Databases Reference
In-Depth Information
With
optimistic processing
techniques, the transaction processor doesn't
check for potential conflicts, like with scheduling or locking methods. Instead,
it assumes that there is little, or no, possibility of conflict between the transac-
tions. If an error is detected during transaction processing, it's necessary to abort
and roll back, then restart the transaction. Most database systems let you use
optimistic processing on an ad hoc basis in addition to other concurrency man-
agement methods. The current trend in database management system and data-
base application design is toward more optimistic methods of transaction man-
agement. This is an area that is continuing to evolve as manufacturers release
new versions of the DBMS products.
FOR EXAMPLE
Why Concurrency Matters
Most businesses have the same basic goal, to make money. That means that
they are looking for ways to either make more money, or reduce their
expenses, the cost of making money. Some expenses are obvious, such as
building, inventory, and salaries. Others are hidden and not always noticed.
That leads us to concurrency errors.
Many experts agree that cutting costs pays off better in profits, dollar-
for-dollar, than increasing revenues. Errors cost, including concurrency
errors. Think about the examples in this chapter, and you can start to see
some of the hidden costs, such as the following:
•
Lost sales due to inaccurate inventory and dirty reads.
•
Data inaccuracies that have to be corrected, such as through a costly,
time-consuming physical inventory.
•
Lost time, which costs more in employee wages, for the time
required to repost orders because of errors.
•
Lost administrator time troubleshooting problems that might or
might not really exist.
Administrators can lose a lot of time, and possibly have to cause addi-
tional server down time, looking for the source of intermittent problems.
Some of these problems could be caused by transient concurrency errors.
Because of the brief error duration and mixed symptoms, problems like this
can be especially hard to isolate and correct.
The cost of any one of these (except possibly the lost order) is minimal
when taken by itself. Over time, and over multiple occurrences, they can
add up to a measurable expense.
