1. December 9, 2001Architectural Design, ECEN 50331 Architecture – Layers, in particular ECEN 5543 / CSCI 5548 SW Eng of Standalone Programs University of Colorado, Boulder

2. December 9, 2001Architectural Design, ECEN 50332 Design Pattern Roots Text describes various arch. patterns as codified learning Older techniques include –Levels of Abstraction –Integrated Top-Down –Information Hiding Overall principles of –abstraction –structure –verification –modularity –business influence and impact -- extensibility

3. December 9, 2001Architectural Design, ECEN 50333 POSA 1 & POSA 2 Patterns-Oriented Software Architecture, Volume 1: A System of Patterns; Frank Buschmann, Regine Meunier, Hans Rohnert, Peter Sommerlad; John Wiley & Sons; 1996; ISBN: 0471958697 (On Amazon, 4.5 stars out of 5) (Volume 2 subtitle: Concurrent and Networked Objects, Douglas Schmidt, Michael Stal, Hans Rohnert, Frank Buschmann, 2000) “Patterns help you build on the collective experience of skilled software engineers. “ (1 st sentence)

4. December 9, 2001Architectural Design, ECEN 50334 Patterns put you on the shoulders of Giants Isaac Newton famously remarked in a letter to Robert Hooke, dated 5 February 1676: "If I have seen a little further it is by standing on the shoulders of Giants.” (Earliest use of the metaphor is John Salisbury, 1159) When Hamming received the Turing award in 1968. he contrasted software developers to Newton saying something like “While Newton may have stood on the shoulders of Giants, in software development, we stand on each other’s feet.”

5. December 9, 2001Architectural Design, ECEN 50335 What is the Layers Pattern? Architectural pattern that helps to structure applications that can be decomposed into groups of subtasks in which each group of subtasks is at a particular level of abstraction. What is high? What is low? Flow –requests from higher layer to lower layer –answers from lower layer to higher layer –incoming data or event notification from low to high

6. December 9, 2001Architectural Design, ECEN 50336 Layers Arch.

7. December 9, 2001Architectural Design, ECEN 50337 Balance the Forces Late source code changes should not ripple Interfaces should be stable Interfaces may be prescribed by a standards body Parts of the system should be exchangeable –Design for change! May be necessary to build other systems at a later date with same low-level issues Similar responsibilities should be grouped to help understandability and maintainability No standard component granularity Complex components may need further decomposition

8. December 9, 2001Architectural Design, ECEN 50338 More forces Crossing component boundaries may impede performance –substantial amount of data transferred over several boundaries The system will be built by actual human beings –Work must be subdivided along clear boundaries –Architecture influences team structure and vice versa

9. December 9, 2001Architectural Design, ECEN 50339 Solution Structure the system into an appropriate number of layers Think of lowest level as Layer 1; highest Layer N This gives a conceptual view – not marching orders regarding sequence of further design Within a layer, all subcomponents work at the same level of abstraction Most of what Layer J provides is composed of services provided by Layer J-1, combined in a meaningful way for the application.

10. December 9, 2001Architectural Design, ECEN 503310 Scenario I

11. December 9, 2001Architectural Design, ECEN 503311 Scenario II

12. December 9, 2001Architectural Design, ECEN 503312 Scenario III

13. December 9, 2001Architectural Design, ECEN 503313 Scenario IV

14. December 9, 2001Architectural Design, ECEN 503314 Scenario V

15. December 9, 2001Architectural Design, ECEN 503315 Implementation Steps 1.Define which of the abstraction criteria you will use 2.Determine the number of abstraction levels according to your criterion 3.Name the layers and assign tasks to each of them 4.Specify the services 5.Refine the layering 1.repeat steps 1-4 until natural, stable layering evolves 2.Finding layers is not orderly – yo-yo development 6.Specify an interface for each layer

16. December 9, 2001Architectural Design, ECEN 503316 Implementation Steps -- continued 7.Structure individual layers 8.Specify communication between adjacent layers 1.push/pull 9.Decouple adjacent layers 1.top-down: J+1 knows about J; J can ignore J+1 2.bottom-up: 1.can use callbacks 2.can decouple the upper from the lower somewhat 10.Design an error-handling strategy

17. December 9, 2001Architectural Design, ECEN 503317 Known Uses Virtual Machines APIs Information systems – lower layer is database –Presentation –Application logic –Domain layer –Database Some operating systems – Windows NT

18. December 9, 2001Architectural Design, ECEN 503318 Benefits

19. December 9, 2001Architectural Design, ECEN 503319 Liabilities

20. December 9, 2001Architectural Design, ECEN 503320 Bibliography (cont.) Pattern-Oriented Software Architecture, A System of Patterns, Volume 1, Frank Buschmann, Regine Meunier, Hans Rohnert, Peter Sommerlad, Michael Stal, Wiley & Sons, 1996, ISBN 0 471 95869 7 – often referred to as the POSA book or “POSA 1” because there are now 2 volumes. Go to Amazon.com and read the reviews to get a feel for how well respected this book is.