NonStop Systems Introduction

Transaction Management

NonStop Systems Introduction—527825-001

5-2

What Is a TMF Transaction?

Figure 5-1 on page 5-2 shows how a failure during the processing of a money-transfer

transaction could cause the database to become inconsistent. In this example, the

customer 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 gets 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, it puts the database in a

new consistent state, where the savings balance is $4000 and the checking balance is

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