Chapter 9
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Transaction Management
and Concurrency Control
Database Systems: Design, Implementation and Management
6th Edition
Peter Rob & Carlos Coronel
What Is a Transaction?
 A transaction is a logical unit of work that must be either
entirely completed or aborted; no intermediate states are
acceptable.
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
Most real-world database transactions are formed by two
or more database requests.

A database request is the equivalent of a single SQL
statement in an application program or transaction.

A database request involving update actually involves at
least one Read and at least one Write operation.

A transaction that changes the contents of the database
must alter the database from one consistent database state
to another.

To ensure consistency of the database, every transaction
must begin with the database in a known consistent state.
Transaction Examples
 A transaction includes read and write
operations to access the database
T2 (“Deposit”)
read_item(X);
X = X + M;
write_item(X);
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T1 (“Transfer”)
read_item(X);
X = X - N;
write_item(X);
read_item(Y);
Y = Y + N;
write_item(Y);
 A bit of terminology 
Read Set = set of all items a transaction reads {X} for T2; {X,Y} for T1
 Write Set = set of all items a transaction writes {X} for T2; {X,Y} for T1
What Is a Transaction?
 Evaluating Transaction Results

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An accountant wishes to register the credit sale of
100 units of product X to customer Y in the amount of
$500.00:


Reducing product X’s Quantity on hand by 100.
Adding $500.00 to customer Y’s accounts receivable.
UPDATE PRODUCT
SET PROD_QOH = PROD_QOH - 100
WHERE PROD_CODE = ‘X’;
UPDATE ACCREC
SET AR_BALANCE = AR_BALANCE + 500
WHERE AR_NUM = ‘Y’;

If the above two transactions are not completely executed, the
transaction yields an inconsistent database.
– Garbage is worse than no data
What Is a Transaction?
 Evaluating Transaction Results

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The DBMS does not guarantee that the semantic
meaning of the transaction truly represents the realworld event.

If we define the transaction to be just this one
statement, then the DBMS doesn’t know that we have
messed up
UPDATE PRODUCT
SET PROD_QOH = PROD_QOH - 100
WHERE PROD_CODE = ‘X’;
What Is a Transaction?
 Transaction Properties (ACID plus)

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
Atomicity requires that all operations of a transaction be
completed; if not, the transaction is aborted.
Consistency Preserving – complete execution of
transaction takes DB from one consistent state to another

Isolation – transaction should appear as though it is
running in isolation – transactions shouldn’t interfere with
each other

Durability – (or permanency) – changes made by
committed transactions must persist – cannot be lost.

Serializability – concurrent transactions are treated as if
they were executed in serial order (one after another)
Transaction Management with SQL

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

ANSI has defined standards that govern SQL database
transactions
Transaction support is provided by two SQL statements:
COMMIT and ROLLBACK.
When a transaction sequence is initiated, it must continue
through all succeeding SQL statements until one of the
following four events occurs:


A COMMIT statement is reached.
A ROLLBACK statement is reached.

The end of a program is successfully reached (equivalent to
COMMIT).

The program is abnormally terminated (equivalent to
ROLLBACK).
The Transaction Log
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
A transaction log keeps track of all transactions that
update the database.

The information stored in the log is used by the DBMS for a
recovery triggered by a ROLLBACK statement, program
crash, or a system failure.

The transaction log stores before-and-after data about the
database and any of the tables, rows, and attribute values
that participated in the transaction. – start of transaction,
reads, writes, commits, aborts all noted.

The transaction log is itself a database, and it is managed
by the DBMS like any other database.
The Transaction Log
 Stores:

A record for the beginning of transaction
 For each transaction component (SQL statement)

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



Type of operation being performed (update, delete, insert)
Names of objects affected by the transaction (the name of the table)
“Before” and “after” values for updated fields
Pointers to previous and next transaction log entries for the same
transaction
The ending (COMMIT) of the transaction
A Snippet of a Transaction Log
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Concurrency Control
 Concurrency control coordinates simultaneous
execution of transactions in a multiprocessing database.
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
The objective of concurrency control is to preserve the
Isolation of transactions – generally by ensuring the
serializability of transactions in a multi-user database
environment.

Important  Simultaneous execution of transactions over
a shared database can create several data integrity and
consistency problems:



Lost Updates.
Uncommitted Data (Dirty Read)
Inconsistent retrievals. (Incorrect Summary)
Normal Sequential Execution of Two Transactions
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T2 (“Deposit”)
read_item(X);
X = X + M;
write_item(X);
T1 (“Transfer”)
read_item(X);
X = X - N;
write_item(X);
read_item(Y);
Y = Y + N;
write_item(Y);
The Lost Update Problem
T2 (“Deposit”)
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T1 (“Transfer”)
read_item(X);
X = X - N;
read_item(X);
X = X + M;
write_item(X);
read_item(Y);
write_item(X);
Y = Y + N;
write_item(Y);
The Dirty Read (Uncommitted Data)
Problem
T2 (“Deposit”)
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T1 (“Transfer”)
read_item(X);
X = X - N;
write_item(X);
read_item(X);
X = X + M;
read_item(Y);
<CRASH>
Recovery sets X back to
original value
write_item(X);
Concurrency Control
 Inconsistent Retrievals (or Incorrect Summary)
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
Inconsistent retrievals occur when a transaction
calculates some summary (aggregate) functions over
a set of data while other transactions are updating
the data.

Example:


T1 calculates the total quantity on hand of the products
stored in the PRODUCT table.
At the same time, T2 updates the quantity on hand
(PROD_QOH) for two of the PRODUCT table’s products.
Retrieval During Update
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Transaction Results: Data Entry Correction
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Inconsistent Retrievals
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Concurrency Control
 The Scheduler
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
The scheduler (part of DBMS) establishes the order
in which the operations within concurrent
transactions are executed.

The scheduler interleaves the execution of database
operations to make sure that the computer’s CPU is
used efficiently – while also ensuring serializability
and isolation.

To determine the appropriate order, the scheduler
bases its actions on concurrency control algorithms,
such as locking or time stamping or optimistic
methods.
Read/Write Conflict Scenarios:
Conflicting Database Operations Matrix
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Concurrency Control with
Locking Methods
 Concurrency can be controlled using locks.
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 A lock guarantees exclusive use of a data item to a
current transaction.
 A transaction acquires a lock prior to data access;
the lock is released (unlocked) when the transaction
is completed.
 All locking of information is managed by a lock
manager.
Concurrency Control with
Locking Methods
 Lock Granularity
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
Lock granularity indicates the level of lock use.

Database level (See Figure 9.3)

Table level (See Figure 9.4)

Page level (See Figure 9.5)

Row level (See Figure 9.6)

Field (attribute) level
A Database-Level Locking Sequence
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An Example Of A Table-Level Lock
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An Example Of A Page-Level Lock
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An Example Of A Row-Level Lock
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Concurrency Control with
Locking Methods
 Binary Locks
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
A binary lock has only two states: locked (1) or
unlocked (0).

If an object is locked by a transaction, no other
transaction can use that object.

If an object is unlocked, any transaction can lock the
object for its use.

A transaction must unlock the object after its
termination.

Every transaction requires a lock and unlock operation
for each data item that is accessed (which could be at
any level of granularity – table, page, row, …).
An Example Of A Binary Lock
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Concurrency Control with
Locking Methods
Exclusive Locks
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 An exclusive lock exists when access is specially
reserved for the transaction that locked the object.
 The exclusive lock must be used when the potential
for conflict exists.
 An exclusive lock is issued when a transaction wants
to write (update) a data item and no locks are
currently held on that data item.
Concurrency Control with
Locking Methods
Shared Locks
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 A shared lock exists when concurrent transactions are
granted READ access on the basis of a common lock.
 A shared lock produces no conflict as long as the
concurrent transactions are read only.
 A shared lock is issued when a transaction wants to
read data from the database and no exclusive lock is
held on that data item.
Concurrency Control with
Locking Methods
 Potential Problems with Locks
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

The resulting transaction schedule may not be
serializable.
The schedule may create deadlocks.
 Solutions


Two-phase locking for the serializability problem.
Deadlock detection and prevention techniques for
the deadlock problem.
Concurrency Control with
Locking Methods
 Two-Phase Locking
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
The two-phase locking protocol defines how
transactions acquire and relinquish locks. It
guarantees serializability, but it does not prevent
deadlocks.

In a growing phase, a transaction acquires all the
required locks without unlocking any data. Once
all locks have been acquired, the transaction is in
its locked point.

In a shrinking phase, a transaction releases all
locks and cannot obtain any new locks.
How A Deadlock Condition Is Created
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Concurrency Control with
Locking Methods
 Three Techniques to Control Deadlocks:
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
Deadlock Prevention
A transaction requesting a new lock is aborted if there
is a possibility that a deadlock can occur. Restarted
later

Deadlock Detection
The DBMS periodically tests the database for
deadlocks. If a deadlock is found, one of the
transactions (“victim”) is aborted, and the other
transaction continues.

Deadlock Avoidance
The transaction must obtain all the locks it needs
before it can be executed.
Database Recovery Management
 Recovery restores a database from a given state, usually
inconsistent, to a previously consistent state.
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 Recovery techniques are based on the atomic
transaction property:

All portions of the transaction must be treated as a single logical unit of
work,


All operations must be applied and completed to produce a consistent
database.
If, for some reason, any transaction operation cannot be completed, the
transaction must be aborted, and any changes to the database must be
rolled back.
Database Recovery Management
 Levels of Backup

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Full backup of the database
It backs up or dumps the whole database.

Differential backup of the database
Only the last modifications done to the database are copied.

Backup of the transaction log only
It backs up all the transaction log operations that are not reflected
in a previous backup copy of the database.
Database Recovery Management
 Database Failures

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Software
Operating system, DBMS, application programs, viruses

Hardware
Memory chip errors, disk crashes, bad disk sectors, disk full
errors

Programming Exemption
Application programs, end users

Transaction
Deadlocks

External
Fire, earthquake, flood
Transaction Recovery
 Makes use of deferred-write and write-through
 Deferred write (or deferred update)
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
Transaction operations do not immediately update the physical
database

Only the transaction log is updated

Database is physically updated only after the transaction reaches
its commit point using the transaction log information
Transaction Recovery (continued)
 Write-through
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
Database is immediately updated by transaction operations during
the transaction’s execution, even before the transaction reaches its
commit point

Transaction log is also updated

If a transaction fails, the database uses the log information to roll
back the database
A Transaction Log for Transaction
Recovery Examples
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End Chapter 9
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