ACID Properties in DBMS

ACID properties in DBMS 

  • A transaction is a single logical action which accesses and modify the contents of the database through reading and write operations
  • For maintaining consistency of the database before and after a transaction, certain properties are followed called as acid properties

Atomicity (A) 

  • An atomic transaction simply means that the transaction happens only if it is able to be completed and achieve its purpose or if not it doesn’t happen at all
  • Atomicity defines that there are no transactions that occur partially hence the atomicity is also known as “all or nothing rule”
ACID properties

It is associated with two operations

  • Abort: If a transaction is aborted i.e. it is incomplete, changes made to the database are not visible
  • Commit: If a transaction is committed i.e. it is complete changes made to the database or visible

Example 

Consider a transaction T which consists of T1 and T2: Task is to transfer 100 from account X to account Y

  • T1: Deduct the amount from account X
  • T2: Credit the amount to account Y

If the transaction fails after the completion of T1 and before completion of T2 then the amount will be deducted from account X but it will not be added to the account Y which ultimately results in inconsistent database state

Atomicity

Consistency(C) 

  • Integrity constraints (maintain certain rules to authenticate database) must be maintained to ensure your database is consistent before and after the transaction
  • Consistency refers to the correctness of the database

Example 

The total amount before and after the transaction must be maintained.

  • Total amount before T occurs = 500 + 200 = 700.
  • Total amount after T occurs = 400 + 300 = 700.
  • The database is said to be inconsistent if T1 is completed but fails as a result total
  • Transaction P is incomplete

Isolation (I) 

  • Isolation ensures that multiple transactions can occur at the same time provided each transaction is independent and shall not interfere in other transaction
  • Changes in one particular transaction are visible to any other transaction until unless a particular change in the transaction is written to the memory

 Durability (D) 

  • Durability ensures that once after the completion of the transaction execution the updates and modifications to the database are stored and return to a disc so that they can be used whenever a system failure occurs
  • So that all the changes become permanent and stored in non-volatile memory so that any action can be referred and never lost
Isolation

What is the purpose of this ACID properties ?

  • Provides a mechanism for correctness and consistency of a database system
  • As a result, each transaction is independent, consistent to each other and all actions are stored properly and permanently and supports failure recovery

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