vranac / tactical-ddd-workshop-code

Code base for the Tactical DDD workshop

Tactical DDD workshop

Getting started

You don't need much to run the code in this project: just PHP 7 and composer. This project comes with a set up for Vagrant using Ansible for provisioning, so if you don't want to or can't install these dependencies on your machine, just run:

vagrant up
vagrant ssh
cd /vagrant

Run the unit tests:

vendor/bin/phpunit

Or run the sandbox script:

php sandbox.php

Entities

• Identity
  • User provided
  • Application generated
  • Persistence mechanism generated
  • Repository encapsulates identity generation
  • Surrogate identity
• Mutability & individuality
• Validation & consistency

Checklist

• Does the object need to be retrieved from a repository by its unique identity?
• Does the object need to track changes?
• Are two instances equivalent if they have the same identity?

Value objects

• Characteristics:

  • Descriptive
  • Immutable
  • Often composite, a conceptual whole
  • Replaceable
  • Comparable
  • Side-effect free behaviors

• Is it irrelevant which exact instance of the object is used?

• Can the object be replaced instead of modified?

• Are two objects of the same type equal if their properties contain equal values?

Aggregates

• Consistency boundary

• Transactional consistency

• Eventual consistency

  When examining the use case (or story), ask whether it's the job of the user executing the use case to make the data consistent.
  

• Rules of thumb

  • Small aggregates
  • Reference aggregates by identity
  • Use eventual consistency

• Implementation

  • Root entity with unique identity
  • Favor value object parts
  • Use Law of Demeter and Tell, Don't Ask
  • Optimistic concurrency
  • Avoid dependency injection

Repositories

• Collection or persistence oriented
• Persistence implementations
  • Unit of Work, changeset calculation
  • Event-sourcing
• Reconstitution
• Querying

Domain events

• Immutable
• Relation with commands
• Authoritative
• Sufficient data