Advanced Database Technologies
Lecture 6: Transactions and
Database Recovery
Content
 What is a Transaction?
 ACID properties
 Transaction Processing
 Database Recovery
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Lecture 6: Transaction Processing and Recovery
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1. What is a Transaction?
Definition
 The sequence of a logically linked actions that access a shared
database, usually on behalf of on-line users
Examples
 Airlines operation: Reserve an airline seat. Buy an airline
ticket. Assemble cabin crew. Fly.
 ATM Cash operation: Check credentials. Check money.
Withdraw amount. Pay amount.
 Credit card sale: Log on the card. Verify credit card details.
Check money. Handle. Issue withdrawal.
 Internet sale: Request an item from an on-line catalogue.
Check availability. Provide credit card details. Check details.
Issue order. Dispatch. Issue withdrawal.
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Origin and Needs for Transactions in DB
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Lecture 6: Transaction Processing and Recovery
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Automated Teller Machines (ATM)
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2. A.C.I.D. properties

Transactions have 4 main properties
 Atomicity - all or nothing
 Consistency - preserve database integrity
 Isolation - execute as if they were run alone
 Durability - results aren’t lost by a failure
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2.1 Atomicity
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All-or-nothing, no partial results.
 E.g. in a money transfer, debit one account, credit the
other. Either both debiting and crediting operation
succeed, or neither of them do.
 Successful completion is called Commit (Commit
statement in SQL), unsuccessfull - Rollback (statement
Rollback in SQL)
 Transaction failure is called Abort
Commit and abort are irrevocable actions.
An Abort undoes operations that already executed
 For database operations, restore the data’s previous
value from before the transaction (Rollback-it);
 But some real world operations are not undoable.
Examples - transfer money, print ticket, fire missile
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2.2 Consistency
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Every transaction should maintain DB consistency
 Referential integrity - E.g. each order references an
existing customer number and existing part numbers
 The books balance (debits = credits, assets = liabilities)
Consistency preservation is a property of a
transaction, not of the database mechanisms for
controlling it (unlike the A, I, and D of ACID)
If each transaction maintains consistency,
then serial executions of transactions do too.
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2.3 Isolation
Intuitively, the effect of a set of transactions should
be the same as if they ran independently.

Formally, an interleaved execution of transactions is
serializable if its effect is equivalent to a serial one.

Implies a user view where the system runs each user’s
transaction stand-alone.
Of course, transactions in fact run with lots of
concurrency, to use device parallelism.

Transactions can use common data (shared data)

They can use the same data processing mechanisms
(time sharing)
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2.4 Durability

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When a transaction commits, its results will survive
failures (e.g. of the application, OS, DB system … even of
the disk).
Makes it possible for a transaction to be a legal
contract.
Implementation is usually via a log
 DB system writes all transaction updates to its log
 to commit, it adds a record “commit(Ti)” to the log
 when the commit record is on disk, the transaction is
committed.
 system waits for disk ack before acking to user
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3. Transaction Processing
Can be automatic (controlled by the RDBMS) or
programmatic (programmed using SQL or other
supported programming languages, like PL/SQL)

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Identifying critical points for database changes through set
of database states
Preparation for the control over transaction progress using
labels of the transaction states
Management of the transactions using explicit
manipulation of transaction states and enforcing
transaction operations
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3.1 Database State and Changes
D1, D2 - Logically consistent states of the database data
TTransaction for changing the database
t1, t2 - Absolute time before and after the transaction
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Transaction Parameters

diff D = D2  D1 can have different scale:

single data item in one memory area
 many items across several files and databases
 structural changes like new database schema, etc.
 t = t2 - t1 is the time for executing T


T occupies real physical resources
 between D1 and D2 there may be intermediate states D11,
D12 …; some of them can be inconsistent


the final state D2 state could be unreachable
When T fails
 we should first come back to D1 (recovery)
 then try again to reach D2 (redo)
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Transaction Operations
For recovery purposes the system needs to keep track of when a
transaction starts, terminates and commits.

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begin: marks the beginning of a transaction execution;
end: specifies that the read and write operations have ended and
marks the end limit of transaction execution;
commit: signals a successful end of the transaction. Any updates
executed by the transaction can be safely committed to the
database and will not be undone;
rollback: signals that the transaction has ended unsuccessfully.
Any changes that the transaction may have applied to the
database must be undone;
undo: similar to rollback but it applies to a single operation
rather than to a whole transaction;
redo: 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;
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Reading and Writing
Specify read or write operations on the database items that are
executed as part of a transaction


read (X): reads a database item named X into a program
variable also named X.
1. find the address of the disk block that contains item X
2. copy that disk block into a buffer in the main memory
3. copy item X from the buffer to the program variable
write (X): writes the value of program variable X into
the database
1. find the address of the disk block that contains item X
2. copy that disk block into a buffer in the main memory
3. copy item X from the program variable named X into its
current location in the buffer
4. store the updated block in the buffer back to disk (this
step updates the database on disk)
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3.2 Transaction State and Progress
A transaction reaches its commit point when all
operations accessing the database are completed
and the result has been recorded in the log. It then
writes a [commit, <transaction-id>] and terminates.
BEGIN
END
active
READ , WRITE
COMMIT
partially
committed
committed
ROLLBACK
ROLLBACK
aborted
terminated
When a system failure occurs, search the log file for entries
[start, <transaction-id>]
and if there are no logged entries [commit, <transaction-id>]
then undo all operations that have logged entries
[write, <transaction-id>, X, old_value, new_value]
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Logging transaction states


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Saving the initial database state D1 before starting the
transaction T: D1->D2 (transaction begins)
Saving all intermediate states D11, D12 … (checkpoint logs)
In the case of a failure at an intermediate state D1i before
reaching D2, restore D1 (rollback); the simplest strategy is to
apply a series of atomic actions R which change the state to the
initial state R: D1i->D1
In the case of successful reach of the last intermediate state D2,
force-write or flush the log file to disk and change the database
state to it (transaction ends);
Note: if the transactions are controlled in SQL(using COMMIT),
the rollback operation should be initiated explicitly (using
ROLLBACK)
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Entries in the log file
Credit (
[start, <transaction-id>]: the start Proceduretrans_id
INTEGER,
of execution of the transaction
accno INTEGER,
bcode CHAR(6),
identified by transaction-id
amount NUMBER)
old NUMBER;
 [read, <transaction-id>, X]: the
new NUMBER;
transaction identified by transaction-id
begin
reads the value of database item X
SELECT balance INTO old
FROM account
 [write, <transaction-id>, X, oldWHERE no = accno and
branch = bcode;
value, new-value]: the transaction
identified by transaction-id changes the
new := old + amount;
value of database item X from oldUPDATE account
value to new-value
SET amount = new
WHERE no = accno and
 [commit, <transaction-id>]: the
branch = bcode;
transaction identified by transaction-id
has completed all data manipulations
COMMIT;
and its effect can be recorded
EXCEPTION
WHEN FAILURE THEN
 [rollback, <transaction-id>]: the
ROLLBACK;
transaction identified by transaction-id
17
has
been
aborted
its6: effect
lost and RecoveryEND credit;
CG096
Advanced
Database
Technologies and
Lecture
Transaction Processing
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
Controlling Subtransactions

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All intermediate states of the transaction which are end states
of the subtransactions included should become consistent
database states
In the case of successful reach of an intermediate state of such
kind the actions are



temporary suspension of transaction execution
forced writing of all updated database blocks in main memory
buffers to disk and flush the log file
resuming of transaction execution
Note: If the transactions are controlled in SQL, the rollback
operation can be stepped to an intermediate state which is
labeled (using ROLLBACK TO <label>)
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Adding checkpoints to the log file

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A [checkpoint, <label>] record is created each time new
checkpoint is encountered
[commit,<transaction-id>] entries for the active
subtransactions are automatically written when the system
writes out to the database the effect of write operations of
the successful transaction
In the case of a rollback to a given checkpoint within a
transaction an entry [commit,<transaction-id>] is logged
against this subtransaction
In the case of a rollback of the global transaction to a given
checkpoint all subtransactions will not be committed either
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4. Database Recovery


Need for recovery from failure during transaction
 for preventing the loss of data
 to avoid global inconsistency of the database
 to analyze the possible reasons for failure
Factors to be accounted for database recovery:
 what is the nature of the failure
 when it happened in the transaction
 what we need to recover
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4.1 Categories of Transactions at
Failure
t c hec k
t fa il
T1
T2
T3
T4
T5
T1 - Can be ignored (committed before the previous checkpoint)
T2 - Must Redo complete (the database will be rolled back to a state
when the transaction was not committed)
T3 - Must Undo (not finished, and rollback to a state when not finished)
T4 - Must Redo if possible (finished, but not committed)
T5 - Must Undo (did not finish and the rollback will lead to a state
before it was even started)
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4.2 Types of Failure
If an error or hardware/software crash occurs between the begin and
end of transaction, the database will be inconsistent


Catastrophic failure
 Restore a previous copy of the database from archival backup
 Apply transaction log to copy to reconstruct more current
state by redoing committed transaction operations up to
failure point
 Incremental dump + log each transaction
Non-catastrophic failure
 Reverse the changes that caused the inconsistency by
undoing the operations and possibly redoing legitimate
changes which were lost
 The entries kept in the system log are consulted during
recovery.
 No need to use the complete archival copy of the database.
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4.3 Recovery Strategy

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Mirroring
 keep two copies of the database and maintain them
simultaneously
Backup
 periodically dump the complete state of the database to
some form of tertiary storage
System Logging
 the log keeps track of all transaction operations affecting
the values of database items. The log is kept on disk so that
it is not affected by failures except for disk and catastrophic
failures.
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Write-ahead Logging

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Deferred Update:
 no actual update of the
database until the
transaction reaches its
commit point

1. Updates recorded in log

2. Transaction commit point 
3. Force log to the disk

4. Update the database


FAILURE!
• REDO database from log
entries
• No UNDO necessary because
database never altered
CG096 Advanced Database Technologies
Immediate Update:
 the database may be
updated by some operations
of a transaction before it
reaches its commit point.
1. Update X recorded in log
FAILURE!
2. Update X in database
• UNDO X
3. Update Y recorded in log
4. Transaction commit point
5. Force log to the disk
FAILURE!
6. Update Y in database
• REDO Y
FAILURE!
• UNDO in reverse order in log
• REDO in committed log order
(uses the write log entry)
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