on disk.
• The idea is to maintain 2 directories (page tables) during the lifetime of a transaction,
Current directory & the Shadow directory.
• The directory is kept in main memory if it is not too large, and all references—reads or
writes—to database pages on disk go through it.
• Initially both the directories are identical. Only current directory is used for data item
accesses during execution of the transaction.
• Whenever any page is about to be written for the first time-a copy of this page is made on
to an unused page.
• When a transaction begins executing, the current directory—whose entries point to the
most recent or current database pages on disk—is copied into a shadow directory. The
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shadow directory is then saved on disk while the current directory is used by the
transaction.
The figure below illustrates the use of Shadow paging techniques:
Advantages:
• The overhead of maintaining the transaction log file is eliminated.
• Since there is no need for undo or redo operations, recovery is significantly fast.
Disadvantages:
• Copying the entire page table is very expensive.
• Complex storage management strategies.
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What is Database Backup? The 3 Important Recovery Techniques from transaction failures

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