Acid Properties Dbms

Transaction Processing and DBMS
Acid Properties
By Pragya Bhatt
Introduction 2
• Data – Any fact that can be recorded
e.g., - texts, numbers, alphanumeric, audio, video, image.
Database – Collection of interrelated data
• Types of database –
 Traditional database
 Multimedia database
 Geographic Information System (GIS) Database
 Real-time database
 Data Warehouse (huge volume of historical data)
DBMS – Collection of interrelated data and a set of programs to access those data. It
helps us to store and retrieve database information in a convenient and efficient
manner.
Database (DB) + Database Management System (DBMS) = Database System (DBS)

What is transaction 3
• A Transection is a unit of program execution that
can access and update various data items in a
database.
• It could be seen as a collection of operations that
form a single logical unit of work.
• A transaction is usually made to change data in a
database which can be done by inserting new
data, updating the existing data, or by deleting
the data that is no longer required.
Transection Operations 4
There are four basic operations
• Read(X) : It is used to read the previous value of
any data item from database.
• Write(X) : This operation is used to
write/update/write back the value of any data
item in the database.
• Commit : Used to finally store the work.
• Rollback : Used to undone the work if required.
Transaction States in DBMS
5
• During the lifetime of a transaction, there are a lot of states to go

through. These states update the operating system about the current
state of the transaction and also tell the user about how to plan further
processing of the transaction. These states decide the regulations
which decide the fate of a transaction whether it will commit or abort.
Acid Properties of Transection 6
• Atomicity : either the entire transaction takes place at once or doesn’t
happen at all. Each transaction is considered as one unit .It involves the
following two operations.
• Abort: If a transaction aborts, changes made to the database are not visible.
Commit: If a transaction commits, changes made are visible.
Consider the following
transaction T consisting of T1 and T2:
Transfer of 100 from account X to account Y.
If the transaction fails after completion
of T1 but before completion of T2.( say,
after write(X) but before write(Y)), then the
amount has been deducted from X but not
added to Y. This results in an inconsistent
database state. Therefore, the transaction must
be executed in its entirety in order to ensure
the correctness of the database state.
• Consistency:integrity constraints must be maintained so that the

database is consistent before and after the transaction.
Consider the following
transaction T consisting of T1 and T2:
Transfer of 100 from account X to
account Y.
The total amount before and after the
transaction must be maintained.
Total before T occurs = 500 + 200 = 700.
Total after T occurs = 400 + 300 = 700.
Therefore, the database is consistent.
Inconsistency occurs in case T1 completes
but T2 fails. As a result, T is incomplete.
• Isolation:Transactions occur independently without interference. Changes occurring in a particular
transaction will not be visible to any other transaction until that particular change in that transaction is
written to memory or has been committed. This property ensures that the execution of transactions
concurrently
Let X= 500, Y = 500.
Consider two transactions T and T”. Suppose T has been executed till Read (Y) and then T’’ starts. As a
result, interleaving of operations takes place due to which T’’ reads the correct value of X but the incorrect
value of Y and sum computed by
T’’:(X+Y=50,000+500=50,500)
is thus not consistent with the sum at end of the transaction:
T: (X+Y = 50, 000 + 450 = 50, 450).
This results in database inconsistency, due to a loss of 50 units. Hence, transactions must take place in
isolation and changes should be visible only after they have been made to the main memory.
• Durability: once the transaction has completed execution,

the updates and modifications to the database are stored in
and written to disk and they persist even if a system
failure occurs. These updates now become permanent and
are stored in non-volatile memory. The effects of the
transaction, thus, are never lost.
Thank You 10

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Transaction Processing and DBMS

Database (DB) + Database Management System (DBMS) = Database System (DBS)

• During the lifetime of a transaction, there are a lot of states to go

• Consistency:integrity constraints must be maintained so that the

• Durability: once the transaction has completed execution,

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