1.
From legacy to DDD
@andrzejkrzywda
Arkency

2.
What is legacy code?

3.
Legacy
• Frameworks
• monolithic
• no architecture
• no tests

4.
DDD

5.
Strategic vs Tactical
DDD

6.
Tactical DDD
• entity
• value objects
• aggregate
• domain event
• services
• repositories
• factory

7.
DDD-lite
• app service
• calls 1 or more domain services
• domain service
• takes repo as input
• load aggregate from the repo
• do sth cool
• may use some value objects
• may trigger some domain events
• save aggregate (repo)

8.
• URL
• Rails action
• App service
• Domain service
• Repo
• ActiveRecord
• Database

9.
One app service can call
multiple domain services

10.
UpdateCustomer
==
MoveToNewAddress
ChangeEmail
IntroduceNewContactPerson
In one of our projects one request was about 15 domain services
“Update” is always hiding a lot

11.
Strategic DDD
• bounded contexts
• context map

12.
http://martinfowler.com/bliki/BoundedContext.html

13.
Software House
management tool

14.
optionally as microservices
partially available
different persistence
data duplication
Context Map
HR Project Management Finances
Invoicing
CRUD
DDD-lite
external SaaS
Communication
developer
project
client
DDD-lite

15.
How to get there from
legacy?
BTW, There’s no such thing as the ultimate DDD codebase

16.
Draw the Context Map
aka what are the boxes in your application
aka you have more apps
They are potential microservices

17.
Decide which contexts are
CRUD and which ones
need more logic (DDD-lite)
the ones with CRUD ideally need to publish events anyway

18.
Ongoing process
No time for 2-months rewrites

19.
Peopleware
the human factor
http://andrzejonsoftware.blogspot.com/2014/01/refactoring-human-factor.html

20.
Escape from the
CRUD verbs
Talk more to the business people

21.
Test coverage
#noregressions
when you’re a client you don’t like them too

22.
Line coverage
is
a joke

23.
Mutation testing
is the real test coverage!
http://blog.arkency.com/2015/04/why-i-want-to-introduce-mutation-testing-to-the-rails-event-store-gem/
http://blog.arkency.com/2015/06/how-good-are-your-ruby-tests-testing-your-tests-with-mutant/

24.
You want to try mutant
(today)
https://github.com/mbj/mutant

25.
mutant
==
testing your tests
==
refactoring with confidence

26.
Test units, not classes!
http://andrzejonsoftware.blogspot.com/2015/02/tdding-unit-not-class.html
http://andrzejonsoftware.blogspot.com/2014/04/tdd-and-rails-what-makes-good-unit.html
http://blog.arkency.com/2014/09/unit-tests-vs-class-tests/

27.
Escape from the
framework ASAP
http://blog.arkency.com/2015/03/extract-a-service-object-in-any-framework/
Call application services from the framework

28.
Active Record doesn’t
make your code happy
AR is not DDD-lite-friendly
Data Mapper is better here

29.
ActiveRecord is a short-term loan
are you able to pay the debt?
what is your credit history?

30.
Don’t just pass the
params/hash everywhere
http://andrzejonsoftware.blogspot.com/2011/12/args-opts-params.html

31.
Wrap your legacy
contexts with the
application services layer

32.
Use events to
communicate between
bounded contexts
https://github.com/arkency/rails_event_store
https://github.com/arkency/ruby_event_store

33.
Sync may be safer
than async

34.
Separate reads/writes
at the services layer
CQRS

35.
Eliminate querying
from BC to another
Push the data!

36.
Keep the Bounded Contexts
in-sync by publishing all the
important events

37.
Easier testing when no
dependency on other
system

38.
Read models
Denormalized data

39.
Treat your frontend as a
separate Bounded
Context

40.
When in doubt, think
in aggregates

41.
Event sourcing
build your aggregates from events
audit log for free
(no, it’s not slow, but thanks for asking)

42.
Distributed
transactions are tricky

43.
Easier setup of the
project when it’s a smaller
piece
Useful when you’re maintaining many projects and need to bugfix them from time to time

44.
Legacy -> DDD books
• red book, Vaughn Vernon
• Implementing DDD
• blue book, Eric Evans
• Refactoring, Fowler
• Legacy code, Feathers
• Fearless Refactoring: Rails Controllers

45.
THANKS!
andrzej@arkency.com