4  PROPERTIES  OF  SOFTWARE  SYSTEMS    

Think  Process…  

Ice  Breaker  

?  

INTRO  

•  Your  two  quesAons  What  do  you  want  to  take  away  

• WHAT  ARE  SYSTEMS…  •  IS    SOFTWARE  IS  A  SYSTEM,  WHAT  KIND  OF  SYSTEM  IS  IT?  

By:BS  

ProperAes  of  soKware  systems  

1.  Complexity  2.  Conformity  3.  Changeability  4.  Invisibility   Fred Brooks Jr.

Mythical Manmonth Father  of  /  IBM  360  OS  

1.  Complexity  

•  Human  Channel  capacity  •  The  magical  number  7  ±  2  

•  AdapAve  System  •  Social  (CollecAve)  Intelligence  :  e.g.  Bees,  Ants  •  Entropy  

2.  Conformity  •  Unlike  Physics,  No  unifying  principle  

•  human  built  system  change  from  –  Interface  to  Interface  –  People  to  people  –  Ame  to  Ame  

•  AI:  I  only  want  to  know  god’s  thought..rest  are  details  •  Unlike  God,..  

•   Human  endeavors  have  skewed  distribuAon  and  not  Normal  Distr.  

–  Conformity  X  Changeability  à  Impact  on  Quality  

3.  Changeability  

•  Perceived..…”SoKness”  à  Change  •  SoKware  embodies  funcAon,  RFC  is  funcAon  itself  •  Not  same  expectaAon  in  Automobile,  Computers  

– where  funcAon  is  not  expected  to  change,  but  other  adribute  –  Effect  of  change  à  Entropy  

•  However  applying  change  in  ProducAon  has  negligible  cost  unlike  HW  /  Automobile  wherein  Product  has  to  be  recalled  expensively  

4.  Invisibility  

•  Not  spaAal  in  nature,  Dimensionless  •  Impossible  to  apply  Geometric  abstracAon  e.g.  map,  exploded  view,  building  plan  

•  Need  Abstract  thinking.    –  Only  5-­‐9  can  abstract  effecAvely  –  Not  easily  automatable  e.g.  VLSI  Design  

•  Design  …SoKware  ProducAon  

BACKUP  SLIDES  

By:BS  

…So$ware  Entropy  

1.  A Program that is used will be modified

2.  When a program is modified, it’s complexity will increase, provided that one does not work against this

-Lehman 1985

Entropy  

Changes  

Physical/  Project  Constraints  to  max  

I-­‐Change   III-­‐Change  

A closed system’s disorder cannot be reduced, it can only increase or possibly remains unchanged. A measure of this disorder is entropy (Thermodynamics)

SoKware  Systems    

Releases  to  effect  required  change  in  funcAonality  

By:BS  

The Magical Number ‘7’± 2 Some Limits on Our Capacity for Processing Information by George A. Miller originally published in The Psychological Review, 1956, vol. 63, pp. 81-97

What about the magical number seven? … For the present I propose to withhold judgment. Perhaps there is something deep and profound behind all these sevens, something just calling out for us to discover it. But I suspect that it is only a pernicious, Pythagorean coincidence.   Handling  Complexity  

What  is  this  report?  

First  Computer  Bug  •  In  1947,  Grace  Murray  Hopper  was  working  on  the  Harvard  

University  Mark  II  Aiken  Relay  Calculator  (a  primiAve  computer).  

•  On  the  9th  of  September,  1947,  when  the  machine  was  experiencing  problems,  an  invesAgaAon  showed  that  there  was  a  moth  trapped  between  the  points  of  Relay  #70,  in  Panel  F.  

•  The  operators  removed  the  moth  and  affixed  it  to  the  log.  The  entry  reads:  "First  actual  case  of  bug  being  found."  

•  The  word  went  out  that  they  had  "debugged"  the  machine  and  the  term  "debugging  a  computer  program"  was  born.