1. CS 491z / CS 791z Topics on Software Engineering
Spring 2004
Introduction to RTS
January 26, 2004

2. RTS: A Characterization……
This section of the presentation is based on [Dascalu01]
“A real-time system must respond to externally generated stimuli within a finite, specifiable time delay” [Everett95]
An RTS differs from a “regular” (non-RTS) system in at least the following aspects [Stankovic88]:
Have deadlines attached to some or all tasks
Faults in the system may lead to catastrophic consequences
Must have the ability to deal with exceptions
Must be fast, predictable reliable, adaptive

3. .RTS: A Characterization.….
“Development of most software focuses on how to handle a normal situation, but real-time, critical-application development also focuses on how to handle the abnormal situation” [Everett95]
RTS “must operate under more severe constraints than ‘normal’ software yet perform reliably for long periods of time” [Douglass99]

4. ..RTS: A Characterization….
A classification of RTS:

5. …RTS: A Characterization…
Requirements for RTS:
Timeliness
Reaction to stimuli “on time” (deadlines must be met)
Relative and absolute timing constraints
Reliability
Many errors have roots in incorrect specification
Formal techniques needed for safety-critical systems
Intensive dynamics
Models to describe behavior are necessary (based on finite state machines)

6. ….RTS: A Characterization..
Requirements for RTS (cont’d):
Exception handling
Priorities should be assigned to stimuli/events
Mechanisms for handling interrupts need be developed
Concurrency
Parallel tasks are inherent in RTS
The environment is also “concurrent” in nature
Distribution & resource allocation
Distribution is not necessarily a characteristic of RTS, but should be taken into consideration in larger applications

7. …..RTS: A Characterization.
Requirements for RTS (cont’d):
Communication and synchronization
Synchronous and asynchronous communication mechanisms should be designed
Size
In larger applications, there are numerous processes and threads
Size is associated with continuous change
Decomposition in smaller units is needed, as are mechanisms for modeling hierarchical structures

8. .…..RTS: A Characterization
Requirements for RTS (cont’d):
Non time-constrained activities
Worst case scenarios cannot be easily evaluated
Computations & data modeling
In process control systems computations can be complex
In RT databases data must have temporal validity
Reuse
RTS are poor candidates for reuse (are too specialized)
However, OO design may provide solutions

9. Additional References
[Dascalu01] Dascalu, S., Combining Semi-forma and Formal Notations in Software Specification: An Approach to Modelling Time-Constrained Systems, PhD thesis, Dalhousie University,
Halifax, NS, Canada, 2001.
[Douglass99] Douglass, B.P., Doing Hard Time: Developing Real-Time Systems with UML, Objects, Frameworks and Patterns, Addison-Wesley,
[Everett95] Everett, W., and Honiden, S., “Reliability and Safety of Real-Time Systems,” IEEE Software, 12(3), May 1995, p
[Gibbs94] Gibbs, W.W., “Software’s Chronic Crisis,” Scientific American, Sep. 1994, p
[Stankovic88] Stankovic, J.A., and Ramamritham, K., Tutorial: Hard Real-Time Systems, IEEE Computer Society Press, 1988.