Write about Concurrency Control ?

The coordination of the simultaneous execution of transactions in a multi user data base system is known as concurrency control.
the objective of concurrency control is to ensure the Serviceability of transactions in a multi –user data base environment. The 3–problems are Lost updates Uncommitted data Inconsistent retrievals
1.lost updates: The problem occurs when two concurrent transactions T1 & T2 are updating the same data element & one of the update is lost. Ex; T1 & T2  are two transactions update the account.
account contains 1000rs . T1 deposit 2000 Rs and at the same time, T2 deposit 3000 Rs either of the following is occur.
2.Uncommitted data :   T1 & T2 transactions are executed concurrently & the first transaction (T1) is  rolled back after the second transaction (T2) has already accessed the uncommitted data.
Thus violate the isolation property of transaction . Ex : T1 : purchase 100 units. T2 : sell 30 units.
Correct execution T1 Read 35 T1 Purchase 35+100 135 T1 Write 135 T1 Roll back 35 T2 Sell 35-30 30 T2 Write 5	Uncommitted data problem T1 Read 35 T1 Purchase 135 T1 Write 135 T2 Read 135 T2 Sell 30 T1 Roll back 35 T2 Write 105
3.Inconsistent retrievals  this occur when a transaction access data base before & after another transaction finish working with such data.