Answer / s.vanitha

Pessimistic concurrency control
Optimistic concurrency control

Answer / afan

When many people attempt to modify data in a database at
the same time, a system of controls must be implemented so
that modifications made by one person do not adversely
affect those of another person. This is called concurrency
control.

Concurrency control theory has two classifications for the
methods of instituting concurrency control:

Pessimistic concurrency control
A system of locks prevents users from modifying data in a
way that affects other users. After a user performs an
action that causes a lock to be applied, other users cannot
perform actions that would conflict with the lock until the
owner releases it. This is called pessimistic control
because it is mainly used in environments where there is
high contention for data, where the cost of protecting data
with locks is less than the cost of rolling back
transactions if concurrency conflicts occur.

Optimistic concurrency control
In optimistic concurrency control, users do not lock data
when they read it. When a user updates data, the system
checks to see if another user changed the data after it was
read. If another user updated the data, an error is raised.
Typically, the user receiving the error rolls back the
transaction and starts over. This is called optimistic
because it is mainly used in environments where there is
low contention for data, and where the cost of occasionally
rolling back a transaction is lower than the cost of
locking data when read.

Answer / priyanka

there are basically two types of concurracnies...
1-> logical concurrency
2-> physical concurrency

Answer / nomaan khan

Concurrency occur when more than one Task want to perform
at same time, so as per my knowledge it will be of two type
1. Concurrent read
2. Concurrent Write
In this concurrent Read permission will be grant by
Admin but Concurrent write permission will not grant by the
Admin because it will give us a Fantom Problem.

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can any one say how to update the following senario I have a table <srabank> in which the table structure is as follows ANAME ACCNO LOCATION ACCTYPE BAL SBanuPrakash 31518746291 Punganur deposit 4000 Sreenivas 31518746292 mahoobnagar current 14000 Ranjith 31518746293 Karimnagar Savings 2000 Giresh 31518746294 Chennai deposit 40000 Boo 31518746295 Chennai Savings 20000 Jay 31518746296 Valachari Savings 1000 tirumalraj 31518746297 Vellore Savings 8000 The senario is We need to select one account number and check the balance after checking the balance if the balance exist we need to transfer to another account . in the from account the amount need to be reduced and in the to account the amount needed to be added. for example for the <accountno> <31518746291> the balance is <4000> for the <accno> <31518746292> the balance is <14000> after transferring the balance the details will look as follows <accno><31518746291> <bal> <2000> <accno><31518746292> <bal> <16000> the above mentioned two statment will come under the final result.

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