Chapter 9 introduction to transaction processing

C H A P T E R 9
INTRODUCTION TO
TRANSACTION PROCESSING
Introduction to Transaction
Processing
1

Introduction
 One criterion for classifying a database system is
according to the number of users who can use the
system concurrently-that is, at the same time.
 A DBMS is single-user if at most one user at a time
can use the system, and it is multiuser if many users
can use the system-and hence access the database-
concurrently.
 Single-user DBMSs are mostly restricted to personal
computer systems; most other DBMSs are multiuser.
Introduction to Transaction Processing
2

Introduction
 Multiple users can access databases-and use
computer systems-simultaneously because of the
concept of multiprogramming, which allows the
computer to execute multiple programs-or
processes-at the same time.
 If only a single central processing unit (CPU) exists,
it can actually execute at most one process at a time.
3

Introduction
 However, multiprogramming operating systems
execute some commands from one process, then
suspend that process and execute some commands
from the next process, and so on.
 A process is resumed at the point where it was
suspended whenever it gets its turn to use the CPU
again.
 Hence, concurrent execution of processes is actually
interleaved.
4

Introduction
 Figure , shows two processes A and B executing
concurrently in an interleaved fashion.
 Interleaving keeps the CPU busy when a process
requires an input or output (r/o) operation, such as
reading a block from disk.
5

Introduction
 The CPU is switched to execute another process
rather than remaining idle during r/o time.
 Interleaving also prevents a long process from
delaying other processes.
 If the computer system has multiple hardware
processors (CPUs), parallel processing of multiple
processes is possible, as illustrated by processes C
and D in Figure.
6

Transactions
 A transaction is an executing program that forms a
logical unit of database processing.
 A transaction includes one or more database access
operations-these can include insertion, deletion,
modification, or retrieval operations.
 The database operations that form a transaction can
either be embedded within an application program
or they can be specified interactively via a high-level
query language such as SQL.
7

 One way of specifying the transaction boundaries is
by specifying explicit begin transaction and end
transaction statements in an application program;
in this case, all database access operations between
the two are considered as forming one transaction.
8

Read and Write Operations
 The basic database access operations that a
transaction can include are as follows:
 read_item(X): Reads a database item named X
into a program variable.
 write_item(X): Writes the value of program
variable X into the database item namedX.
9

DBMS Buffers
 The DBMS will generally maintain a number of
buffers in main memory that hold database disk
blocks containing the database items being
processed.
 When these buffers are all occupied, and additional
database blocks must be copied into memory, some
buffer replacement policy is used to choose which of
the current buffers is to be replaced.
 If the chosen buffer has been modified, it must be
written back to disk before it is reused.
10

Why Concurrency Control Is Needed
 Several problems can occur when concurrent
transactions execute in an uncontrolled manner.
 Figure 2( a) shows a transaction T1 that transfers N
reservations from one flight whose number of
reserved seats is stored in the database item named
X to another flight whose number of reserved seats is
stored in the database item named Y.
 Figure 2(b) shows a simpler transaction T2 that just
reserves M seats on the first flight (X) referenced in
transaction T1.
11

 The Lost Update Problem: This problem occurs
when two transactions that access the same database
items have their operations interleaved in a way that
makes the value of some database items incorrect.
12

13

 The Temporary Update (or Dirty Read)
Problem.
 This problem occurs when one transaction updates a
database item and then the transaction fails for some
reason.
 The updated item is accessed by another transaction
before it is changed back to its original value.
14

The value of item X that is read by T2 is called dirty data,
because it has been created by a transaction that has not
completed and committed yet; hence, this problem is also
known as the dirty read problem.
15

 The Incorrect Summary Problem:
 If one transaction is calculating an aggregate
summary function on a number of records while
other transactions are updating some of these
records, the aggregate function may calculate some
values before they are updated and others after they
are updated.
16

17
 For example: Suppose that a transaction T3 is
calculating the total number of reservations on all
the flights; meanwhile, transaction T1 is executing.
 If the interleaving of operations occurs, the result of
T3 will be off by an amount N because T3 reads the
value of X after N seats have been subtracted from it
but reads the value of Y before those N seats have
been added to it.

18
 Another problem that may occur is called
unrepeatable read, where a transaction T reads an
item twice and the item is changed by another
transaction T' between the two reads.
 Hence, T receives different values for its two reads of
the same item.

Why Recovery Is Needed
19
 Whenever a transaction is submitted to a DBMS for
execution, the system is responsible for making sure
that either
 all the operations in the transaction are completed
successfully and their effect is recorded permanently
in the database, or
 the transaction has no effect whatsoever on the
database or on any other transactions.
 The DBMS must not permit some operations of a
transaction T to be applied to the database while
other operations of T are not.

Types of Failures
20
 A computer failure (system crash):
 A hardware, software, or network error occurs in the computer
system during transaction execution.
 Hardware crashes are usually media failures-for example,
main memory failure.
 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 erroneous
parameter values or because of a logical programming error.
 In addition, the user may interrupt the transaction during its
execution

Types of Failures
21
 Local errors or exception conditions
detected by the transaction:
 During transaction execution, certain conditions may occur
that necessitate cancellation of the transaction.
 For example, data for the transaction may not be found. Notice
that an exception condition," such as insufficient account
balance in a banking database, may cause a transaction, such
as a fund withdrawal, to be canceled.
 This exception should be programmed in the transaction itself,
and hence would not be considered a failure.

Types of Failures
22
 Concurrency control enforcement:
 The concurrency control method may decide to abort the transaction,
to be restarted later, because it violates serializability or because
several transactions are in a state of deadlock.
 Disk failure:
 Some disk blocks may lose their data because of a read or write
malfunction or because of a disk read/write head crash. This may
happen during a read or a write operation of the transaction.
 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.

TRANSACTION AND SYSTEM CONCEPTS
23
 A transaction is an atomic unit of work that is either
completed in its entirety or not done at all.
 For recovery purposes, the system needs to keep
track of when the transaction starts, terminates, and
commits or aborts.

Transaction Operations
24
 BEGIN_TRANSACTION: This marks the beginning
of transaction execution.
 READ DR WRITE: These specify read or write
operations on the database items that are executed
as part of a transaction.
 END_TRANSACTION: This specifies that READ and
WRITE transaction operations have ended and
marks the end of transaction execution.

Transaction Operations
25
 COMMIT_TRANSACTION: This signals a successful
end of the transaction so that any changes (updates)
executed by the transaction can be safely committed
to the database and will not be undone.
 ROLLBACK (OR ABORT): This signals that the
transaction has ended unsuccessfully, so that any
changes or effects that the transaction may have
applied to the database must be undone.

Transaction States
26
Figure shows a state transition diagram that
describes how a transaction moves through its
execution states.

Transaction States
27
 A transaction goes into an active state immediately
after it starts execution, where it can issue READ and
WRITE operations.
 When the transaction ends, it moves to the partially
committed state.
At this point, some recovery protocols need to ensure
that a system failure will not result in an inability to
record the changes of the transaction permanently.
 Once this check is successful, the transaction is said
to have reached its commit point and enters the
committed state.

Transaction States
28
 Once a transaction is committed, it has concluded its
execution successfully and all its changes must be
recorded permanently in the database.
 However, a transaction can go to the failed state if
one of the checks fails or if the transaction is aborted
during its active state.
 The transaction may then have to be rolled back to
undo the effect of its WRITE operations on the
database.
 The terminated state corresponds to the transaction
leaving the system.

The System Log
29
 To be able to recover from failures that affect
transactions, the system maintains a log to keep
track of all transaction operations that affect the
values of database items.
 This information may be needed to permit recovery
from failures.

Commit Point of a Transaction
30
 A transaction T reaches its commit point when all
its operations that access the database have been
executed successfully and the effect of all the
transaction operations on the database have been
recorded in the log.
 Beyond the commit point, the transaction is said to
be committed, and its effect is assumed to be
permanently recorded in the database. The
transaction then writes a commit record [commit,T]
into the log.

DESIRABLE PROPERTIES OF
TRANSACTIONS
31
 Atomicity: A transaction is an atomic unit of
processing; it is either performed in its entirety or
not performed at all.
 Consistency preservation: A transaction is
consistency preserving if its complete execution
takes the database from one consistent state to
another.

DESIRABLE PROPERTIES OF
TRANSACTIONS
32
 Isolation: A transaction should appear as though it
is being executed in isolation from other
transactions. That is, the execution of a transaction
should not be interfered with by any other
transactions executing concurrently.
 Durability or permanency: The changes applied
to the database by a committed transaction must
persist in the database. These changes must not be
lost because of any failure.

Chapter 9 introduction to transaction processing

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Chapter 9 introduction to transaction processing