 Chapter 13 – Dependability Engineering 1 Chapter 12 Dependability and Security Specification 1

Dependability through Redundancy and Diversity  Redundancy – having multiple components that perform the same function  Diversity – usage of secondary components that are distinct in structure from each other Chapter 12 Dependability and Security Specification 2

Redundancy Examples  Power Supply  Disc (RAID)  Processor  Database server  Web server  DNS servers  Authentication servers Chapter 12 Dependability and Security Specification 3

Redundancy  Redundancy solves problems of failure  Redundancy does not solve problems of design  Same password authetication  Same processing of database Chapter 12 Dependability and Security Specification 4

Diversity  Separate versions of software to accomplish the same thing  Software to check output of one set of software against that of another Chapter 12 Dependability and Security Specification 5

Arguments Against Diversity  Too costly  Extra effort put into second set of software could have been applied to more validation, verification Chapter 12 Dependability and Security Specification 6

Diversity Overhead  Mechanism to compare outputs, decide on correct output Chapter 12 Dependability and Security Specification 7

Dependability and Agile Methods  Agile often considered not suitable for critical systems  Some debate exists Chapter 12 Dependability and Security Specification 8

Dependability Through Architecture  Replication  Protection System  Self Monitoring System  N-Version programming Chapter 12 Dependability and Security Specification 9

Replication  Same functionality  Repeated  Data replicated to other servers  Examples  Database server  Web server  DNS server Chapter 12 Dependability and Security Specification 10

Protection System  Control of system by one set of software  Independent monitoring of key parameters  Override capability  Only has to look at override  Example:  One system to control train at airport  Separate system shuts down car if overspeed Chapter 12 Dependability and Security Specification 11

Self Monitoring Systems  Primary system performs primary operations  Second system constantly checks primary system  Effectiveness depends on:  Diverse hardware  Diverse software  Example:Airbus (5 processors) Chapter 12 Dependability and Security Specification 12

N- Version  Multiple versions  Output selector / fault manager  Monitors results from all concurrent systems Chapter 12 Dependability and Security Specification 13

Diversity Issues  Separate teams:  No interaction between teams  Require different methodologies (oo vs functional), programming languages, tools, algorithms  Challenges  Teams may have same background, knowledge and may make the same mistakes  Requirements failures not resolved  Requirements must leave no room for question Chapter 12 Dependability and Security Specification 14

Dependability Through Processes  Requirements reviews  Requirements management  Formal Specification  System modeling – graphical  Design and program inspections  Static analysis of code  Test planning, management Chapter 12 Dependability and Security Specification 15

Dependability Through Programming  Sommerville’s summary of some key development strategies  #1 Control Visibility of Data  #2 Check All inputs for Validity  #3 Provide handlers for exception Chapter 12 Dependability and Security Specification 16

Dependability Through Programming  #4 Minimize error prone constructs  Unconditional branch  Floating point numbers  Pointers  Dynamic memory allocation  Parralelism  Recursion  Interrupts Chapter 12 Dependability and Security Specification 17

Dependability Through Programming  (Minimize error prone constructs)  Inheritance  Aliasing  Unbounded Arrays  Default input processing Chapter 12 Dependability and Security Specification 18

Dependability Through Programming  #5 Provide restart (desktop apps)  #6 Check array bounds  #7 Include timeouts when calling external components  #8 Name constants Chapter 12 Dependability and Security Specification 19