1. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 1 Informatics 43 Introduction to Software Engineering Lecture 2 Duplication of course material for any commercial purpose without the explicit written permission of the professor is prohibited.

2. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 2 Today’s lecture Programming versus software engineering Complexity, conformity, changeability, intangibility Software architecture Examples Some slides adopted and adopted from “Software Architecture: Foundations, Theory, & Practice” by Taylor, Medvidovic, and Dashofy

3. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 3 Today’s lecture Programming versus software engineering Complexity, conformity, changeability, intangibility Software architecture Examples

4. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 4 Essential characteristics Software engineering concerns the development of large programs The central theme is mastering complexity The efficiency with which software is developed is of crucial importance Software evolves Regular cooperation between people is an integral part of programming-in-the-large The software has to support its users effectively Software engineering is a field in which members of one culture create artifacts on behalf of members of another culture Software engineering is a balancing act

5. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 5 Programming versus software engineering Small projectLarge to huge project YouTeams Build what you wantBuild what they want One productFamily of products Few sequential changesMany parallel changes Short-livedLong-lived CheapCostly Small consequencesLarge consequences ProgrammingSoftware engineering

6. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 6 From programming to software engineering People – who else would do the work? – range from novice to very experienced Processes – to organize and manage the efforts of individuals – range from informal to very formal Tools – to support the people and the processes – range from simple to very advanced

7. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 7 People The single most important factor in the success/failure of a product Scarce resource – quality – suitability – cost Many different kinds of people – managers – programmers – technical writers

8. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 8 Processes Essential to achieve a quality product Scarce resource – quality – suitability – cost Many different kinds of processes – bug tracking – change approval – quality assurance

9. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 9 Tools Needed to support people and processes Scarce resource – quality – suitability – cost Many different kinds of tools – drawing – analysis – project management – source code management

10. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 10 Today’s lecture Programming versus software engineering Complexity, conformity, changeability, intangibility Software architecture Example #1: Linux Example #2: iRADS Example #3: HADOOP Architectural styles Principles of software engineering

11. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 11 Brooks – Mythical Man Month Accidental versus essential difficulties Accidental difficulties – people shortage – not using the right tools – wrong design choice – … Essential difficulties – complexity – conformity – changeability – intangibility

12. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 12 Complexity No two software parts are alike – if they are, they are abstracted away into one Complexity grows non-linearly with size – e.g., it is impossible to enumerate all states of a program – except perhaps “toy” programs

13. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 13 Conformity Software is required to conform to its – operating environment – hardware Often “last kid on block” Perceived as most conformable

14. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 14 Changeability Change originates with – new applications, users, machines, standards, laws – hardware problems Software is viewed as infinitely malleable

15. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 15 Intangibility Software is not embedded in space – often no constraining physical laws No obvious representation – e.g., familiar geometric shapes

16. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 16 Drastic consequences Deceased patients – x-ray machine delivered very high doses because of a timing problem in its control software Crashed planes – software prevented pilots from performing emergency maneuvers – software had similar codes for different airports Decreased national security – NSA computers down for four days due to a “software problem” Peter Neumann’s Risks Digest: http://catless.ncl.ac.uk/Risks

17. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 17 Today’s lecture Programming versus software engineering Complexity, conformity, changeability, intangibility Software architecture Examples

18. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 18 Software architecture Requirements Code

19. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 19 Software architecture Requirements Code

20. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 20 An analogy to building architectures We all live in them (We think) We know how they are built – requirements – design (blueprints) – construction – use This is similar (though not identical) to how we build software

21. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 21 Parallels Design before build Satisfaction of customers’ needs Specialization of labor Multiple perspectives of the final product Intermediate points where plans and progress are reviewed

22. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 22 The architect A distinctive role and character in a project Very broad training Amasses and leverages extensive experience A keen sense of aesthetics Deep understanding of the domain – properties of structures, materials, and environments – needs of customers

23. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 23 Limitations of analogy Software serves a much broader range of purposes We know a lot about buildings, much less about software The nature of software is different from that of building architecture Software is much more malleable than physical materials Software is a machine; a building is not

24. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 24 But still very real power of architecture Giving preeminence to architecture offers the potential for – intellectual control – conceptual integrity – effective basis for knowledge reuse – realizing experience, designs, and code – effective project communication – management of a set of variant systems Limited-term focus on architecture will not yield significant benefits!

25. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 25 Defining software architecture A software system’s architecture is the set of principal design decisions about the system Software architecture is the blueprint for a software system’s construction and evolution Design decisions encompass every facet of the system under development – structure – behavior – interaction – non-functional properties

26. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 26 “Principal” “Principal” implies a degree of importance that grants a design decision “architectural status” – it implies that not all design decisions are architectural – that is, they do not necessarily impact a system’s architecture How one defines “principal” will depend on what the stakeholders define as the system goals

27. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 27 Architecture in action: WWW This is the Web

28. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 28 Architecture in action: WWW So is this

29. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 29 29 Architecture in action: WWW And this

30. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 30 WWW in a (Big) Nutshell The Web is a collection of resources, each of which has a unique name known as a uniform resource locator, or “URL” Each resource denotes, informally, some information URI’s can be used to determine the identity of a machine on the Internet, known as an origin server, where the value of the resource may be ascertained Communication is initiated by clients, known as user agents, who make requests of servers. – Web browsers are common instances of user agents

31. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 31 WWW in a (big) nutshell (continued) Resources can be manipulated through their representations – HTML is a very common representation language used on the Web All communication between user agents and origin servers must be performed by a simple, generic protocol (HTTP), which offers the command methods GET, POST, etc. All communication between user agents and origin servers must be fully self-contained (so-called “stateless interactions”)

32. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 32 WWW’s architecture Architecture of the Web is wholly separate from the code There is no single piece of code that implements the architecture There are multiple pieces of code that implement the various components of the architecture – e.g., different Web browsers

33. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 33 WWW’s architecture (continued) Stylistic constraints of the Web’s architectural style are not apparent in the code – the effects of the constraints are evident in the Web One of the world’s most successful applications is only understood adequately from an architectural vantage point

34. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 34 Today’s lecture Programming versus software engineering Complexity, conformity, changeability, intangibility Software architecture Examples

35. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 35 Prescriptive versus descriptive architecture A system’s prescriptive architecture captures the design decisions made prior to the system’s construction – it is the as-conceived or as-intended architecture A system’s descriptive architecture describes how the system has been built – it is the as-implemented or as-realized architecture

36. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 36 Linux – prescriptive architecture

37. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 37 Linux – descriptive architecture

38. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 38 iRODS – prescriptive architecture

39. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 39 iRODS – descriptive architecture

40. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 40 HADOOP – prescriptive architecture

41. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 41 HADOOP – descriptive architecture

42. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 42 HADOOP + MapReduce

43. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 43 HADOOP – complete architecture

44. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 44 Gap A gap remains between the prescriptive architecture, which concerns decisions, and the descriptive architecture, which concerns programmatic elements

45. Department of Informatics, UC Irvine SDCL Collaboration Laboratory Software Design and sdcl.ics.uci.edu 45 Assignment 2 Will be out on Wednesday