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In order to
understand why CICS is necessary, you must understand what a transaction
is. The definition presented in the
article by Comaford is quite common and is known as is ACID definition. ACID stands for Atomic, Consistent,
Isolated, Durable. [Comaford]
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Atomic means that a
transaction will operate on one or more tables and rows such that either all
of the changes occur or none of the changes occur. This group
is known as a unit of work (UOW). A Commit occurs when all of the
changes succeed and the changes are made permanent. A Rollback occurs to reverse the changes
and the database is restored to how it was before the start of the UOW.
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Consistency means
that the database conforms to all of its rules at the end of any work. For example, this might mean that no two
row identifiers are the same.
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Isolated means that
changes performed by one person cannot be seen or interfere with anyone else
until they are committed. It is as though you have your own private database.
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Durable means that
once changes to the database are committed, the change can live through a
system failure. Uncommitted changes
cannot survive a system failure. [Nusphere]
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