BERNADETTE BRZEZINA

1
Observer and Reactor A Real-Time Comparison

• GROUP 2
• BERNADETTE BRZEZINA
• PATRICK GAVIGAN
• NADINE HILL
• WILLIAM KNISELY
• SYSC 4805/4806
• WINTER 2008

2
Outline

• Refresher on Design Patterns
• Observer
• Description
• Design Considerations
• Real-Time Applications
• Reactor
• Description
• Design Considerations
• Real-Time Applications
• Compare and Contrast

3
What is a Design Pattern?

• Facilitates reuse of successful designs, not code
  reuse
• Common solutions to similar problems
• Standard strategies
• Interface reuse

4
Observer Description

• Define a one to many dependency between objects
  so that when one object changes state, all its
  dependencies are notified and updated
  automatically
• Allow multiple clients to effectively share a
  server and autonomously be updated

5
Observer Class Diagram
6
Observer Sequence Diagram
7
Observer Design Considerations

• Advantages
• Decouples observer from subject
• Reuse observers and subjects independently
• Easily add or take away observer

8
Observer Design Considerations

• Disadvantages
• Decreased performance when there are many
  observers
• Possible memory leaks
• Hidden dependencies
• Unexpected updates

9
Observer Real-Time Applications

• Graphical user interfaces.
• Can be used for data acquired through sensors
• Can add heart-beat functionality

10
Reactor - Description

• Handles events from multiple sources/clients a
  many to one relationship
• Dispatches event handlers automatically in
  response to external events
• Allow multiple clients to effectively connect to
  a server and independently access its services

11
Reactor UML Class Diagram
12
Reactor UML Sequence Diagram
13
Reactor Design Considerations

• Advantages
• Allows for the combination of multiple events,
  without introducing additional threads avoiding
  expensive context switching
• Simplifies event-driven applications by
  decoupling event demultiplexing and handler
  dispatching from event application services
• Easily add or take away event handlers (compile
  time)

14
Reactor Design Considerations

• Disadvantages
• Performance limited by the number and computation
  time of handlers
• Scalability limited by the demultiplexing and
  synchronous calling of handlers
• Can be very hard to debug due to the
  non-procedural design

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Reactor Real-Time Applications

• Event logging servers
• Graphics systems
• Telephone switchboard systems
• Asset tracking systems

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Conclusion

• Observer and Reactor are design patterns often
  used in real-time applications
• Observer
• Can be used for client-server interaction
• Notifies clients of updates
• One to many
• Reactor
• Can be used for client-server interaction
• Responds to external events (clients)
• Many to one

17
References

• Schmidt, Douglas C., Using Design Patterns to
  Develop Reusable Object-Oriented Communication
  Software, COMMUNICATIONS OF THE ACM October
  1995/Vol. 38, No. 10
• Porter, A., Design Patterns, March 21, 2002
• Shirazi, J., Java Performance Tuning, np
  OReilly, nd
• http//pocoproject.org/poco/docs/Poco.Net.SocketRe
  actor.html
• http//citeseer.ist.psu.edu/cache/papers/cs/13106/
  httpzSzzSzwww.esconline.comzSzdb_areazSz98springz
  Sz225.pdf/real-time-design-patterns.pdf
• http//www.cs.umd.edu/class/spring2002/cmsc433-020
  1/Lectures/designPatterns.pdf
• http//www.dofactory.com/Patterns/PatternObserver.
  aspx
• http//www.ibm.com/developerworks/webservices/libr
  ary/ws-dbarch/

18
Thank You

• Questions?