CSC 321.01, Class 03: An introduction to software engineering

Overview

• Preliminaries
• Notes and news
• Upcoming work
• Extra credit
• Questions
• Mixins
• Some questions from the reading.
• Service-oriented architecture (SOA)
• Software as a service (SaaS)
• SOA vs. SaaS vs. Cloud
• Models of software engineering
• Important agile practices
• Why agile?
• Why not agile?

Preliminaries

• Take a playing card. It will determine where you sit in class.
• Why? I want you to practice discussing things with other people.
• Please take the journals seriously.
• Correct grammar and spelling.
• Thoughtful responses.
• On the last question, “Explain it in your own words” means “explain the concept” not just “explain your confusion”.
• Example: “I did not fully understand the differences in the different types of testing described on page 70 e.g BlackBox Testing, Regression Testing, etc. The descriptions in the book aren’t very informative and hard to understand.”
• Page number: Great!
• e.g., … etc.: Shows carelessness.
• Does not attempt to explain.
• Your mentor will be offering one-hour mentor sessions the night assignments are due.

Upcoming work

• Met Opera Saturday
• CS table Tuesday: Spectre and Meltdown (I think)
• CS extras Thursday: Sam’s students

Questions

Where do I find the videos?
We are relying on a MOOC on EdX

Ruby reflections resume: Mixins

Question: Find the total distance a critter has jumped, not just the amount the critter jumped.

• Can you figure the Ruby involved?
• What does your design look like? In particular, how did you decide to report it?

Why don’t I like the following code? (Apologies to the author.)

module Action
def jump
@distance += rand(4) + 2
puts "I jumped forward #{@distance} feet!"
end
end

class Rabbit
include Action
def initialize(name)
@name = name
@distance = 0
end
end

class Cricket
include Action
def initialize(name)
@name = name
@distance = 0
end
end


Problems

• We’ve lost the “I’ve jumped this far” knowledge. We should not have lost that.
• @distance is used in the mixin, but not defined in the mixin. This issue is not documented. Hidden assumptions that cross class barriers are hard to track down. They should be rare, even when documented.
• Rabbit and Cricket have the same internals. (Perhaps we plan to use them differently). We should probably make a superclass.
class Jumper
include Action
def initialize(name)
@name = name
@distance = 0
end
end

class Rabbit < Jumper
end

class Cricket < Jumper
end


The following code “fixes” the primary problem, but introduces another problem. What is it?

module Action
@@total_distance = 0
def jump
@distance = rand(4) + 2
puts "I jumped forward #{@distance} feet!"
end

def total_distance
total_distance = @@total_distance + @distance
end
end

class Rabbit
include Action
def initialize(name)
@name = name
end
end

class Cricket
include Action
def initialize(name)
@name = name
end
end

• @@total_distance is a class variable. It is shared by all instances.
• If it’s shared by all instances, one thing jumping probably changes the total distance of another.
• Better testing would have caught this issue.

This code fixes that problem, but introduces another. Can you tell what it is? (You may not be able to tell from inspection.)

module Action
@overall_dist = 0

def jump
@distance = rand(4) + 2
@overall_dist += @distance
puts "I jumped forward #{@distance} feet!"
end
end

class Rabbit
include Action
def initialize(name)
@name = name
end
end

class Cricket
include Action
def initialize(name)
@name = name
end
end

• Doesn’t run.

• Ruby mixins aren’t quite as nice as you would hope.
• Things that should be simple and “obvious” aren’t.

Sam’s solution

module Action
def jump
@distance = rand(4) + 2
@overall_dist ||= 0
@overall_dist += @distance
puts "I jumped forward #{@distance} feet!"
end

def total
puts "I have now jumpted a total of #{@overall_dist} feet!"
end
end


Some shorter questions

Note: SOA was the most common question, so I’ll leave that to later.

What is synthesis? Why is it helpful?

• Automatic code generation. The attr_reader above is a simple example.
• Can provide clearer code.
• Can provide more uniform code.
• But can obscure important issues.

Can you explain the different types of testing?

• We’ll talk about them on the testing day (and probably elsewhere)

Why don’t people just throw away and rebuild legacy code?

• The cost.
• Sometimes they discover that it’s harder to build the software than they thought.
• And some people do throw away legacy software and start again.
• “Every piece of software feels like legacy software to someone new to the project.”

Service-oriented architecture (SOA)

• Traditional model: All that parts of the application live together and interact with each other.
• That model leads to assumptions about availability of data, availability of features, etc. etc.
• Good object design says: Segment with clear interfaces (interface: things you can call and meaning of returns)
• SOA: Does this at the larger application level
• N (at least) big advantages
• Easier to distribute work
• Easier to scale
• So our services can become services to other applications
• Can affect efficiency

Software as a Service (SaaS)

• How your program relates to the user.
• Great for rolling out updates
• Great for spying on your users
• Not so great as a user if someone decides to cancel the “service”

Contrasting SOA, SaaS, and cloud

• SOA: Software design from the architect’s perspective
• SaaS: Where does my program live?
• Cloud: Potentially, a lower-level SOA. “Storage” and “CPU”

Models of software engineering

The book covers three models of software engineering.

• Plan and document approaches
• Waterfall
• Spiral
• Agile

What do you see as their similarities and differences?

Important agile practices

What do you see as the most important agile practices?