Answer Posted / swapna
The term ACID conveys the role transactions play in mission-
critical applications. Coined by transaction processing
pioneers, ACID stands for atomicity, consistency,
isolation, and durability.
These properties ensure predictable behavior, reinforcing
the role of transactions as all-or-none propositions
designed to reduce the management load when there are many
variables.
Atomicity
A transaction is a unit of work in which a series of
operations occur between the BEGIN TRANSACTION and END
TRANSACTION statements of an application. A transaction
executes exactly once and is atomic ? all the work is done
or none of it is.
Operations associated with a transaction usually share a
common intent and are interdependent. By performing only a
subset of these operations, the system could compromise the
overall intent of the transaction. Atomicity eliminates the
chance of processing a subset of operations.
Consistency
A transaction is a unit of integrity because it preserves
the consistency of data, transforming one consistent state
of data into another consistent state of data.
Consistency requires that data bound by a transaction be
semantically preserved. Some of the responsibility for
maintaining consistency falls to the application developer
who must make sure that all known integrity constraints are
enforced by the application. For example, in developing an
application that transfers money, you should avoid
arbitrarily moving decimal points during the transfer.
Isolation
A transaction is a unit of isolation ? allowing concurrent
transactions to behave as though each were the only
transaction running in the system.
Isolation requires that each transaction appear to be the
only transaction manipulating the data store, even though
other transactions may be running at the same time. A
transaction should never see the intermediate stages of
another transaction.
Transactions attain the highest level of isolation when
they are serializable. At this level, the results obtained
from a set of concurrent transactions are identical to the
results obtained by running each transaction serially.
Because a high degree of isolation can limit the number of
concurrent transactions, some applications reduce the
isolation level in exchange for better throughput.
Durability
A transaction is also a unit of recovery. If a transaction
succeeds, the system guarantees that its updates will
persist, even if the computer crashes immediately after the
commit. Specialized logging allows the system's restart
procedure to complete unfinished operations, making the
transaction durable.
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