Domain Driven Rails

@skwp @reverbdotcom #wcr14
Yan Pritzker
CTO, Reverb.com 
@skwp @reverbdotcom
Domain Driven Rails
https://speakerdeck.com/skwp

Are you happy with the  
SIZE and COMPLEXITY  
of your models?

SCALABILITY 
of your team?

ADAPTABILITY 
of your business?

What are you
building?

Rails is a
detail!"
Decouple
all things"
Is the code
better?"
Keep it
Railsy"

Somewhere
in between
Simple 
CRUD"
Apps"
Complex 
Enterprise 
Logic"

I want to discover  
relevant compromises  
rather than defend ideals

Over
115 
Model
Classes

Over
1000 
Total
Classes

Not great, but we have only
three chubby models

Are monoliths
bad for business?

Quick Iterations
Low Operational  
Complexity
Refactoring

Monolith
ServiceService
Service
Service
Service
Early 2013 - Startup / Proof of Concept

Monolith
ServiceService
Service
Service
Service
2014 - Growth Phase

http://martinfowler.com/articles/distributed-objects-
microservices.html
“I'm wary of distribution and  
my default inclination is to prefer
a monolithic design”
“While small microservices are certainly simpler to reason
about, I worry that this pushes complexity into the
interconnections between services”
“Refactoring becomes much harder when you have to do it
across remote boundaries.”

Quick Iterations
Low Operational  
Complexity
May lead to a BBOM

Maintainable
Monoliths
Can Be
Achieved

Product 
400 LOC 
~150 LOC non-ActiveRecord
Churn: 49 changes this year"
Order 
333 LOC 
Churn: 36 changes this year
User 
338 LOC 
Churn: 29 changes this year

Commonly used
classes are hard
to refactor

Stop  
modifying  
code!
(Open/Closed Principle)

Don’t put
different rates
of change
together"
Kent Beck - Smalltalk Best Practice Patterns
(see also: Single Responsibility Principle)

Don’t put
different rates
of change
together"
Kent Beck - Smalltalk Best Practice Patterns
Data Model
Business Logic

Separate  
what the system is "
from what the
system does"
James Copelien & Trygve Reenskaug
(Data, Context, Interaction)

Where does business
logic go?

Controller
2005
ActiveRecord
Mailers
Services
User

Console?"
Rake task?"
Background jobs?"
API layer?"
Testing?
What about…

Order
Fat models?

Order
Refund

Order
Refund
Ship

Order
Refund
Ship
Check Fraud Risk

“Active Record is a good choice
for domain logic that isn't too
complex, such as creates, reads,
updates, and deletes”
Martin Fowler

If Controller and Model
are all you have then  
one has to be skinny and
one has to be fat

Domain Layer
skinny framework,  
healthy business logic,"
no fat anywhere
http://joncairns.com/2013/04/fat-model-
skinny-controller-is-a-load-of-rubbish/

Model
Controller
View
The Rails Way
The
Way
Active
Record
Use Cases
Grape
API
ControllersCron
Redis
Rake
HTTP
Services
Workers
R
Entities
Roles
DB
Listeners
Events

app/reverb
for app speciﬁc

lib/reverb
for Open Source / Generic

Reverb::
Namespace to avoid
collisions with gems

This is a 
marketplace!
http://blog.
8thlight.com/uncle-
bob/2011/09/30/
Screaming-
Architecture.html

http://blog.8thlight.com/uncle-bob/2012/08/13/the-clean-
architecture.html

see also Hexagonal, Ports & Adapters, DCI

Clean  
Enough  
Architecture"

Choosing the
right ﬁght

http://blog.8thlight.com/mike-ebert/2013/03/23/the-
repository-pattern.html

http://blog.8thlight.com/mike-ebert/2013/03/23/the-
repository-pattern.html
(but you should still read this)

User.where(…)
User.active
User.ﬁnd(1)
This is easy to replace with 
something other than AR.  
Repository not required.
Don’t leak SQL outside of AR

Domain Logic in
ActiveRecord and
Controllers 
leads to Churn

Use Cases
Reify complex business logic 
into classes

This is not"
Rails

Explicitly Require Dependency
http://myronmars.to/n/dev-blog/2012/12/5-reasons-to-avoid-
bundler-require
Invoke It

Code Reuse!
Thin Shell

Stubby
Happy Path

Testing conditionals and side
effects has nothing to do with Rails

Naming
The hardest problem in computer science

OrderService

Order 
Service

Order 
Service
NounService Grows Unbounded

Use verbs to
narrow your scope

Order

Order
ShipOrder

Order
ShipOrder
Cancel 
Order

Order
ShipOrder
Cancel 
Order
Refund 
Order

Order
ShipOrder
Cancel 
Order
Refund 
Order
Does not 
grow over time

ShipOrder
Cancel 
Order
Refund 
Order
Don’t change"
once you write them

Ubiquitous Language

What is this?

Roles
Add methods to objects on demand
in the context of a Use Case

Decorator

We added these methods

Methods related to each"
other but loosely related"
to the parent concept"
and used only in a few"
Use Cases

Events
Add behavior with listeners
without modifying code

Order
ActiveRecord Callbacks
Send Email

Order
Send Email
Call External Service

Order
Send Email
Call External Service
Conditional 
Callbacks

AR callbacks become
more complex as the
system supports more
use cases

Different use cases may
trigger different events
even when working with
the same model
Instead

Add another listener to add behavior 
The core class doesn’t change

Global listeners for cross-cutting concerns 
without littering code
Wisper::GlobalListeners.add(Reverb::Listeners::AnalyticsListener.new)"

Controller is a listener too

Policy Objects
Reify complex business rules
into objects

Likely to change
Unlikely to change

refactored for readability

Injectable, but has a default
Simple code, only need to test one path 
for the imperative side effect"
(sending an email)

Distillation Time

Language of codebase  
reﬂects language of business

Separate behaviors (what the system
does) from models (what the system is)

Rates of Change

Is this all Unicorns
and Rainbows?

Sprawl?"
Onboarding?"
Naming?"
Documentation?"

http://blog.mattwynne.net/category/hexagonal-rails/
http://clean-ruby.com/
Resources
http://confreaks.com/videos/759-rubymidwest2011-keynote-
architecture-the-lost-years
http://blog.8thlight.com/uncle-bob/2011/09/30/Screaming-
Architecture.html
http://www.artima.com/articles/dci_vision.html
http://blog.8thlight.com/uncle-bob/2012/08/13/the-clean-
architecture.html
https://speakerdeck.com/skwp

We Are Hiring Always"
jobs.reverb.com
Ruby, ElasticSearch, DevOps,
Designers, Android, and more!
jobs@reverb.com

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