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Lecture on System Design Principles in Software Development - CSCI 250

This lecture from CSCI 250 at Clark University, held on February 20, 2001, covers essential concepts in system design as outlined in Chapter 6 of the course. The session includes discussions on design terminology, decomposition, subsystem management, and the balance of coherence and coupling in software architecture. Students are reminded of upcoming deadlines, including the Functional Specifications due on Thursday. The focus remains on high-level service definitions rather than implementation details to effectively manage complexity in software systems. An exercise involving self-driving car design is also introduced.

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Lecture on System Design Principles in Software Development - CSCI 250

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  1. CSci 250Software Design & Development Lecture #11 Tuesday, Feb. 20, 2001 CSci 250 - Clark University

  2. Class Format for Today • Announcements • Questions • Lecture on Chapter 6 • Design Exercise CSci 250 - Clark University

  3. Announcements • EMC Career Fair @ Museum of Science in Boston: Weds., Feb. 28th, 4-7pm • Technology Research: • JNI • File system change notification • Filtering - see link on course web page • Reminder: Functional Specs due Thursday CSci 250 - Clark University

  4. Questions? • From last week: • Analysis • Rational Rose - How did installation go? • About Content Index • Anything else? CSci 250 - Clark University

  5. Chapter 6 System Design CSci 250 - Clark University

  6. System Design Terminology • Decomposition • Subsystem (Vs. Class) • Control Flow • Access Control • Services Vs. Interfaces • Coupling Vs. Coherence / Cohesion • Layers & Partitions CSci 250 - Clark University

  7. System Design • Focus: • Processes • Data Structures • Components • Challenges: • Managing conflicting criteria • Managing constraints CSci 250 - Clark University

  8. Impact on Implementation • Coding complexity of subsystems • Cost of rework if design changes • Design of individual components should be delayed - Why? CSci 250 - Clark University

  9. Software Architecture • Subsystem decomposition in terms of responsibilities • Dependencies among subsystems • Subsystem mapping to hardware • Policy decisions • Read & understand bullet list on page 171 CSci 250 - Clark University

  10. Subsystems Vs. Classes • Decompose system into simpler parts • Subsystems are composed of some number of classes • Complex subsystems may be further decomposed into smaller subsystems • How do you know when to stop? CSci 250 - Clark University

  11. Services Vs. Interfaces • Service is a set of related operations that share a common purpose (Example?) • Interface is a specification (API) that includes: • Names of operations • Parameter names & types • Return values CSci 250 - Clark University

  12. Focus of System Design • System Design focuses on defining services • Object Design focuses on defining interfaces • During System Design, keep it high level - focus on services rather than implementation. • Why? CSci 250 - Clark University

  13. Trade-off of conflicting goals • Sometimes increasing coherence also increases coupling - why? • Want a balance between high coherence and low coupling - how can we do both? CSci 250 - Clark University

  14. Layers Vs. Partitions • Both are ways to manage complexity • Layers are hierarchical; Partitions are not • Each layer uses services provided by which other layer(s)? • Define: Closed Architecture Vs. Open • See diagrams on pages 180, 181 • Layers have no knowledge of which other layer(s)? CSci 250 - Clark University

  15. Layers & Partitions Cont’d • Partitions are responsible for different classes of services • Which to use: Layers or Partitions? • Usually both • How do we know which to use when? • How much is too much? “Excessive partitioning or layering can lead to increased complexity.” Explain. CSci 250 - Clark University

  16. System Design Exercise Another look at The Car That Drives Itself CSci 250 - Clark University

  17. For Next Time: Object Design Chapter 7 CSci 250 - Clark University

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