NonStop Systems Introduction

Transaction Management
NonStop Systems Introduction—527825-001
5-8
Transaction Backout
What has really happened to the database now that the transaction is successfully
completed? After transaction commitment, all the changed records are changed
permanently. TMF can no longer undo the changes. Another transaction can now
access the changed records and change them in its turn.
Transaction Backout
You have seen how TMF handles successful transactions. Essentially, at the end of
such transactions, TMF says, “All the updates have been done, so they can stay in the
database.” TMF can no longer undo such transactions, but it can use its audit trails to
reapply the transactions to the database, if necessary.
But what happens when a transaction cannot complete all its changes successfully?
What does TMF do then? Figure 5-5 on page 5-9 shows how TMF handles a
transaction that has failed before being able to complete all its database changes.
Figure 5-4. Commitment of a Successful Transaction
Sales Office
Headquarters
Warehouse
Mirrored audit trail
Databases
Before After End
Unlocked
Unlocked
Unlocked
VST059.vsd
End Transaction
Begin