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What is a transaction and what are ACID properties?
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Answer Posted / pardeep jangra
A transaction is a unit of program execution that
accesses and possibly updates various data items.
! A transaction must see a consistent database.
! During transaction execution the database may be
inconsistent.
! When the transaction is committed, the database must
be consistent.
ACID PROPERTIES
Atomicity- requires that each transaction is "all or
nothing": if one part of the transaction fails, the entire
transaction fails, and the database state is left unchanged.
An atomic system must guarantee atomicity in each and every
situation, including power failures, errors, and crashes.
Consistency
The consistency- property ensures that any transaction will
bring the database from one valid state to another. Any data
written to the database must be valid according to all
defined rules, including but not limited to constraints,
cascades, triggers, and any combination thereof.
Isolation
Isolation- property ensures that the concurrent execution of
transactions results in a system state that could have been
obtained if transactions are executed serially i.e. one
after the other.[citation needed]
Durability
Durability- means that once a transaction has been
committed, it will remain so, even in the event of power
loss, crashes, or errors. In a relational database, for
instance, once a group of SQL statements execute, the
results need to be stored permanently (even if the database
crashes immediately thereafter).
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