Software Engineering

asks, ``How can we develop solutions to industrial-size problems to be

• quickly produced,
• cheaply produced,
• safer,
• better?''

The Software Life Cycle

First we try to understand how software development proceeds.

A typical software system passes through five stages of life, called the software life cycle.

/----------------\
|   conception   |
\----------------/
        |
        |
        v
/----------------\
|   development  |
\----------------/
        |
        |
        v
/----------------\
|   deployment   |
\----------------/
        |
        |
        v
/----------------\
|  modification  |
\----------------/
        |
        |
        v
/----------------\
|   retirement   |
\----------------/

This normally lasts a few decades.

Conception

1. Concept: Alice thinks of a useful capability the computer system does not provide.
2. Communication: Alice tells Bob, a developer whose job it will be to implement the idea.
3. Requirements analysis: Bob asks questions to find what Alice really wants and needs.

Development

Alice waits while Bob's team develops the system.

1. Design: Bob, Spot, and Carrie discuss how to build the system.
   • features to include
   • user interface
   • system organization
   • task division
   • project schedule
2. Implementation: Each developer builds (and tests!) piece individually.
3. Prototype: In the middle they show a prototype to get Alice's thoughts and directions.
4. Alpha-testing: The developers try to find anything wrong.
5. Documentation: Carrie writes documents describing interface (for users) and design (for developers).

Deployment

1. Beta-testing: Eve, somebody not involved with the design, tries to discover errors in the system.
2. Delivery: Bob gives the completed system to Alice. She may request changes.
3. Distribution: Alice gives/sells system to potential users.

Modification

1. Error discovery: Alice finds how the system can be improved.
2. Error report: She tells Bob about it.
3. Error fix: If significant, Bob and Spot find where the error is and fix it.
4. Redistribution: Bob eventually gives Alice a fixed system.
5. Repeat.

Retirement

1. Decision: Alice or Bob decide the system is better replaced than modified, because
   • better technology is available,
   • the need for the system is gone,
   • the system has grown impossible to maintain.
2. Suspension: Modification ceases except for most critical problems. An alternative is developed.
3. Destruction: Once alternative is viable, Alice destroys the outdated system.

Why Homework is Not the Real World

• Scale
• Teamwork
• Professionalism
• Small piece of system
• Complexities (especially interface)
• Long-term
• Ill specifications
• Correctness criticality
• Money

Task Division

Large projects are typically split into tasks, each for a team of four to five.

example:

      ________________________
     /                        \
     |       spreadsheet      |
     \________________________/ 
      /     /     |     \    \
interface  / calculation \  graphs
          /               \
        files        communication

A prominent company uses the following teams for a product.

• Planning team
• Development teams
• Testing teams
• Design teams
• Documentation team

Issues for Software Engineering

• How to represent design clearly?
• What are good programming practices?
• What errors should be identified automatically?
• What other development tools are useful?
• How can we verify correctness?
• How can we reuse already-written code?