whatis transaction? (its behaviour) ACID properties

consistent state vs inconsistent state

Transactions are the features that keep database in a consistent state.
It is shift change from file base system to database system.


Database system implements transactions.

suppose we have two acounts

c:5000---checking
s:4000---savin
you are said to add from c into s 1000

1>read how much new amt in checking act
2>deduct 1000 from c act
3>write the new amt in c act
4>reas s
5>add 1000 to s
6>write new amt in s

these steps are transaction


observations

total sum at start in 9000
total sum at end is 9000

this is called consistency


now lets suppose after step 3 something happens and you are not able to do next steps,
this will lead to unconsistent data
so we have to undo the 1st three steps

either do all steps or do none--atomicity
after the transactions the data is permament #durability



If two transactions works concurrently
we have to make sure their work not interfere with each other.
i.e the transactions should work in isolation


so every transaction should follow ACID properties of (fundamental prperties of transaction)

A---atomicity
C---consistency
I-----isolation
D----durability


all databases implement these four properties .

how cuncurrent transactions are maintained

transaction control language TCL
commit
savepoint
rollback