Page 171 - DCAP305_PRINCIPLES_OF_SOFTWARE_ENGINEERING

Unit 8: Detailed Design

inform the caller that an error has occurred. The calling module can then check the error flag for Notes
successful completion of the called module. It should be unnecessary for the caller to check the
other module outputs. It is possible for a subordinate to fail to return control. Modules should
normally set a ‘timeout’ when waiting for a message, a rendezvous to be kept, or an event flag
to be set. Modules should always act appropriately after a timeout (e.g. retry the operation).
The principle of immediate detection means that possible errors should be checked for
immediately. If the reaction is not immediate, the error should be flagged for later action.
Conventions for taking action after detection of an error should be established. Normally error
control responsibilities of a module should be at the same level as normal control responsibilities.
Error logging, if done at all, should be done at the point where action is taken, and not at the
point of error detection, since the significance of the error may not be apparent at the point of
detection. It may be appropriate to insert diagnostic code at the point of detection, however.
Only the top level module should have responsibility for stopping the program. Developers
should always check that there are no ‘STOP’ statements lurking in an otherwise apparently
useful module. It may be impossible to reuse modules that suddenly take over responsibility
for the control flow of the whole system. Halting the program and giving a trace back may be
acceptable in prototype software (because it helps fault diagnosis), but not in operational software.
In detailed design, redundancy considerations can lead designers to include checksums in records
and identity tags to confirm that an item is really what it is assumed to be (e.g. header record).
Myers also makes a useful distinction between ‘passive fault detection’ and ‘active fault detection’.
The passive fault detection approach is to check for errors in the normal flow of execution.
Examples are modules that always check their input, and status codes returned from system
calls. The active fault detection approach is to go looking for problems instead of waiting for
them to arise. Examples are ‘monitor’ programs that continuously check disk integrity and
attempts to violate system security. Often most system code is dedicated to error handling.
Library modules for error reporting should be made available to prevent duplication of error
handling code. When modules detect errors, they should call error library modules to perform
standard error handling functions such as error logging and display.
Defensive design principles have influenced the design of most modern languages. Strong
type-checking languages automatically check that the calling data type matches the called data
type, for example. Analysis and design algorithm goes further and builds range checking into
the language. The degree to which a language supports defensive design can be a major factor
in its favours.

8.1.6 Optimization
Conventionally, optimization earnings to make the best compromise between opposing
tendencies. Improvement in one area is often associated with degradation in another. Software
performance is often traded-off against maintainability and portability, for example.
The optimization process is to:
• Define the attributes to change (e.g. execution time);
• Measure the attribute values before modifying the software;
• Measure the attribute values after modifying the software;
• Analyze the change in attribute values before deciding whether to
• Modify the software again.
Optimization can stop when the goals set in the SRD have been met. Every change has some
risk, and the costs and benefits of each change should be clearly defined. The law of diminishing
returns can also be used to decide when to stop optimization. If there are only slight improvements
in the values of attribute values after optimization, the developers should stop trying to seek
improvements.

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