Answer Posted / ravikumar
ACID properties
When a transaction processing system creates a transaction,
it will ensure that the transaction will have certain
characteristics. The developers of the components that
comprise the transaction are assured that these
characteristics are in place. They do not need to manage
these characteristics themselves. These characteristics are
known as the ACID properties. ACID is an acronym for
atomicity, consistency, isolation, and durability.
Atomicity
The atomicity property identifies that the transaction is
atomic. An atomic transaction is either fully completed, or
is not begun at all. Any updates that a transaction might
affect on a system are completed in their entirety. If for
any reason an error occurs and the transaction is unable to
complete all of its steps, the then system is returned to
the state it was in before the transaction was started. An
example of an atomic transaction is an account transfer
transaction. The money is removed from account A then
placed into account B. If the system fails after removing
the money from account A, then the transaction processing
system will put the money back into account A, thus
returning the system to its original state. This is known
as a rollback, as we said at the beginning of this chapter..
Consistency
A transaction enforces consistency in the system state by
ensuring that at the end of any transaction the system is
in a valid state. If the transaction completes
successfully, then all changes to the system will have been
properly made, and the system will be in a valid state. If
any error occurs in a transaction, then any changes already
made will be automatically rolled back. This will return
the system to its state before the transaction was started.
Since the system was in a consistent state when the
transaction was started, it will once again be in a
consistent state.
Looking again at the account transfer system, the system is
consistent if the total of all accounts is constant. If an
error occurs and the money is removed from account A and
not added to account B, then the total in all accounts
would have changed. The system would no longer be
consistent. By rolling back the removal from account A, the
total will again be what it should be, and the system back
in a consistent state.
Isolation
When a transaction runs in isolation, it appears to be the
only action that the system is carrying out at one time. If
there are two transactions that are both performing the
same function and are running at the same time, transaction
isolation will ensure that each transaction thinks it has
exclusive use of the system. This is important in that as
the transaction is being executed, the state of the system
may not be consistent. The transaction ensures that the
system remains consistent after the transaction ends, but
during an individual transaction, this may not be the case.
If a transaction was not running in isolation, it could
access data from the system that may not be consistent. By
providing transaction isolation, this is prevented from
happening.
Durability
A transaction is durable in that once it has been
successfully completed, all of the changes it made to the
system are permanent. There are safeguards that will
prevent the loss of information, even in the case of system
failure. By logging the steps that the transaction
performs, the state of the system can be recreated even if
the hardware itself has failed. The concept of durability
allows the developer to know that a completed transaction
is a permanent part of the system, regardless of what
happens to the system later on.
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