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

December 9, 2001Architectural Design, ECEN 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

December 9, 2001Architectural Design, ECEN 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: (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)

December 9, 2001Architectural Design, ECEN 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 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.”

December 9, 2001Architectural Design, ECEN 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

December 9, 2001Architectural Design, ECEN Layers Arch.

December 9, 2001Architectural Design, ECEN 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

December 9, 2001Architectural Design, ECEN 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

December 9, 2001Architectural Design, ECEN 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.

December 9, 2001Architectural Design, ECEN Scenario I

December 9, 2001Architectural Design, ECEN Scenario II

December 9, 2001Architectural Design, ECEN Scenario III

December 9, 2001Architectural Design, ECEN Scenario IV

December 9, 2001Architectural Design, ECEN Scenario V

December 9, 2001Architectural Design, ECEN 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

December 9, 2001Architectural Design, ECEN 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

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

December 9, 2001Architectural Design, ECEN Benefits

December 9, 2001Architectural Design, ECEN Liabilities

December 9, 2001Architectural Design, ECEN 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 – 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.