Multiuser concurrency and Contention

[K Anand]

Dear sir,
I know this is a very famous problem and something must have been already discussed on it on this forum. The issue i am dealing with is a kind of a typical online shopping mall. Two kind of users (administrators and customers) try to read or/and update the database at the same time and result in inconsistent data.
Eg.
A(User) B(User)
1 Read Operation 10(Product qty) 10(Product qty)
Sees 10, makes 15 Sees 10, makes 20
2 Update 15
3 Commit 15
4 Update 20
5 Commit 20
As you see the changes made by user A have been lost.
Can anyone share his valuable experience in this regard or if possible give me a link to some related topic in this forum
Thanking in advance
K Anand

[K Anand]

sorry for that illustration. This should be a better presentation.
-------------------------A(User)========================== B(User)
1 Read Operation---------10(Product qty)=============== 10(Product qty)
-------------------------Sees 10, makes 15 ============ Sees 10, makes 20
2 Update---------------- 15
3 Commit---------------- 15
4 Update ===============================================20
5 Commit ===============================================20