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CIS 068
Welcome to CIS 068 !Lesson 2:
Software Engineering
or
Why not only code it ?
CIS 068
CIS 068
Why not only code it ?• Which event happens more frequently ?
• Which is deadlier ?
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Why not only code it ?
Famous Software Failures
• AT&T long distance service fails for nine hours(Wrong BREAK statement in C-Code, 1990)
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Why not only code it ?Famous Software Failures cont’d:
• Mars Climate Orbiter (September 23rd, 1999)The 125 million dollar Mars Climate Orbiter is assumed lost by officials at NASA. The failure responsible for loss of the orbiter is attributed to a failure of NASA’s system engineer process. The process did not specify the system of measurement to be used on the project. As a result, one of the development teams used Imperial measurement while the other used the metric system of measurement. When parameters from one module were passed to another during orbit navigation correct, no conversion was performed, resulting in the loss of the craft.
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Why not only code it ?
Famous Software Failures cont’d:
• Hi-tech toilet swallows woman (2001) [Source: Article by Lester Haines, 17 Apr 2001] A 51-year-old woman was subjected to a harrowing two-hour ordeal when she was imprisoned in a hi-tech public convenience. She was captured by the toilet, which boasts state-of-the-art electronic auto-flush and door sensors, which steadfastly refused to release it’s victim, and further resisted attempts by passers-by to force the door. Finally the fire brigade ripped the roof off the cantankerous crapper.
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Why not only code it ?Famous Software Failures cont’d:
• E-mail buffer overflow (1998)Several E-mail systems suffer from a "buffer overflow error", when extremely long e-mail addresses are received. The internal buffers receiving the addresses do not check for length and allow their buffers to overflow causing the applications to crash. Hostile hackers use this fault to trick the computer into running a malicious program in its place.
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Why not only code it ?Software is• a critically important infrastructure component• a key enabler
– militaryly– economically– scientifically– culturally
But usually• expensive• of poor quality
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Common Software Problems
– Software can cost hundreds or thousands of dollars per line
– Lifetime maintenance costs are higher still– Software is late or fails– Software is not performant (too slow)– Software is incomprehensible– Software is more trouble to use than it is
worth
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Past Approaches to Solutions
•Use more people•Create better programming languages•Write software tools to help create software•Design before writing
•Start by baselining requirements•Train people better
•Create more chaos•Bad programs can be written in any language•(who finds the error in
reasoning in here ?)•Are you designing the
right program ?•but they change !
•…to do what ?
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The Software Crisis
Summary:
Millions are spent for an
incomprehensible tool that comes
late just to cause trouble, and we don’t
have answers
or:
THE SOFTWARE CRISIS
(1968)
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History1968: NATO Software Engineering Conference in Garmisch (Germany):
Why cannot bridge-building techniques be used to build operating systems
(‘engineering’) ?
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The Nature of Software
• It is a component of a larger system that “fits” with hardware, people, mechanical devices
• It transforms data using computers
• It has a complex structure
• It is usually very large, expensive, and lengthy to build
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The Nature of Software• Software is extremely malleable – we can modify the product all too easily
• Software construction is human-intensive, there are no real costs of materials
• Software is intangible: no laws of physics are applicable
• Software is not detectable by any of the five human senses
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The Nature of SoftwareBut:
The are characteristics analogue to physical engineering processes
Studying such analogs can be useful:
• Help us learn about computer software • Find points of similarity • Suggest successful approaches to be emulated • Avoid known mistakes
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Engineering Example
Building a house:•Land and finances•garden, garage, you are used to age wine, enjoy to sit by the fireplace, lots of storage, don’t like bauhaus•Architect will define number of floors and rooms, orientation of the driveway, size of the garage …•type of bricks, colour of the walls,…•Construction•Entering•Living in the house•Fixing minor problems, leaking in the roof …
System Feasibility
Software Plans and Requirements
Product Design
Detailed Design
Code
Integration (Product Verification)
Integration (System Test)
Operations and Maintenance
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The Waterfall ModelSystem Feasibility Validation
Plans +
RequirementsValidation
Product Design Verification
Detailed Design Verification
Code Unit Test
IntegrationProduct Verification
Integration System Test
Operation + Maintenance
RevalidationDidn’t we forget something ?
CIS 068
The Waterfall ModelSystem Feasibility Validation
Plans +
RequirementsValidation
Product Design Verification
Detailed Design Verification
Code Unit Test
IntegrationProduct Verification
Integration System Test
Operation + Maintenance
Revalidation
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User
SOFTWARE
Customer
Programmer
Designer
The Human Factor
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User Programmer
SOFTWARE
Customer Designer
Programmer‘s view:• Some (holy) lines of code• A technical challenge• A pet• ...
The Human Factor
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User Programmer
SOFTWARE
Customer Designer
User‘s view:• A miracle• A wonderful tool making things easier• An incombprehensible tool unnecessarilly complicating life• Something that simply should work !
The Human Factor
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User Programmer
SOFTWARE
Customer DesignerCustomer‘s view:• A hopefully affordable tool to enhance profit.
The Human Factor
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User Programmer
SOFTWARE
Customer DesignerDesigner‘s view:• A reasonably complicated tool to fulfill the needs• A technical challenge
The Human Factor
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The Human Factor
• Usually one person plays multiple roles
• Separation of different roles needs discipline !
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Review of Waterfall Model
Weaknesses:– Usually requirements change, are
incomplete or even not known– Communication ! (…see Mars Orbiter…)
Result: ‘That’s not what I meant !’ ( go back to last step )
WF-Model reacts very statically:– Each stage must be completed before next
one starts
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Total Feedback System Feasibility Validation
Plans +
RequirementsValidation
Product Design Verification
Detailed Design Verification
Code Unit Test
IntegrationProduct Verification
Integration System Test
Operation + Maintenance
Revalidation
•Too expensive•Doesn’t force to discipline•Don’t show this to your boss !
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The Unified Model• A tradeoff, unifying different models• Shows the basic message of different
approaches
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The Unified ModelTime
Act
iviti
esCustomer
User
Designer
Programmer
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Models: Review•There are a lot of models, each with it’s strong- and weaknesses•Keep in mind:
• There is a necessity to manage the workflow• There are different views of software• Smaller projects can be managed by the waterfall model• Review your programming process, check which phase you are in• Play different roles by yourself
• And NEVER forget the testing !
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Software Life Cycle ActivitiesIndependent of how they are organized, the following activities are involved in the development of software:
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Example: The Phone Directory
Example will show activities:• Requirements
• Analysis• Design
• Implementation
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Phone Directory: Requirements
Phone Directory:
• Interactive Program containing collection of names and phone-numbers
• Insert new entries • Retrieve entries• Change Entries
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Phone Directory: Detailed Req’s
• Import existing data ?• Read from file or enter interactively• If file: file-type ?• If text-file: comma separated ?• Final directory: filetype spec’s ?• Limited namelength ?• Numbers as string ?• Order alphabetically ?• Printout required ?• Double entries possible ?• …
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Phone Directory: Analysis
Requirement – related:
• Cluster requirements to different levels of detail
• Understand ALL requirements• Explore EVERY uncertainty
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Phone Directory: Analysis
Implementation-strategy:
• Use commercial software ?• Design specific or reusable software ?• Outsource different tasks (if specified) ?• Which language ?• Impact on existing software-packages ?• …
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Phone Directory: Analysis
High level design:
• Again: make sure you understand the problem !
• Different methodologies:• Top Down Design• Object Oriented Approach
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Analysis: Top Down Design
Stepwise Refinement, Divide and Conquer
• Start at top level• Divide into subproblems• For each subproblem:
• Divide into subproblems, solving the higher level problem
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Analysis: Top Down Design
Structure chart, indicating the relationship
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Analysis: Top Down DesignRefinement
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Analysis: Top Down DesignRefinement
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Analysis: Top Down Design
Question: When should we stop the refinement ?
Answer:Each subproblem should be RESPONSIBLEfor exactly ONE activity
(…in it’s description, there’s no AND)
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How to go on ?What happens if proceeding with
refinement, e.g. going down to flowchart ?
• the problem description then will focus on PROCEDURES
• Definition of data structures ?
This is a major problem in procedural driven design !
Alternative: Object Oriented Design
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Object Oriented Design1. Identify objects participating in the system
• Look at nouns in the problem statement to identify objects:
…create phone directory …containing entries… read from/write to file… interact with user …
Objects:• Directory• Entry• File• User
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Object Oriented Design
2. Identify INTERACTIONS between objects• Messages between objects• Look at verbs in the problem statement
to identify interactions:
…create phone directory …containing entries… read from/write to file… interact with user …
• Messages must be processed by object’s methods
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Object Oriented DesignClass Diagram for Phone Book Example:
Defined by UML (Unified Modeling Language)
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Object Oriented DesignClass Diagram for Phone Book Example:
Defined by UML (Unified Modeling Language)
Actor Class Aggregation (“part of”)
Navigability: Source Target
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AbstractionDefinition:Abstraction = process of separating inherent qualities or properties of something from theactual physical representation.
• Procedural Abstraction • separate what a procedure does from how it is done
• Data Abstraction• describe what information is stored, not how• logical view instead of physical view
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AbstractionLeads to Information Hiding:
Abstract data types are only defined by their methods, the actual implementation is hidden.
Advantage: •separation of definition and implementation•Maintenance simplification
Data protected by methods
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Abstraction
• JAVA interfaces define Abstract Data Types.
• Specification of names, parameters, return values
• No implementation in interfaces but in classes
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Use CasesDefinition:
Use Case = Closed loop interaction with the user
The refinement process of the top down approach is replaced by listing all use cases, or: “write down
everything the system is supposed to do”
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Use CasesUse Cases for phone book example:
The program must be able to:
• load initial directory from file
• insert new entry or change existing one
• retrieve and display entry
• save modified directory back to file
• exit
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Use CasesDetailed Description ( 1 of 5):
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Use CasesDetailed Description ( 2 of 5):
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Use CasesDetailed Description ( 3 of 5):
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Use CasesDetailed Description ( 4 of 5):
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Use CasesDetailed Description ( 5 of 5):
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Use Cases
Compare Use Cases to results of refinement
• of course they seem similar (this is a simple example !)• refinement didn’t contain any data-structure related information• Use Cases contain messages, these messages contain implicit information about data• Use Cases and objects do not need explicit information about data• Data structures should even be hidden to other classes !
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Use Cases
(The following slides differ from the Textbook)
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Abstraction
The class structure redefined:
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Sequence Diagrams
Each Use Case corresponds to a
Sequence Diagram
• shows the flow of messages between classes• defined by UML standard
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Use CasesAgain: Use Case 1
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Sequence DiagramsSequence Diagram of Use Case 1:
Load data from file
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Sequence DiagramsSequence Diagram of Use Case 1:
Load data from fileactor object
Object’s Lifeline:active / inactive
message
self call
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Use CasesAgain: Use Case 2
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Sequence DiagramsSequence Diagram of Use Case 2:
Insert / change entry
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Use CasesAgain: Use Case 3:
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Sequence DiagramsSequence Diagram of Use Case 3:
Retrieve and Display entry
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From Diagrams to ObjectsRemind:
• all messages must be processed by object’s method
• the message-processing requires data types
• the messages received and sending from an object in all use cases define the object’s methods explicitely
• data structures for implementation are defined by the needs of methods, hidden to other objects
• the objects are defined by collecting all messages for/from each object
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From Diagrams to ObjectsCollect all messages to define object’s methods !
Phone Directory
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Reviewreasons for thinking about software
software 'engineering'
different views of software
software life cycle models
waterfall model
unified model
phone directory example:
requirements
analysis
top down (divide and conquer)
object oriented
use cases
abstraction
sequence diagrams to objects
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Good Bye !