Undergraduate
Database Management
Systems
Lecture Transaction Managemnet
Hamza Shaukat
Hamza.Shaukat@itu.edu.pk
Information Technology University (ITU)
Faculty of Engineering
Indexes as Access Paths
A single-level index is an auxiliary file that makes it more efficient to
search for a record in the data file.
The index is usually specified on one field of the file (although it could
be specified on several fields)
One form of an index is a file of entries <field value, pointer to
record>, which is ordered by field value
The index is called an access path on the field.
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1 Introduction to Transaction Processing
Single-User System:
▪ At most one user at a time can use the system.
Multiuser System:
▪ Many users can access the system concurrently.
Concurrency
▪ Interleaved processing:
– Concurrent execution of processes is interleaved in a single
CPU
▪ Parallel processing:
– Processes are concurrently executed in multiple CPUs.
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Introduction to Transaction Processing (2)
A Transaction:
▪ Logical unit of database processing that includes one or more
access operations (read -retrieval, write - insert or update,
delete).
A transaction (set of operations) may be stand-alone specified in a
high level language like SQL submitted interactively, or may be
embedded within a program.
Transaction boundaries:
▪ Begin and End transaction.
An application program may contain several transactions separated
by the Begin and End transaction boundaries.
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Introduction to Transaction Processing (3)
SIMPLE MODEL OF A DATABASE (for purposes of discussing
transactions):
A database is a collection of named data items
Basic operations are read and write
▪ read_item(X): Reads a database item named X into a program
variable. To simplify our notation, we assume that the program
variable is also named X.
▪ write_item(X): Writes the value of program variable X into the
database item named X.
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Introduction to Transaction Processing (4)
READ AND WRITE OPERATIONS:
Basic unit of data transfer from the disk to the computer main memory
is one block. In general, a data item (what is read or written) will be
the field of some record in the database, although it may be a
larger unit such as a record or even a whole block.
read_item(X) command includes the following steps:
▪ Find the address of the disk block that contains item X.
▪ Copy that disk block into a buffer in main memory (if that disk
block is not already in some main memory buffer).
▪ Copy item X from the buffer to the program variable named X.
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Introduction to Transaction Processing (5)
READ AND WRITE OPERATIONS (contd.):
write_item(X) command includes the following steps:
▪ Find the address of the disk block that contains item X.
▪ Copy that disk block into a buffer in main memory (if that disk
block is not already in some main memory buffer).
▪ Copy item X from the program variable named X into its correct
location in the buffer.
▪ Store the updated block from the buffer back to disk (either
immediately or at some later point in time).
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Two sample transactions
FIGURE 17.2 Two sample transactions:
▪ (a) Transaction T1
▪ (b) Transaction T2
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Introduction to Transaction Processing (6)
Why Concurrency Control is needed:
The Lost Update Problem
▪ This occurs when two transactions that access the same database
items have their operations interleaved in a way that makes the
value of some database item incorrect.
The Temporary Update (or Dirty Read) Problem
▪ This occurs when one transaction updates a database item and
then the transaction fails for some reason (see Section 17.1.4).
▪ The updated item is accessed by another transaction before it is
changed back to its original value.
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.
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Concurrent execution is uncontrolled: (a) The lost update problem.
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Concurrent execution is uncontrolled: (b) The temporary update problem.
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Introduction to Transaction Processing (12)
Why recovery is needed:
(What causes a Transaction to fail)
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 erroneous parameter values or
because of a logical programming error. In addition, the
user may interrupt the transaction during its execution.
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Introduction to Transaction Processing (13)
Why recovery is needed :
(What causes a Transaction to fail)
3. Local errors or exception conditions detected by the transaction:
Certain conditions necessitate cancellation of the transaction.
For example, data for the transaction may not be found. A
condition, such as insufficient account balance in a
banking database, may cause a transaction, such as a
fund withdrawal from that account, to be canceled.
A programmed abort in the transaction causes it to fail.
4. 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
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Introduction to Transaction Processing (14)
Why recovery is needed :
(What causes a Transaction to fail)
5. 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.
6. 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.
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2 Transaction and System Concepts
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 states:
▪ Active state
▪ Partially committed state
▪ Committed state
▪ Failed state
▪ Terminated State
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Transaction and System Concepts (2)
Recovery manager keeps track of the following operations:
▪ begin_transaction: This marks the beginning of transaction
execution.
▪ read or 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 limit
of transaction execution.
– At this point it may be necessary to check whether the
changes introduced by the transaction can be permanently
applied to the database or whether the transaction has to
be aborted because it violates concurrency control or for
some other reason.
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Transaction and System Concepts (3)
Recovery manager keeps track of the following operations (cont):
▪ 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.
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Transaction and System Concepts (4)
Recovery techniques use the following operators:
▪ undo: Similar to rollback except that it applies to a
single operation rather than to a whole transaction.
▪ redo: 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.
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State transition diagram illustrating the states for
transaction execution
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Transaction and System Concepts (6)
The System Log
▪ Log or Journal: The log keeps track of all
transaction operations that affect the values of
database items.
– This information may be needed to permit recovery from
transaction failures.
– The log is kept on disk, so it is not affected by any type of
failure except for disk or catastrophic failure.
– In addition, the log is periodically backed up to archival
storage (tape) to guard against such catastrophic failures.
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Transaction and System Concepts (7)
The System Log (cont):
▪ T in the following discussion refers to a unique
transaction-id that is generated automatically by the
system and is used to identify each transaction:
▪ Types of log record:
– [start_transaction,T]: Records that transaction T has started
execution.
– [write_item,T,X,old_value,new_value]: Records that
transaction T has changed the value of database item X
from old_value to new_value.
– [read_item,T,X]: Records that transaction T has read the
value of database item X.
– [commit,T]: Records that transaction T has completed
successfully, and affirms that its effect can be committed
(recorded permanently) to the database.
– [abort,T]: Records that transaction T has been aborted.
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Transaction and System Concepts (9)
Recovery using log records:
If the system crashes, we can recover to a consistent database state
by examining the log.
1. Because the log contains a record of every write operation
that changes the value of some database item, it is possible to
undo the effect of these write operations of a transaction T by
tracing backward through the log and resetting all items
changed by a write operation of T to their old_values.
2. We can also redo the effect of the write operations of a
transaction T by tracing forward through the log and setting all
items changed by a write operation of T (that did not get done
permanently) to their new_values.
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Transaction and System Concepts (10)
Commit Point of a Transaction:
Definition a Commit Point:
▪ 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 has 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 an entry [commit,T] into the log.
Roll Back of transactions:
▪ Needed for transactions that have a [start_transaction,T] entry
into the log but no commit entry [commit,T] into the log.
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Transaction and System Concepts (11)
Commit Point of a Transaction :
Redoing transactions:
▪ Transactions that have written their commit entry in the log
must also have recorded all their write operations in the log;
otherwise they would not be committed, so their effect on the
database can be redone from the log entries. (Notice that the
log file must be kept on disk.
▪ At the time of a system crash, only the log entries that have
been written back to disk are considered in the recovery
process because the contents of main memory may be lost.)
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Desirable Properties of Transactions
ACID properties:
Atomicity: A transaction is an atomic unit of processing; it is either
performed in its entirety or not performed at all.
Consistency preservation: A correct execution of the transaction
must take the database from one consistent state to another.
Isolation: A transaction should not make its updates visible to other
transactions until it is committed; this property, when enforced
strictly, solves the temporary update problem and makes cascading
rollbacks of transactions unnecessary (see Chapter 21).
Durability or permanency: Once a transaction changes the database
and the changes are committed, these changes must never be lost
because of subsequent failure.
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