NonStop Systems Introduction for H-Series RVUs

Transaction Management

NonStop Systems Introduction for H-Series RVUs—540083-001

5-2

What Is a TMF Transaction?

transfers $1000 from a $5000 savings account to a $50 checking account. When the

transaction ends, the customer should have $4000 in the savings account and $1050

in the checking account.

Note that if a failure occurs after the transaction has debited the savings account by

$1000 but before it has credited the checking account with $1000, the database is

inconsistent because the checking account record no longer has a correct relationship

with the savings account record. The decrease in the savings balance is not matched

by an increase in the checking balance.

This is where TMF plays an important role. It ensures that a group of update

operations is applied as one logical unit of work. Either they all happen or none of them

happen. TMF takes the concept of a transaction as a group of computer operations

and redefines it as a single unit of work that is recovered as a whole in the event of a

failure.

For example, if a failure interrupts the money-transfer transaction at the point shown in

Figure 5-1, TMF backs out the change made to the savings account record. That is, it

cancels the new savings balance by restoring the old balance. As a result, the

database looks exactly the way it looked before the transaction started. It shows $5000

in the savings account and $50 in the checking account. The database is back in a

consistent state because the savings account record and the checking account record

agree with each other.

When the transaction is redone and successfully completed, the database is in a new

consistent state, with a savings balance of $4000 and a checking balance of $1050. To

summarize, the purpose of TMF is to enable transactions to transform the database

from one consistent state to another consistent state.

Figure 5-1. Money-Transfer Transaction

Money-transfer transaction

Database records

Transaction begins

Savings balance Checking balance

Savings account debited

$1000

Checking account credited

$1000

Transaction ends

$5000

$4000

$50

$1050

Failure point

VST056.vdd