ACID Properties
Transaction Concept :
A transaction is a unit of program execution that accesses and possibly updates various data items.
A transaction is delimited by statements of the form ‘begin transaction’ and the ‘end transaction’. The transaction consists of all operations executed between the ‘begin transaction’ and the ‘end transaction’.
Properties of Transactions : (ACID properties)
- Atomicity : Either all operations of the transaction are reflected properly in the database or none.
- Consistency : Execution of a transaction in isolation (i.e., with no other transaction executing concurrently) preserves the consistency of the database.
- Isolation : Even though multiple transactions may execute concurrently, the system guarantees that, for every pair of transactions Ti and Tj, it appears to Ti that
– either Tj finished execution before Ti started or
– Tj started execution after Ti finished.
Thus, each transaction is unaware of other transactions executing concurrently in the system.
- Durability : After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures.
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