A transaction can be defined as a unit or part of any program at the time of its execution. During transactions data items can be read or updated or both.
1. ACID Properties of Transaction
The database system is required to maintain the following properties of transactions to ensure the integrity of the data.
- Atomicity : A transaction must be Atomic transaction means either all operations within a transaction are completed or none.
- Consistency : A transaction must be executed in It means variables used by a transaction cannot be changed by any other transaction concurrently.
- Isolation : During concurrent transaction each transaction must be unaware of other For any transaction Ti, it appears toTi that any other transaction Tk is either finished before starting of Ti or started after Ti finished.
- Durability : Changes are made permanent to database after successful completion of transaction even in the case of system failure or crash.
2. Transaction States
A transaction must be in one of the following states as shown in Figure 8.1.
- Active state : It is the initial state of During execution of statements, a transaction is in active state.
- Partially committed : A transaction is in partially committed state, when all the statements within transaction are executed but transaction is not committed.
- Failed : In any case, if transaction cannot be proceeded further then transaction is in failed state.
- Committed : After successful completion of transaction, it is in committed state.
Source: Gupta Satinder Bal, Mittal Aditya (2017), Introduction to Basic Database Management System, 2nd Edition-University Science Press (2017)