Code Reading - Learn ❤️ Domain Driven Design

Published Thursday, June 14, 2018

Challenge: introducing DDD in a legacy codebase

What is DDD?


“Sphere of knowledge…”


Model domain in our software


  • Informed by people who actually do the work (stakeholders)
  • Domain experts are key for bridging gap between domain and software


  • Better code
  • Better product
  • More happy

What is DDD in Learn?

  • It’s always kind of been there
  • DDD encompasses a lot of general best practices, some of which we’ve already been following (ex. ubiquitous language)

First attempts: LearnApps

  • Not true DDD
  • Doesn’t follow all the core principles

Then DDD “Lite”

  • Domains directory
  • Closer to setting up microservices in our monolith
  • Aggregate domain module for each domain
    • Insulation around implementation
    • Provides clearer context
    • Minimize reaching across domains
    • Primitives in, primitives out
  • Still encourages collaboration
    • Not just a tool for engineers
    • Use company-wide ubiquitous language (talk to product, stakeholders)


  • Domains are NOT project namespaces
  • Domains should map to business departments (in general)
  • Watch out for early abstraction
  • Different names for the same things
  • vs. Same name for different things
  • Rails autoloading


  • DDD-lite is one tool in our codebase
  • Meant to make our code easier to reason about
  • AIBAM? (“Am I building a microservice?”)
  • If yes, WWIDIIWBAM? (“What would I do if I were building a microservice?”)