1 cs 501 spring 2008 cs 501: software engineering lecture 20 reliability 2
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1 CS 501 Spring 2008
CS 501: Software Engineering
Lecture 20
Reliability 2
2 CS 501 Spring 2008
Administration
Projects
Four weeks to the end of the semester.
Leave time for system testing and to make small changes discovered when the complete system is assembled.
Better to deliver a limited first phase done well than a fuller system that is incomplete, untested, or without documentation.
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Quiz 3: Sports equipment online
A company that makes sports equipment decides to create a system for selling sports equipment online. The company already has a product database with specification, marketing information, and prices of the equipment that it manufactures.
To sell equipment online the company will need to create: a customer database, and an ordering system for online customers.
The plan is to develop the system in two phases. During Phase 1, simple versions of the customer database and ordering system will be brought into production. In Phase 2, major enhancements will be made to these components.
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Quiz 3 Q1
(a) For the system architecture of Phase 1:
i Draw a UML deployment diagram.
WebBrowser
PersonalCompOrdering system
DeptServer
Product DB
Customer DB
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Quiz 3 Q1
(a) For the system architecture of Phase 1:
i Draw a UML interface diagram.
WebBrowser Ordering system
Product DB
Customer DB
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Quiz 3 Q1
(b) For Phase 1:
i What architectural style would you use for the customer database?
Repository with Storage Access Layer
ii Why would you choose this style?
It allows the DB to be replaced without changing the applications that use the DB.
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Quiz 3 Q1
(b) For Phase 1:
iii Draw an UML diagram for this architectural style showing its use in this application.
Data Store
Input components
Ordering System
Storage Access
Customer DB
optional
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Quiz 3 Q2
Carefully design during Phase 1 will help the subsequent development of new components in Phase 2.
(a) For the interface between the ordering system and the customer database:
i Select a design pattern that will allow a gradual transition from Phase 1 to Phase 2.
Bridge design pattern
(b) Draw a UML class diagram that shows how this design pattern will be used in Phase 1.
If your diagram relies on abstract classes, inheritance, delegation or similar properties be sure that this is clear on your diagram.
[See next two slides]
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Quiz 3 Q2
Abstract class
Abstract classes are superclasses which contain abstract methods and are defined such that concrete subclasses extend them by implementing the methods. Before a class derived from an abstract class can become concrete, i.e. a class that can be instantiated, it must implement particular methods for all the abstract methods of its parent classes.
The incomplete features of an abstract class are shared by a group of subclasses which add different variations of the missing pieces.
Wikipedia 4/2/08
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Quiz 3 Q2
OrderingAbstraction DBImplementor
Ordering System
ConcreteDBImplementorA
ConcreteDBImplementorB
RefinedOrderingAbstraction
Client
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Quiz 3 Q2
(c) How does this design pattern support:
i Enhancements to the ordering system in Phase 2?
By subclassing OrderingAbstraction
ii A possible replacement of the customer database in Phase 2?
By allowing several ConcreteBDImplementor classes
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Static Validation & Verification
Carried out throughout the software development process.
Validation & verification
Requirements specification Design Program
REVIEWS
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Reviews: Design and Code
Concept
Colleagues review each other's work:
can be applied to any stage of software development
can be formal or informal
Design and code reviews are a fundamental part of good software development
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Review Team (Full Version)
A review is a structured meeting, with the following people
Moderator -- ensures that the meeting moves ahead steadily
Scribe -- records discussion in a constructive manner
Developer -- person(s) whose work is being reviewed
Interested parties -- people above and below in the software process
Outside experts -- knowledgeable people who are not working on this project
Client -- representatives of the client who are knowledgeable about this part of the process
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Example: Program Design
Moderator
Scribe
Developer -- the design team
Interested parties -- people who created the system design and/or requirements specification, and the programmers who will implement the system
Outside experts -- knowledgeable people who are not working on this project
Client -- only if the client has a strong technical representative
In a small team, an individual may have several roles
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Static and Dynamic Verification
Static verification: Techniques of verification that do not include execution of the software.
• May be manual or use computer tools.
Dynamic verification:
• Testing the software with trial data.
• Debugging to remove errors.
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Static Verification: Program Inspections
Formal program reviews whose objective is to detect faults
• Code may be read or reviewed line by line.
• 150 to 250 lines of code in 2 hour meeting.
• Use checklist of common errors.
• Requires team commitment, e.g., trained leaders
So effective that it is claimed that it can replace unit testing
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Inspection Checklist: Common Errors
Data faults: Initialization, constants, array bounds, character strings
Control faults: Conditions, loop termination, compound statements, case statements
Input/output faults: All inputs used; all outputs assigned a value
Interface faults: Parameter numbers, types, and order; structures and shared memory
Storage management faults: Modification of links, allocation and de-allocation of memory
Exceptions: Possible errors, error handlers
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Static Analysis Tools
Program analyzers scan the source of a program for possible faults and anomalies (e.g., Lint for C programs).
• Control flow: loops with multiple exit or entry points
• Data use: Undeclared or uninitialized variables, unused variables, multiple assignments, array bounds
• Interface faults: Parameter mismatches, non-use of functions results, uncalled procedures
• Storage management: Unassigned pointers, pointer arithmetic
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Static Analysis Tools (continued)
Static analysis tools
• Cross-reference table: Shows every use of a variable, procedure, object, etc.
• Information flow analysis: Identifies input variables on which an output depends.
• Path analysis: Identifies all possible paths through the program.
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Security in the Software Development Process
The security goal
The security goal is to make sure that the agents (people or external systems) who interact with a computer system, its data, and its resources, are those that the owner of the system would wish to have such interactions.
Security considerations need to be part of the entire software development process. They may have a major impact on the architecture chosen.
Example. Integration of Internet Explorer into Windows
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Agents and Components
A large system will have many agents and components:
• each is potentially unreliable and insecure
• components acquired from third parties may have unknown security problems
• commercial off-the-shelf (COTS) problem
The software development challenge:
• develop secure and reliable components
• protect whole system from security problems in parts of it
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Techniques: Barriers
Place barriers that separate parts of a complex system:
• Isolate components, e.g., do not connect a computer to a network
• Firewalls
• Require authentication to access certain systems or parts of systems
Every barrier imposes restrictions on permitted uses of the system
Barriers are most effective when the system can be divided into subsystems with simple boundaries
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Techniques: Authentication & Authorization
Authentication establishes the identity of an agent:
• What the agent knows (e.g., password)
• What the agent possess (e.g., smart card)
• Where does the agent have access to (e.g., crt-alt-del)
• What are the physical properties of the agent (e.g., fingerprint)
Authorization establishes what an authenticated agent may do:
• Access control lists
• Group membership
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Example: An Access Model for Digital Content
Digital material
Attributes
User
Roles
Actions
OperationsAccess
Policies
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Techniques: Encryption
Allows data to be stored and transmitted securely, even when the bits are viewed by unauthorized agents
• Private key and public key
• Digital signatures
Encryption
Decryption
X Y
Y X
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Security and People
People are intrinsically insecure:
• Careless (e.g, leave computers logged on, use simple passwords, leave passwords where others can read them)
• Dishonest (e.g., stealing from financial systems)
• Malicious (e.g., denial of service attack)
Many security problems come from inside the organization:
• In a large organization, there will be some disgruntled and dishonest employees
• Security relies on trusted individuals. What if they are dishonest?
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Design for Security: People
• Make it easy for responsible people to use the system
• Make it hard for dishonest or careless people (e.g., password management)
• Train people in responsible behavior
• Test the security of the system
• Do not hide violations
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Suggested Reading
Trust in Cyberspace, Committee on Information Systems Trustworthiness, National Research Council (1999)http://www.nap.edu/readingroom/books/trust/
Fred Schneider, Cornell Computer Science, was the chair of this study.
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Failures and Faults
Failure: Software does not deliver the service expected by the user (e.g., mistake in requirements, confusing user interface)
Fault (BUG): Programming or design error whereby the delivered system does not conform to specification (e.g., coding error, interface error)
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Faults and Failures
Actual examples
(a) A program dies because the programmer typed: x = 1 instead of x == 1.
(b) A mathematical function loops for ever from rounding error.
(c) A distributed system hangs because of a concurrency problem.
(d) After a network is hit by lightning, it crashes on restart.
(e) The head of an organization is paid $5 a month instead of $10,005 because the maximum salary allowed by the program is $10,000.
(f) An operating system fails because of a page-boundary error in the firmware.
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Terminology
Fault avoidance
Build systems with the objective of creating fault-free (bug-free) software
Fault tolerance
Build systems that continue to operate when faults (bugs) occur
Fault detection (testing and validation)
Detect faults (bugs) before the system is put into operation.
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Fault Avoidance
Software development process that aims to develop zero-defect software.
• Formal specification• Incremental development with customer input• Constrained programming options• Static verification• Statistical testing
It is always better to prevent defects than to remove them later.
Example: The four color problem.
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Defensive Programming
Murphy's Law:
If anything can go wrong, it will.
Defensive Programming:
• Redundant code is incorporated to check system state after modifications.
• Implicit assumptions are tested explicitly.
• Risky programming constructs are avoided.
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Defensive Programming: Error Avoidance
Risky programming constructs
• Pointers
• Dynamic memory allocation
• Floating-point numbers
• Parallelism
• Recursion
• Interrupts
All are valuable in certain circumstances, but should be used with discretion
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Defensive Programming Examples
• Use boolean variable not integer
• Test i <= n not i == n
• Assertion checking (e.g., validate parameters)
• Build debugging code into program with a switch to display values at interfaces
• Error checking codes in data (e.g., checksum or hash)
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Maintenance
Most production programs are maintained by people other than the programmers who originally wrote them.
(a) What factors make a program easy for somebody else to maintain?
(b) What factors make a program hard for somebody else to maintain?
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Fault Tolerance
General Approach:
• Failure detection
• Damage assessment
• Fault recovery
• Fault repair
N-version programming -- Execute independent implementation in parallel, compare results, accept the most probable.
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Fault Tolerance
Basic Techniques:
• Timers and timeout in networked systems
• After error continue with next transaction (e.g., drop packet)
• User break options (e.g., force quit, cancel)
• Error correcting codes in data
• Bad block tables on disk drives
• Forward and backward pointers in databases
Report all errors for quality control
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Fault Tolerance
Backward Recovery:
• Record system state at specific events (checkpoints). After failure, recreate state at last checkpoint.
• Backup of files
• Combine checkpoints with system log (audit trail of transactions) that allows transactions from last checkpoint to be repeated automatically.
• Test the restore software!
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Software Engineering for Real Time
The special characteristics of real time computing require extra attention to good software engineering principles:
• Requirements analysis and specification
• Special techniques (e.g., locks on data, semaphores, etc.)
• Development of tools
• Modular design
• Exhaustive testing
Heroic programming will fail!
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Software Engineering for Real Time
Testing and debugging need special tools and environments
• Debuggers, etc., can not be used to test real time performance
• Simulation of environment may be needed to test interfaces -- e.g., adjustable clock speed
• General purpose tools may not be available
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Some Notable Bugs
Even commercial systems may have horrific bugs
• Built-in function in Fortran compiler (e0 = 0)
• Japanese microcode for Honeywell DPS virtual memory
• The microfilm plotter with the missing byte (1:1023)
• The Sun 3 page fault that IBM paid to fix
• Left handed rotation in the graphics package
• The preload system with the memory leak
Good people work around problems.The best people track them down and fix them!