NonStop Systems Introduction

Transaction Management

NonStop Systems Introduction—527825-001

5-6

Audited Files and Audit Trails

The audit trail shown in Figure 5-3 is a separate group of disk files that contain

information about each TMF transaction against audited database files. The audit trail

information for each transaction includes a before-image and an after-image of each

database record that was changed by the transaction. The before-image shows the

value of each field in the record before the update. The after-image shows the value of

each field in the record after the update.

Note that the audit trail for each audited database file in Figure 5-3 is mirrored, which

means that the audit trail is stored on mirrored disk volumes (two disk volumes that

hold identical data). The mirroring of the audit trail disks ensures that the audit trail

information will still be available if one of the disks fails. It is also possible to mirror the

disks that hold the database.

TMF uses audit trail information for two purposes:

•

To undo transactions from the database when they fail before completion

•

To reapply successful transactions to the database when a system or disk fails

But TMF has to do more than maintain an audit trail to keep its control over a

transaction. It relies on the operating system to lock all the database records affected

by the transaction for the duration of the transaction. Locking a record for a transaction

simply means preventing other transactions from accessing and changing the record

until the transaction is over.

Because of record locking, TMF has a stable before-image and after-image of each

record changed by a transaction. It can undo or redo the database changes made by a

Figure 5-3. Audited Files and Audit Trails

Sales office

Headquarters

Warehouse

Begin

Mirrored audit trail

Update #1

Update #3

Update #2

Order entry system databases

Before After

Locked

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