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YAGNI You ain't gonna need it

  • YAGNI is all about avoiding [[overengineering]]
  • There is no a well-defined criterion to measure the "YAGNI-ness".

"Always implement things when you actually need them, never when you just foresee that you need them" - Ron Jeffries

Worse is Better

A model of software design and implementation which has the following characteristics: - Simplicity - [[Correctness]] - [[Consistency]] - [[Completeness]]

If you do something for a future need that doesn't actually increase the complexity of the software, then there's no reason to invoke YAGNI

YAGNI

  • Don't follow any principles blindly
  • YAGNI depends on supporting practices, follow other practices such as [[Continous Integration]], Refactoring, [[Unit Testing]] as well.

YAGNI Violation

  • A team is working on a payment system which interoperates with many devices.
  • Working on a current version, you need to implement a driver for interoperation with the model "Z" of a bill dispenser
  • At the same time, a project manager expects that in four months they will need to support the "Y" model of a bill dispenser.

Deciding to implement such presumptive features is a classic violation of YAGNI principle!

Useless Feature => all the efforts spent on analyzing, programming and testing.
Useful Feature > stolen time + cost of carry + risk of wrong implementation

YAGNI Violation 2

  • You're writing a fuinction within a class which should parse a custom string.

    • Implemenmt that function right there, in that class
    • Implement that function and extract a utility class which contains that parsing function.

  • Ask yourself if the function (or feature) is needed to be implemented right now

  • If the answer is "yes", then implement it
  • If the answer is "no", ask yourself another question
    • How much effort will it take to implement that function or feature in the future in case it will be required.
    • If the answer is "It will definitely be very simple" then keep all the things as they are, don't introduce anything additional.
    • If the answer is "It will take enormous amount of time to rewrite many things to tweak the design to make it enough supple to introduce that feature" - then consider performing some refactoring which will allow you to avoid massive rewriting in the future.
  • You should always carefully plan the upcoming features
  • No big upfront design
  • The goal applying YAGNI is to save some time
  • Perform refactoring to enable YAGNI
  • Be ready to fail applying YAGNI

Open-Closed Principle vs YAGNI