2. DATABASE RECOVERY
1 Purpose of Database Recovery
• To bring the database into the last consistent
state, which existed prior to the failure.
• To preserve transaction properties (Atomicity,
Consistency, Isolation and Durability).
• Example:
• If the system crashes before a fund transfer
transaction completes its execution, then either
one or both accounts may have incorrect value.
Thus, the database must be restored to the state
before the transaction modified any of the
accounts.
3. DATABASE RECOVERY
2 Types of Failure
• The database may become unavailable for use due to
• Transaction failure: Transactions may fail because of
incorrect input, deadlock, incorrect synchronization.
• System failure: System may fail because of addressing error,
application error, operating system fault, RAM failure, etc.
• Media failure: Disk head crash, power disruption, etc.
4. DATABASE RECOVERY
T ID Back P Next P Operation Data item BFIM AFIM
T1 0 1
T1 1 4
T2 0 8
T1 2 5
T1 4 7
T3 0 9
T1 5 nil
Begin
Write
W
R
R
End
Begin
X
Y
M
N
X = 200
Y = 100
M = 200
N = 400
X = 100
Y = 50
M = 200
N = 400
3 Transaction Log
• For recovery from any type of failure data values prior to
modification (BFIM - BeFore Image) and the new value after
modification (AFIM – AFter Image) are required.
• These values and other information is stored in a sequential file
calledTransaction log. A sample log is given below. Back P and
Next P point to the previous and next log records of the same
transaction.
5. DATABASE RECOVERY
4 Data Update
• Immediate Update: As soon as a data item is modified in
cache, the disk copy is updated.
• Deferred Update: All modified data items in the cache is
written either after a transaction ends its execution or after a
fixed number of transactions have completed their execution.
• Shadow update: The modified version of a data item does not
overwrite its disk copy but is written at a separate disk location.
• In-place update:The disk version of the data item is
overwritten by the cache version.
6. DATABASE RECOVERY
5 Data Caching
• Data items to be modified are first stored into database cache by the
Cache Manager (CM) and after modification they are flushed
(written) to the disk.
• The flushing is controlled by Modified and Pin-Unpin bits.
• Pin-Unpin: Instructs the operating system not to flush the
data item.
• Modified: Indicates the AFIM of the data item.
7. DATABASE RECOVERY
6 Transaction Roll-back (Undo) and Roll-Forward
(Redo)
• To maintain atomicity, a transaction’s operations are redone or
undone.
• Undo: Restore all BFIMs on to disk (Remove all AFIMs).
• Redo: Restore all AFIMs on to disk.
• Database recovery is achieved either by performing only Undos or
only Redos or by a combination of the two.These operations are
recorded in the log as they happen.
11. DATABASE RECOVERY
Write-Ahead Logging
• When in-place update (immediate or deferred) is used then
log is necessary for recovery and it must be available to
recovery manager. This is achieved by Write-Ahead
Logging (WAL) protocol. WAL states that
• For Undo: Before a data item’s AFIM is flushed to the database
disk (overwriting the BFIM) its BFIM must be written to the log
and the log must be saved on a stable store (log disk).
• For Redo: Before a transaction executes its commit operation,
all its AFIMs must be written to the log and the log must be
saved on a stable store.
12. DATABASE RECOVERY
7 Checkpointing
• Time to time (randomly or under some criteria) the database
flushes its buffer to database disk to minimize the task of
recovery. The following steps defines a checkpoint operation:
1. Suspend execution of transactions temporarily.
2. Force write modified buffer data to disk.
3. Write a [checkpoint] record to the log, save the log to disk.
4. Resume normal transaction execution.
• During recovery redo or undo is required to transactions
appearing after [checkpoint] record.
13. DATABASE RECOVERY
Steal/No-Steal and Force/No-Force
• Possible ways for flushing database cache to database
disk:
1. Steal: Cache can be flushed before transaction commits.
2. No-Steal: Cache cannot be flushed before transaction
commit.
3. Force: Cache is immediately flushed (forced) to disk.
4. No-Force: Cache is deferred until transaction commits
• These give rise to four different ways for handling
recovery:
• Steal/No-Force (Undo/Redo)
• Steal/Force (Undo/No-redo)
• No-Steal/No-Force (Redo/No-undo)
• No-Steal/Force (No-undo/No-redo)
14. DATABASE RECOVERY
8 Recovery Scheme
• Deferred Update (No Undo/Redo)
• The data update goes as follows:
• A set of transactions records their updates in the log.
• At commit point under WAL scheme these updates are saved on
database disk.
• After reboot from a failure the log is used to redo all the
transactions affected by this failure. No undo is required because no
AFIM is flushed to the disk before a transaction commits.
15. DATABASE RECOVERY
• Deferred Update in a single-user system
There is no concurrent data sharing in a single user system.
The data update goes as follows:
• A set of transactions records their updates in the log.
• At commit point under WAL scheme these updates are
saved on database disk.
• After reboot from a failure the log is used to redo all the
transactions affected by this failure. No undo is required
because no AFIM is flushed to the disk before a transaction
commits.
17. DATABASE RECOVERY
Deferred Update with concurrent users
• This environment requires some concurrency control
mechanism to guarantee isolation property of transactions. In
a system recovery transactions which were recorded in the log
after the last checkpoint were redone. The recovery manager
may scan some of the transactions recorded before the
checkpoint to get the AFIMs.
19. DATABASE RECOVERY
Deferred Update with concurrent users
• Two tables are required for implementing this protocol:
• Active table: All active transactions are entered in this
table.
• Commit table:Transactions to be committed are
entered in this table.
• During recovery, all transactions of the commit table are
redone and all transactions of active tables are ignored
since none of their AFIMs reached the database. It is
possible that a commit table transaction may be redone
twice but this does not create any inconsistency because of
a redone is “idempotent”, that is, one redone for an AFIM
is equivalent to multiple redone for the same AFIM.
20. DATABASE RECOVERY
Recovery Techniques Based on Immediate Update
• Undo/No-redo Algorithm
• In this algorithm AFIMs of a transaction are flushed to the database
disk under WAL before it commits.
• For this reason the recovery manager undoes all transactions during
recovery.
• No transaction is redone.
• It is possible that a transaction might have completed execution and
ready to commit but this transaction is also undone.
21. DATABASE RECOVERY
Recovery Techniques Based on Immediate Update
• Undo/Redo Algorithm (Single-user environment)
• Recovery schemes of this category apply undo and also redo
for recovery.
• In a single-user environment no concurrency control is
required but a log is maintained under WAL.
• Note that at any time there will be one transaction in the
system and it will be either in the commit table or in the active
table.
• The recovery manager performs:
• Undo of a transaction if it is in the active table.
• Redo of a transaction if it is in the commit table.
22. DATABASE RECOVERY
Recovery Techniques Based on Immediate Update
• Undo/Redo Algorithm (Concurrent execution)
• Recovery schemes of this category applies undo and also
redo to recover the database from failure.
• In concurrent execution environment a concurrency
control is required and log is maintained underWAL.
• Commit table records transactions to be committed and
active table records active transactions. To minimize the
work of the recovery manager checkpointing is used.
• The recovery performs:
• Undo of a transaction if it is in the active table.
• Redo of a transaction if it is in the commit table.
23. DATABASE RECOVERY
X Y
Database
X' Y'
Shadow Paging
• The AFIM does not overwrite its BFIM but recorded at another place
on the disk. Thus, at any time a data item has AFIM and BFIM
(Shadow copy of the data item) at two different places on the disk.
X and Y: Shadow copies of data items
X' and Y': Current copies of data items
24. DATABASE RECOVERY
Shadow Paging
• To manage access of data items by concurrent transactions two
directories (current and shadow) are used.
• The directory arrangement is illustrated below. Here a page is a
data item.
![DATABASE RECOVERY
7 Checkpointing
• Time to time (randomly or under some criteria) the database
flushes its buffer to database disk to minimize the task of
recovery. The following steps defines a checkpoint operation:
1. Suspend execution of transactions temporarily.
2. Force write modified buffer data to disk.
3. Write a [checkpoint] record to the log, save the log to disk.
4. Resume normal transaction execution.
• During recovery redo or undo is required to transactions
appearing after [checkpoint] record.](https://image.slidesharecdn.com/unit-vrecoveryppts-250303020857-efdae9bd/85/Unit-V-Recovery-software-wngineering-should-learn-software-12-320.jpg)
