ACID property Atomicity Either all operations of the transaction are properly reflected in the database or none are. Means either all the operations of a transaction are executed or not a single operation is executed. For example consider below transaction to transfer Rs. 50 from account A to account B: read(A) A := A – 50 write(A) read(B) B := B + 50 write(B) In above transaction if Rs. 50 is deducted from account A then it must be added to account B. Consistency Execution of a transaction in isolation preserves the consistency of the database. Means our database must remain in consistent state after execution of any transaction. In above example total of A and B must remain same before and after the execution of transaction. Isolation Although multiple transactions may execute concurrently, each transaction must be unaware of other concurrently executing transactions. Intermediate transaction results must be hidden from other concurrently executed transactions. In above example once your transaction start from step one its result should not be access by any other transaction until last step (step 6) is completed. Durability After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures. Once your transaction completed up to step 6 its result must be stored permanently. It should not be removed if system fails.
