Definition. A type of computer processing in which the computer responds immediately to user requests. Each request is cindered to be a transaction. Automatic teller machines for banks are an example of transaction processing.

The opposite of transaction processing is batch processing, in which a batch of requests is stored and then excited all at one time. Transaction processing requires interaction with a user, whereas batch processing can take place without a user being present.

A transaction is a unit of work that has the following properties:

(i)    ATOMICITY: A transaction should be done or undone completely and unambiguously. In the event of a failure of any operation, effects of all operations that make up the transaction should be undone, and data should be rolled back to its previous state.

(ii)    CONSISTENCY: A transaction should preserve all the invariant properties (such as integrity constraints) defined on the data. On completion of successful transaction, the data should be in a consistent state. In other words, a transaction should transform the system from one consistent state to another consistent state. For example, in the case of relational databases, a consistent transaction should preserve all the integrity constraints defined on the data.

(iii)    ISOLATION: Each transaction should appear to execute independently of other transactions that may be executing concurrently in the same environment. The effect of executing a set of transaction serially should be the same as that of running them concurrently. This requires two things:

(a)    During the course of a transaction, intermediate (possibly inconsistent) state of the data should not be exposed to all other transactions.

(b)    Two concurrent transactions should not be able to operate on the same data. Database management systems usually implement this feature using locking.

(iv) DURABILITY: The effects of completed transaction should always be persistent.

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