31 Oct 2010 at 10:51 PM
A transaction is an atomic unit of work that must be
completed in its entirety.The transaction succeeds if it committed and fails if
it is aborted.Transactions have four essential
properties:atomicity,consistency,isolation, and durability(known as the ACID
Atomicity:The work cannot be broken into smaller parts.Although a transaction
might contain many SQL statements,it must be run as all-or-nothing
proposition,which means that,if a transaction is only partially complete when
an error occurs,the work revertss to its state prior to the start of the
Consistency:A transaction must operate on a consistent view of the data and also
leave the data in a consistency state.Any work in progress must not be visible
to other transactions until the transaction has been committed.
Isolation:A transaction should appear to be running by itself,the effects of
other ongoing transactions must be invisible to this transaction,and the
effects of this transaction must be invisible to other ongoing transaction.
Durability:When the transaction is committed,it must be persisted so it is not
lost in the event of a power failure.Only committed transaction are recovered
during power-up and crash recovery;uncommitted work is roll back.