The document discusses several topics related to maintaining computer systems. It addresses the importance of documenting maintenance activities, the need to replace aging systems rather than modify them, and some common maintenance problems like dealing with unpredictable changes and using maintenance teams for training. It also covers database backup strategies, recovery techniques for transaction errors or full system failures, reasons for enhancing or replacing existing systems, the need for security against both accidental and deliberate threats, and an IBM study ranking errors and omissions as the largest threats.

1. SYSTEMS MAINTENANCE
Maintenance activities must be fully documented to assist future
maintenance programmers.
As a rule, we can say that experience teaches that a system which is
five to six years old is likely to need complete replacement, rather
than modification.
MAINTENANCE PROBLEMS
• Program Maintenance: Whilst predictable changes should be
built-in to the system, unpredictable changes such as latent bugs, law
changes, changes in business practice will all have to be dealt with.
• Maintenance Team: The maintenance team is an excellent
training ground for new programmers and analysts. Sometimes the
development staff rotate with the maintenance team for specific projects.
• Database Back-up and Recovery:

2. Database Back-up and Recovery
Databases are subject to much activity of a complex
nature. From time to time, something will go wrong.
• A transaction error such as the wrong record being
retrieved or a programming error being
highlighted.
• The whole system going down due to power loss
or software failure.
• Complete or partial database destruction due to
some natural disaster or major operator error.

3. Database Back-up and Recovery
cont…
• Whichever fault occurs, it is necessary that the database
be recovered. To make this possible, a back-up strategy is
necessary and a simple model is regularly to dump the
database contents in a safe area (e.g. copy it to separate
disks), and to keep a log of all subsequent transactions to
the next dump, and so on
The following recovery techniques, are in general use:
• Following a transaction error, roll the database back (back
out) to the state prior to the transaction using the original
unaltered versions of the records and start again.
• Following a whole system failure roll back the database, as
for a transaction error, through all the uncompleted
transactions and start again.

4. SYSTEM ENHANCEMENTS
• Every system will require enhancement.
• Sooner or later, a choice must be made between enhancement (of the existing
system) and development (of a new system).
(a) Reasons for Change
• Several reasons for change lead to this point. Examples could be:
• General frustration with the limitations of the present system.
• The attraction of new technology.
• A relatively new system that has several severe design faults.
• Management decision on an expansion of the computing function.
• It will depend very much on the particular circumstances
(b) Points to Consider There are several points to consider.
• Enhancement entails less risk, as it involves a proven system.
• Documentation is easier – most is already in use.
• Documentation can be quite informal, as staff are already familiar with the system.
• However, amending and testing existing code is far more time-consuming than
writing brand new code.

5. NEED FOR SYSTEM SECURITY
• Publicity is frequently given to damage to
computer equipment through fire or flood,
and to computer-related crimes. These
breaches of security bring to our attention the
need to protect computer systems.
• A computer system cannot be 100% secure,
but measures must be taken to reduce the risk
to an acceptable level.

6. Categories of Threat
• There are two main categories of threat to security – accidental and
deliberate threats. These can be split into the following sub-
categories:
(a) Accidental Threats These include:
• Malfunctioning of a hardware component
• Modification of software
• Naturally occurring threats such as fire/flood/earthquake, and
interruptions or surges in power supply
• Death or injury of people, affecting their capacity to do normal work
• Interruption of data transmission lines
(b) Deliberate Threats These are:
• Removal or corruption of programs/data
• Disclosure of information
• Withdrawal of labour

7. Order of threats
The following order of threats was produced as the
result of a study by IBM. It is significant in that over
50% of all threats originate from errors and
omissions:
• Errors and omissions (over 50%)
• Dishonest employees
• Fire
• Disgruntled employees
• Water
• Intruders and others (less than 5%)