Introduction to Data Management

Ensuring Database Consistency With TMF
7-2 15873 Tandem Computers Incorporated
Features TMF offers application developers many powerful features, all of which support the
overall goal of fault tolerant protection for your applications and their databases. The
main benefits of TMF are outlined as follows.
Protection Against
Transaction Failures
An individual transaction can fail for a variety of reasons. For example, a transaction
can fail because a system hardware or software error occurs, because a user program
error takes place, because a user program deliberately issues a request to abort the
transaction, or because a system manager or operator intervenes while the transaction
is in process. For example, any of the following events can terminate a transaction
prematurely:
The CPU on which the server is running fails, and the TCP must restart the
transaction on another CPU
The TCP becomes disabled, and the backup TCP must restart the transaction
In an application accessing a distributed database, network communication
between participating nodes is lost in mid-transaction
To protect your database against this kind of failure, TMF ensures that either all
changes requested by a transaction are permanently recorded in the database, or that
none of them are. Thus, if a transaction fails while in progress, TMF backs out the
changes that the transaction imposed up to that point, restoring the database to the
state it was in prior to the beginning of the transaction. This backout undoes the
effects of any transaction that aborts before all of its changes become permanent. The
backout action is illustrated in Figure 7-1.