Designing and Maintaining Software (DAMS) Louis Rose
Habitable Software Leaner
Avoids Duplication
Less Complex
Clearer
Loosely Coupled
More Extensible
More Cohesive
???
Lean “Perfection is finally achieved not when there is no longer anything to add, but when there is no longer anything to take away.” - Antoine de Saint-Exupery
Lean software… Has no extra parts Solves the problem at hand and no more Is often easier to change (i.e., is more habitable)
YAGNI The argument in favour of lean software (part 1)
You Ain’t Gonna Need It!
“Always implement things when you actually need them, never when you just foresee that you need them.” - Ron Jeffries
YAGNI Examples Don’t implement a feature today that you won’t be shipping for a long time. Don’t use a database when flat files will do. Avoid “speculative generality”.
UNIX Philosophy
Favour small, focused and composable tools over large, monolithic and integrated tools.
UNIX Example Problem: Count number of DAMS students named “Bob” Solution: Write a bespoke program
UNIX Philosophy Problem: Count number of DAMS students named “Bob” Solution: Write a bespoke program
UNIX Philosophy Problem: Count number of DAMS students named “Bob” Solution: Reuse existing programs: cat dams_students.csv | grep Bob | wc -l
Worse is Better The argument in favour of lean software (part 2)
The Right Thing Simplicity
The implementation and the interface must be simple.
Correctness
The design must be correct in all observable aspects. Incorrectness is not allowed.
Consistency
The design must not be inconsistent. A design can be less simple / complete to allow consistency.
Completeness
The design must cover as many use cases as is practical. Simplicity is not allowed to overly reduce completeness.
http://dreamsongs.com/WIB.html
Worse is Better Simplicity
The implementation and the interface must be simple. Simplicity is the most important design consideration.
Correctness
The design must be correct in all observable aspects. It is slightly better to be simple than correct.
Consistency
The design must not be overly inconsistent. A design can sometimes be inconsistent to allow simplicity.
Completeness
The design must cover as many use cases as is practical. Completeness can be sacrificed for any other quality.
http://dreamsongs.com/WIB.html
“Worse is Better” is Better Quality does not necessarily increase with functionality WIB software is more likely to “survive” and be improved over time “Unix and C are the the ultimate computer viruses”
KISS
Keep it simple, stupid!
When lean is mean The argument against lean software
Overheads If I split a class, I have to edit >1 file If I split a program, I have to manage >1 project If I split an application, I have to manage >1 server
Legacy Systems I have a huge, bloated, monolith. Where do I start? Huge: refactor as you go. Bloated: run coverage in production. Monolith: start to identify boundaries in the domain.
Designing and Maintaining Software (DAMS) - GitHub
Habitable Software. Leaner. Less Complex. Loosely Coupled. More Cohesive. Avoids Duplication. Clearer. More Extensible ??? Page 3. Lean. âPerfection is finally achieved not when there is no longer anything to add, but when there is no longer anything to take away.â - Antoine de Saint-Exupery. Page 4. Lean softwareâ¦
ASTs are tree data structures that can be analysed for meaning (following JLJ in SYAC 2014/15) ... More Cohesive. Avoids Duplication. Clearer. More Extensible.
Open-source. Influenced by Perl, Smalltalk, Eiffel, Ada and Lisp. Dynamic. Purely object-oriented. Some elements of functional programming. Duck-typed class Numeric def plus(x) self.+(x) end end y = 5.plus(6) https://www.ruby-lang.org/en/about · http
Page 1. Getting Lean. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Lean software⦠Has no extra parts. Solves the problem at hand and no more. Is often easier to change (i.e., is more habitable). Page 3. The Advice I Want to Give.
Why not duplicate? Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Habitable Software. Leaner. Less Complex. Loosely Coupled. More Cohesive. Avoids Duplication. Clearer. More Extensible ??? Page 3. Bad Practice. Page 4. Don't Repeat Yo
âWe have tried to demonstrate that it is almost always incorrect to begin the decomposition of a system into modules on the basis of a flowchart. We propose instead that one begins with a list of difficult design decisions or design decisions which
Tools: Vagrant. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Bugs that appear in production and that can't be reproduced by a developer on their machine are really hard to fix. Problem: âIt works on my machineâ. Page 3. Why does
Clear Documentation. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Bad documentation. Misleading or contradictory find_customer(id). CustomerGateway. Used to look up a customer by their customer number. Page 3. Bad documentation. Red
%w.rack tilt date INT TERM..map{|l|trap(l){$r.stop}rescue require l};. $u=Date;$z=($u.new.year + 145).abs;puts "== Almost Sinatra/No Version has taken the stage on #$z for development with backup from Webrick". $n=Module.new{extend. Rack;a,D,S,q=Rack
R&D: sketch habitable solutions on paper, using UML. 4. Evaluate solutions and implement the best, using TDD. Probably start again at 3. 5. Give to the product owner to validate. Probably start again at 1. 6. Put into production for customers to eval
Fixes issue #42. Users were being redirected to the home page after login, which is less useful than redirecting to the page they had originally requested before being redirected to the login form. * Store requested path in a session variable. * Redi
What is it? Several pieces of data are often used together. Why is it problematic? Behaviour that operates on the clump has no home. (and consequently is often duplicated). When does it arise? High cohesion of the clump has not been detected. D
Observers. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Page 3. Delivery people need to know when pizzas are ready class Pizza def initialize(delivery_person). @delivery_person = delivery_person end def bake cook # blocking call. @d
âWe want the reading of code to be easy, even it makes the writing harder. (Of course, there's no way to write code without also reading it, soâ¦)â - Bob Martin. Clean Code. Prentice Hall, 2009. Page 5. Page 6. User Experience. âA person of av
Getting loose coupling. Designing and Maintaining Software (DAMS). Louis Rose ... should not depend on low-level modules. Both should depend on abstractions.â âAbstractions should not depend on details. Details should depend on abstractions.â -
Automatically detect similar fragments of code. class StuffedCrust def title. "Stuffed Crust " +. @toppings.title +. " Pizza" end def cost. @toppings.cost + 6 end end class DeepPan def title. "Deep Pan " +. @ingredients.title +. " Pizza" end def cost
Ruby Testing Frameworks. 3 popular options are: RSpec, Minitest and Test::Unit. We'll use RSpec, as it has the most comprehensive docs. Introductory videos are at: http://rspec.info ...
Clear Names. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Naming is hard. âThere are only two hard things in Computer. Science: cache invalidation and naming things.â - Phil Karlton http://martinfowler.com/bliki/TwoHardThings.ht
Coupling Between Objects. Counts the number of other classes to which a class is coupled (other than via inheritance). CBO(c) = |d â C - (1cl U Ancestors(C))| uses(c, d) V uses(d, c). - Chidamber and Kemerer. A metrics suite for object-oriented des
Reducing duplication. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Tactics. Accentuate similarities to find differences. Favour composition over inheritance. Know when to reach for advanced tools. (metaprogramming, code generation).
Plug-ins. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Problem. Page 3. Current Architecture. Shareable. Likeable. Food. Pizza. Liking and sharing foods are primary business concerns, so shouldn't be implemented as delegators. Page
When we are testing the way that a unit behaves when a condition is met, use a stub to setup the condition. Solution: use stubs for queries class Subscription ... def bill(amount) unless payments.exists(subscription_id: id) payments.charge(subscripti
Getting Cohesion. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Single Responsibility. Principle. A class should have only one reason to change. - Martin and Martin. Chapter 8, Agile Principles, Patterns and Practices in C#, Prentice
Size != Complexity. âImagine a small (50 line) program comprising. 25 consecutive "IF THEN" constructs. Such a program could have as many as 33.5 million distinct control paths.â - Thomas J. McCabe. IEEE Transactions on Software Engineering, 2:4,