lifecycle.ppt

1、1/24/2001,1,The Software Life Cycle,Professor James Landay CS169: Software Engineering Spring 2001 January 24, 2001,1/24/2001,2,Outline,Review Life cycle models Administrivia Improving the SE process,1/24/2001,3,Review,What is the software process? the activities, techniques, tools, and individuals

2、to produce software Documentation important for each phase. Why? people potential of software unknown more complex systems attempted more features, more sophistication greater complexity, more chances for error heterogeneous users,1/24/2001,6,Lifecycle Models,Build-and-fix develop system without spe

3、cs or design modify until customer is satisfied Why doesnt build-and-fix scale? changes during maintenance most expensive!,1/24/2001,7,Lifecycle Models,Staged models successive stages of development e.g., waterfall model,1/24/2001,8,Lifecycle Models,Rapid prototyping build something users can unders

4、tand in each spiral: identify potential risks plan next step based on risk analysis refine design in highest-risk areas Explicitly attempts to identify potential problems not just in initial stages of design also later, when more has been learned about the problem and the design What are the “risky”

5、 parts of the system relies on developer experience,1/24/2001,21,Spiral Model,1/24/2001,22,Assessing the Spiral Model,Risk assessment how does one do it? how is it known that “proper” risks have been identified? “experience” is a critical factor what if a new software system is being attempted? mode

6、l fails if risks are inaccurately assessed Not clear it works for contract projects how do you cancel a contract in the middle? Mainly for very large projects risk assessment could cost more than development!,1/24/2001,23,Perils & Promises of Lifecycles,Managers love em defines a set of “deliverable

7、s” sound bites can tell upper management that “stage so-and-so is completed” formally mandated by many government agencies Programmers find them inadequate not representative of what happens in the “trenches” customers cant adequately state requirements up-front stages become intermixed w/ others e.

8、g., design reveals that part of specification cant be cost-effectively implemented,1/24/2001,24,Administrivia,Roll call No webcast until at least next week (at earliest) Any other admin issues?,1/24/2001,25,Improving the Process,Why does it need improvement? we still struggle with creating/maintaini

9、ng software 87 Department of Defense report 20 years of “unfulfilled promises about productivity & quality” problem is “inability to manage the software process” global economy dependent on computer software improved software process - ? improved software quality delivery on time, within budget Resp

10、onses to this problem Software Engineering Institute (SEI) at Carnegie Mellon capability maturity model (CMM) - 1986,1/24/2001,26,Capability Maturity Models,Strategies for improving software process irrespective of life-cycle model Main tenet SW techniques not the problem, management is! improved ma

11、nagement improved techniques Change cannot occur overnight induce changes incrementally five levels of “maturity” defined what is maturity ?,1/24/2001,27,Maturity Levels,1/24/2001,28,Maturity Level 1: Initial Level,Ad hoc approach entire process unpredictable management consists of responses to cris

12、es Most organizations world-wide are here! pattern of time and cost overruns Success depends totally on the staff as the staff changes, so does the process,1/24/2001,29,Maturity Level 2: Repeatable Level,What is the key to this level? metrics! Basic software management planning, management based on

13、past experience measurements (“metrics”) realistic costs and schedules can be used for predictions in next project problems identified take immediate corrective action,1/24/2001,30,Maturity Level 3: Defined Level,Software process is fully documented managerial & technical aspects clearly defined con

14、tinual efforts to improve quality, productivity reviews to improve software quality CASE tools Many orgs have made it to level 2 & 3,1/24/2001,31,Maturity Level 4: Managed Level,Quality & productivity goals set for each project Quality, productivity continually monitored corrective action taken upon

15、 deviation rely on statistical quality controls distinguish random deviations & meaningful violations e.g., #faults detected per 1000 lines of code,1/24/2001,32,Maturity Level 5: Optimizing Level,Continuous process improvement Statistical quality & process controls Feedback knowledge from project to

16、 next leads to continued improvement How does level 5 differ from level 2? level 2 tries to find & correct faults level 5 orgs practice defect prevention i.,e. ensure there are no faults in the first place,1/24/2001,33,Climbing Levels & Case Studies,Moving up the process hierarchy 3 to 5 years to le

17、vel 2 1.5 to 3 years to level 3 SEI questionnaires highlight ways to improve process Experience original idea: defense contracts only to capable firms CMM has been profitable Hughes Aircraft (Fullerton, CA) spent $500K (198790) savings: $2M per year, moving from level 2 to level 3 Raytheon moved fro

18、m level 1 in 1988 to level 3 in 1993 productivity doubled return of $7.70 per dollar invested in process improvement,1/24/2001,34,Process Improvement at Motorola,1/24/2001,35,Summary,Life-cycle models waterfall model too inflexible document-driven evolutionary & prototyping models rely on decisions made before the problem is understood good solution is to integrate rapid proto & waterfall