Tag: transaction management in dbms

ACID Properties | ACID Properties in DBMS

Database Management System

Transaction in DBMS-

 

Before you go through this article, make sure that you have gone through the previous article on Transactions in DBMS.

 

We have discussed-

  • A transaction is a set of logically related operations.
  • A transaction goes through different states throughout its life cycle.
  • The life cycle of a transaction is-

 

 

In this article, we will discuss ACID properties of a transaction.

 

ACID Properties-

 

  • It is important to ensure that the database remains consistent before and after the transaction.
  • To ensure the consistency of database, certain properties are followed by all the transactions occurring in the system.
  • These properties are called as ACID Properties of a transaction.

 

 

1. Atomicity-

 

  • This property ensures that either the transaction occurs completely or it does not occur at all.
  • In other words, it ensures that no transaction occurs partially.
  • That is why, it is also referred to as “All or nothing rule“.
  • It is the responsibility of Transaction Control Manager to ensure atomicity of the transactions.

 

2. Consistency-

 

  • This property ensures that integrity constraints are maintained.
  • In other words, it ensures that the database remains consistent before and after the transaction.
  • It is the responsibility of DBMS and application programmer to ensure consistency of the database.

 

3. Isolation-

 

  • This property ensures that multiple transactions can occur simultaneously without causing any inconsistency.
  • During execution, each transaction feels as if it is getting executed alone in the system.
  • A transaction does not realize that there are other transactions as well getting executed parallely.
  • Changes made by a transaction becomes visible to other transactions only after they are written in the memory.
  • The resultant state of the system after executing all the transactions is same as the state that would be achieved if the transactions were executed serially one after the other.
  • It is the responsibility of concurrency control manager to ensure isolation for all the transactions.

 

4. Durability-

 

  • This property ensures that all the changes made by a transaction after its successful execution are written successfully to the disk.
  • It also ensures that these changes exist permanently and are never lost even if there occurs a failure of any kind.
  • It is the responsibility of recovery manager to ensure durability in the database.

 

Next Article- Concurrency Problems in DBMS

 

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Transaction States in DBMS

Database Management System

Transaction in DBMS-

 

“Transaction is a set of operations which are all logically related.”

OR

“Transaction is a single logical unit of work formed by a set of operations.”

 

Operations in Transaction-

 

The main operations in a transaction are-

  1. Read Operation
  2. Write Operation

 

1. Read Operation-

 

  • Read operation reads the data from the database and then stores it in the buffer in main memory.
  • For example- Read(A) instruction will read the value of A from the database and will store it in the buffer in main memory.

 

2. Write Operation-

 

  • Write operation writes the updated data value back to the database from the buffer.
  • For example- Write(A) will write the updated value of A from the buffer to the database.

 

Transaction States-

 

A transaction goes through many different states throughout its life cycle.

These states are called as transaction states.

Transaction states are as follows-

  1. Active state
  2. Partially committed state
  3. Committed state
  4. Failed state
  5. Aborted state
  6. Terminated state

 

 

1. Active State-

 

  • This is the first state in the life cycle of a transaction.
  • A transaction is called in an active state as long as its instructions are getting executed.
  • All the changes made by the transaction now are stored in the buffer in main memory.

 

2. Partially Committed State-

 

  • After the last instruction of transaction has executed, it enters into a partially committed state.
  • After entering this state, the transaction is considered to be partially committed.
  • It is not considered fully committed because all the changes made by the transaction are still stored in the buffer in main memory.

 

3. Committed State-

 

  • After all the changes made by the transaction have been successfully stored into the database, it enters into a committed state.
  • Now, the transaction is considered to be fully committed.

 

NOTE-

 

  • After a transaction has entered the committed state, it is not possible to roll back the transaction.
  • In other words, it is not possible to undo the changes that has been made by the transaction.
  • This is because the system is updated into a new consistent state.
  • The only way to undo the changes is by carrying out another transaction called as compensating transaction that performs the reverse operations.

 

4. Failed State-

 

  • When a transaction is getting executed in the active state or partially committed state and some failure occurs due to which it becomes impossible to continue the execution, it enters into a failed state.

 

5. Aborted State-

 

  • After the transaction has failed and entered into a failed state, all the changes made by it have to be undone.
  • To undo the changes made by the transaction, it becomes necessary to roll back the transaction.
  • After the transaction has rolled back completely, it enters into an aborted state.

 

6. Terminated State-

 

  • This is the last state in the life cycle of a transaction.
  • After entering the committed state or aborted state, the transaction finally enters into a terminated state where its life cycle finally comes to an end.

 

Next Article- ACID Properties of Transaction

 

Get more notes and other study material of Database Management System (DBMS).

Watch video lectures by visiting our YouTube channel LearnVidFun.