Answer Posted / lokesh
Atomicity
Atomicity requires that database modifications must follow
an all or nothing rule. Each transaction is said to be
atomic if when one part of the transaction fails, the entire
transaction fails and database state is left unchanged. It
is critical that the database management system maintains
the atomic nature of transactions in spite of any
application, DBMS, operating system or hardware failure.
An atomic transaction cannot be subdivided, and must be
processed in its entirety or not at all. Atomicity means
that users do not have to worry about the effect of
incomplete transactions.
Transactions can fail for several kinds of reasons:
1. Hardware failure: A disk drive fails, preventing some
of the transaction's database changes from taking effect
2. System failure: The user loses their connection to the
application before providing all necessary information
3. Database failure: E.g., the database runs out of room
to hold additional data
4. Application failure: The application attempts to post
data that violates a rule that the database itself enforces,
such as attempting to create a new account without supplying
an account number
[edit] Consistency
The consistency property ensures that the database remains
in a consistent state; more precisely, it says that any
transaction will take the database from one consistent state
to another consistent state.
The consistency property does not say how the DBMS should
handle an inconsistency other than ensure the database is
clean at the end of the transaction. If, for some reason, a
transaction is executed that violates the database’s
consistency rules, the entire transaction could be rolled
back to the pre-transactional state - or it would be equally
valid for the DBMS to take some patch-up action to get the
database in a consistent state. Thus, if the database schema
says that a particular field is for holding integer numbers,
the DBMS could decide to reject attempts to put fractional
values there, or it could round the supplied values to the
nearest whole number: both options maintain consistency.
The consistency rule applies only to integrity rules that
are within its scope. Thus, if a DBMS allows fields of a
record to act as references to another record, then
consistency implies the DBMS must enforce referential
integrity: by the time any transaction ends, each and every
reference in the database must be valid. If a transaction
consisted of an attempt to delete a record referenced by
another, each of the following mechanisms would maintain
consistency:
* abort the transaction, rolling back to the consistent,
prior state;
* delete all records that reference the deleted record
(this is known as cascade delete); or,
* nullify the relevant fields in all records that point
to the deleted record.
These are examples of Propagation constraints; some database
systems allow the database designer to specify which option
to choose when setting up the schema for a database.
Application developers are responsible for ensuring
application level consistency, over and above that offered
by the DBMS. Thus, if a user withdraws funds from an account
and the new balance is lower than the account's minimum
balance threshold, as far as the DBMS is concerned, the
database is in a consistent state even though this rule
(unknown to the DBMS) has been violated.
[edit] Isolation
Isolation refers to the requirement that other operations
cannot access or see data that has been modified during a
transaction that has not yet completed. The question of
isolation occurs in case of concurrent transaction, (i.e.
transaction occurring at the same time) and with the same
database. To preserve the database consistency, the need of
isolation courses. Each transaction must remain unaware of
other concurrently executing transactions, except that one
transaction may be forced to wait for the completion of
another transaction that has modified data that the waiting
transaction requires. If the isolation system does not
exist, then the database may remain in an inconsistent
state. This may happen as in case of concurrent transaction,
one transaction may leave some data-items in mid process and
at the same time another concurrent transaction may try to
access/alter the same data-item which may cause data
inconsistency and may leave the database in an inconsistent
state.
[edit] Durability
Durability is the ability of the DBMS to recover the
committed transaction updates against any kind of system
failure (hardware or software). Durability is the DBMS's
guarantee that once the user has been notified of a
transaction's success, the transaction will not be lost. The
transaction's data changes will survive system failure, and
that all integrity constraints have been satisfied, so the
DBMS won't need to reverse the transaction. Many DBMSs
implement durability by writing transactions into a
transaction log that can be reprocessed to recreate the
system state right before any later failure. A transaction
is deemed committed only after it is entered in the log.
Durability does not imply a permanent state of the database.
A subsequent transaction may modify data changed by a prior
transaction without violating the durability principle.
| Is This Answer Correct ? | 20 Yes | 3 No |
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