The MPE/iX Transaction Management Facility
provides the following functions for transaction-oriented
applications:
Automatic transaction locking
Automatic transaction logging
Automatic rollback recovery from "soft" failures
Semi-automatic rollforward recovery from "hard"
failures
These functions are described in detail in the subsections below.
Transaction Locking |
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A transaction is a series of data updates that must be entirely
completed to obtain logical consistency; either all or none of
the updates in the series must be done.
To the user, a transaction may appear to be a single change.
However, internal to the system, it may require many changes
to records in several files or data sets.
For example, if you enter a customer order on the system
with a simple command, it may internally
require updating several files
containing data on orders, customers, and inventory requirements.
The multiple changes, all of which are required
for logical consistency, form one transaction.
The MPE/iX Transaction Management Facility ensures data
consistency and integrity by providing
automatic transaction locking.
Transaction locking meets two criteria:
Only one transaction at a time is allowed to update a given
portion of data.
All changes that are part of a transaction must be
completed before the changes are committed to disc (permanently
recorded). When a transaction is abnormally terminated before it is
completed, the changes made up to that point are not committed.
MPE/iX transaction locking is based on page-level protection
of the system architecture and does not require significant CPU overhead.
Transaction Logging and Recovery |
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The Transaction Management Facility automatically generates
and maintains a transaction log file that records all transaction
updates. Maintaining a log file facilitates recovery from
the abnormal end of transactions and system failures.
In these cases, files can be restored to a consistent state
by copying the contents of the log file into the data file.
This "undoes" the actions of partially committed
transactions.
System failures are either hard or soft.
When a soft failure occurs, data is not altered or lost, but
some incomplete transactions may exist.
In this case, the Recovery Manager portion of the
Transaction Management Facility is automatically invoked
to perform recovery of the data file when the system is restarted.
Files are restored to their original state by copying
the "before image" of data from the log file for incomplete
transactions. This is called rollback recovery.
Recovery from a soft failure is efficient and takes a maximum
of only a few minutes.
Rollback recovery is automatically performed for
abnormally ended transactions.
When a hard failure occurs (for example, a
disc media failure), data is lost.
At system restart, you must mount a backup tape and
issue the MPE/iX command :RECOVER.
The Transaction Management Facility returns data to a
consistent state by reapplying all committed transactions
in the log file to the checkpoint presented by the
backup tape. This is called "rollforward recovery."
Transaction logging requires little CPU overhead because
it is designed as an integral part of the MPE/iX File System,
utilizing the addressing and protection features of HP
Precision Architecture (HP-PA).
Simplifying a Program |
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The Transaction Management Facility simplifies
development and maintenance of transaction-oriented applications
by:
Maintaining recovery routines, so a programmer does not have
to develop and maintain custom ones.
Simplifying the programming task of ensuring
data integrity and increasing its efficiency.
Providing locking and logging, thus requiring the
programmer to mark only the beginning and end of transactions.
