Tim Harsch ([EMAIL PROTECTED]) wrote:
> What is ACID?
>From http://openacs.org/philosophy/why-not-mysql.html (I highly
recommend this article for anyone who is wondering what RDBMS
to choose):
Atomicity
Results of a transaction's execution are either all
committed or all rolled back. All changes take effect,
or none do. Suppose that a user is editing a
comment. A Web script tells the database to "copy the
old comment value to an audit table and update the
live table with the new text". If the hard
drive fills up after the copy but before the update,
the audit table insertion will be rolled back.
Consistency
The database is transformed from one valid state to
another valid state. A transaction is legal only if
it obeys user-defined integrity constraints. Illegal
transactions aren't allowed and, if an integrity
constraint can't be satisfied the transaction is
rolled back. For example, suppose that you define a rule
that postings in a discussion forum table must be
tied to a valid user ID. Then you hire Joe Novice
to write some admin pages. Joe writes a delete-user
page that doesn't bother to check whether or not the
deletion will result in an orphaned discussion forum
posting. Oracle will check, though, and abort
any transaction that would result in you having
a discussion forum posting by a deleted user.
Isolation
The results of a transaction are invisible to other
transactions until the transaction is complete.
For example, suppose you have a page to show new
users and their photographs. This page is coded in
reliance on the publisher's directive that there
will be a mugshot for every user and will present a
broken image if there is not. Jane Newuser is
registering at your site at the same time that Bill
Olduser is viewing the new user page. The script
processing Jane's registration does inserts into
several tables: users, mugshots, users_demographics.
This may take some time if Jane's mugshot
is large. If Bill's query starts before Jane's
transaction commits, Bill won't see Jane at all
on his new-users page, even if Jane's insertion
into some of the tables is complete.
Durability
Once committed (completed), the results of a
transaction are permanent and survive future
system and media failures. Suppose your ecommerce
system inserts an order from a customer into a
database table and then instructs CyberCash to
bill the customer $500. A millisecond later,
before your server has heard back from CyberCash,
someone trips over the machine's power cord.
Oracle will not have forgotten about the new order.
Furthermore, if a programmer spills coffee into a
disk drive, it will be possible to install a new
disk and recover the transactions up to the coffee
spill, showing that you tried to bill someone for
$500 and still aren't sure what happened over at
CyberCash.
--
Mike Slack
[EMAIL PROTECTED]
http://www.aworldthatworks.com/
--
"If we knew what it was we were doing, it wouldn't
be called research, would it?" --Albert Einstein
