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DDD-rail Your Monorail
Breaking up the Rails monolith
with domain driven design
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Hi, I’m Andrew
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Meet Delorean
“It’s like Uber, for time travel!”
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The situation
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%
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What happened?
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In the beginning, there
was an app.
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The business decides…
• Drivers deliver passengers from year A to
year B. ⌛
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The business decides…
• Drivers deliver passengers from year A to
year B. ⌛
• Passengers choose the type of Delorean
service they want.
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The business decides…
• Drivers deliver passengers from year A to
year B. ⌛
• Passengers choose the type of Delorean
service they want.
• Time travel carpools! 01
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The business decides…
• Drivers deliver passengers from year A to year B. ⌛
• Passengers choose the type of Delorean service they
want.
• Time travel carpools! 01
• DeloreanEATS: Customers order food, drivers pick
up from time period and deliver to customer time
period!
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Hm.
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Regressions
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The Payments team regresses
Trip while refactoring ServiceTier
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The Restaurants team deploys a
new pricing algorithm that
regresses rideshare pricing
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Responsibilities?
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The Mobile team requests a new
mobile login API, but which team
should implement it?
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Business won’t stop
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Outsourced dev shop is rebuilding
the marketing home page and
needs a pricing API.
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CEO wants to launch "Airbnb for
time travel” feature in, let’s say, 2
months!
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Does this sound familiar?
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There are deeper insights to be had
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What is DDD?
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A set of techniques to arrive at a
flexible design that cleanly maps
to the business model
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Strong, expressive domain
models
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There is no such thing as a
One True Perfect model
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Embrace the chaos
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Domain-Driven
Design: Eric Evans
Implementing Domain-
Driven Design: Vaughn
Vernon
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Let’s get designing!
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Step 1: Visualize your domain models
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Rails ERD
https://github.com/voormedia/rails-erd
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Ruby gem to generate UML diagrams from
your ActiveRecord models
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This helps you get the entire system into
your mind.
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Print it out!
✏
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Step 2: Find your core- and sub-domains
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Concept: Core Domain
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What a business does
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Transportation Core Domain
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Concept: Subdomains
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A supporting unit within the
business
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Rideshare Subdomain
Food Delivery Subdomain
Financial Transaction Subdomain
Identity and Access Subdomain
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Ridesharing
Food Delivery
Financial
Transactions
Identity/
Access
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Step 3: Get the team talking in the same
language
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Concept: Ubiquitous Language
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A defined language that is used
consistently across business and
technical contexts
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Bring in the domain experts
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A Driver picks up a Passenger
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A Passenger requests a Pickup
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An Owner drives a Vehicle
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An Operator? drives a Vehicle
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A User logs in and changes her Password
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An Invoice is delivered to the Passenger
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A Customer orders an item from a Menu,
which is picked up and delivered by the
Delivery Agent
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Step 4: Make a glossary
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Ridesharing
• Passenger: “…”
• Driver: “…”
• Trip: “…”
• Pickup: “…”
• Vehicle: “…”
• Vehicle Owner: “…”
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Financial Transaction
• Invoice: “…”
• Order: “…”
• Payment: “…”
• Royalty: “…”
• Salary: “…”
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Identity and Access
• User: “…”
• Secure password: “…”
• Role: “…”
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Print ‘em out!
✏
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Step 5: Draw out your software systems
(bounded contexts)
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Concept: Bounded Contexts
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A software system that defines the
applicability of a ubiquitous
language
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FoodDeliveryService TripRoutingEngine
“menu” “trip”
“restaurant” “plan”
“delivery” “pickup”
“customer date” “destination”
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Software systems are natural boundaries for
these linguistic terms
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If a term leaks into a different software
system/bounded context, you have a smell
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Ridesharing
Financial
Transactions
Identity/
Access
Food Delivery
DeloreanMonolith
StripeAPI
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DeloreanMonolith
Ridesharing
Financial
Transactions
Identity/
Access
Stripe API
Food Delivery
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DeloreanMonolith
Identity/
Access
Food Delivery
Driver
Routing
Service
Fraud
Detection
Service (etc)
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DeloreanMonolith
Ridesharing
Financial
Transactions
Identity/
Access
Stripe API
Food Delivery
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Step 6: Now add directional dependencies
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DeloreanMonolith
Ridesharing
Financial
Transactions
Identity/
Access
Stripe API
Food Delivery
U
D
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DeloreanMonolith
Identity/
Access
Food Delivery
Driver
Routing
Service
Fraud
Detection
Service (etc)
U
U
D
U
D U
D
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This helps you see dependencies between
teams, where communication will be most
important.
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Context Map: A tool to visualize how your
software systems relate to each other and
the domain(s)
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Our goal is to map our bounded contexts
directly to our subdomains.
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Ridesharing
Financial
Transactions
Identity/
Access
Stripe API
Food Delivery
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Show me the code!
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Tactic 1: Get the names right
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Adjust your class and method names to
reflect the ubiquitous language
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# old
User.without_drivers
# new
Passenger.hailing_drivers
# old
order.calculate_cost
# new
order.calculate_billing_total
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Tactic 2: Namespace and
modulize your domains
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Break Rails’ folder structure conventions
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app/domains/financial
app/domains/financial/inflation_adjustment.rb
app/domains/financial/invoice.rb
app/domains/food_delivery
app/domains/food_delivery/menu.rb
app/domains/food_delivery/menu_item.rb
app/domains/food_delivery/menu_items_controller.rb
app/domains/food_delivery/menus_controller.rb
app/domains/identity
app/domains/identity/user.rb
app/domains/identity/users_controller.rb
app/domains/rideshare
app/domains/rideshare/driver.rb
app/domains/rideshare/passenger.rb
app/domains/rideshare/service_tier.rb
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class Menu < ActiveRecord::Base
belongs_to :restaurant
end
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module FoodDelivery
class Menu < ActiveRecord::Base
belongs_to :restaurant
end
end
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class Trip < ActiveRecord::Base
belongs_to :service_tier
belongs_to :vehicle
end
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module Rideshare
class Trip < ActiveRecord::Base
belongs_to :service_tier
belongs_to :vehicle
end
end
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Domain code stays together
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Tactic 3: Design with
Aggregate Roots
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Aggregate: A collection of domain objects
that can be treated as a single unit
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Use aggregates to model real-world entities
that belong together
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Aggregate Root: An entity at the “top” of the
collection that can represent the whole
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FoodDelivery aggregate root: Order
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Food Delivery
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A user adds a menu item to their cart
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A User adds a MenuItem into their
ShoppingCart
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module Financial
class ShoppingCart
def order
@order ||= FoodDelivery::Order.new
end
def amount
@order.menu_items.sum(&:cost) +
@order.menu_items.sum(&:tax_amount)
end
def add(item)
order.menu_items << item
end
def remove(item)
order.menu_items.delete(item); order.save
end
end
end
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module FoodDelivery
class Order < ActiveRecord::Base
belongs_to :user
has_many :order_menu_items
has_many :menu_items, through: :order_menu_items
end
end
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module FoodDelivery
class Order < ActiveRecord::Base
belongs_to :user
has_many :order_menu_items
has_many :menu_items, through: :order_menu_items
def total_cost
item_cost + tax_cost
end
def item_cost
menu_items.sum(&:cost)
end
def tax_cost
menu_items.sum(&:tax_amount)
end
def add_item!(menu_item)
menu_items << menu_item
end
def remove_item!(menu_item)
menu_items.delete(menu_item); save
end
end
end
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module Financial
class ShoppingCart
def order
@order ||= FoodDelivery::Order.new
end
def amount
@order.total_cost
end
def add(item)
order.add_item!(item)
end
def remove(item)
order.remove_item!(item)
end
end
end
Calculation logic kept in
FoodDelivery domain
Implementation-
agnostic
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The root items of these aggregates are the
only entities that external callers may fetch
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Ridesharing
Financial Transactions
Identity/Access
Food Delivery
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Building good interfaces for a service
oriented future!
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Tactic 4: Break database
joins between domains
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`has_many`-itis!
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module Rideshare
class Trip < ActiveRecord::Base
belongs_to :service_tier
has_many :trip_pool_trips
has_one :trip_pool, through: :trip_pool_trips
belongs_to :driver, foreign_key: :driver_id, class_name: I
belongs_to :passenger, foreign_key: :passenger_id, class_n
belongs_to :vehicle
belongs_to :order, class_name: FoodDelivery::Order
has_one :payment
end
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These tend to happen in your God Objects
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module Rideshare
class Trip < ActiveRecord::Base
belongs_to :service_tier
has_many :trip_pool_trips
has_one :trip_pool, through: :trip_pool_trips
belongs_to :driver, foreign_key: :driver_id, class_name: I
belongs_to :passenger, foreign_key: :passenger_id, class_n
belongs_to :vehicle
belongs_to :order, class_name: FoodDelivery::Order
has_one :payment
end
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module Rideshare
class Trip < ActiveRecord::Base
# belongs_to :order, class_name: FoodDelivery::Order
def order
FoodDeliveryAdapter.new.order_from_trip(self)
end
end
end
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module Rideshare
class FoodDeliveryAdapter
def order_from_trip(trip)
FoodDelivery::Order.find_by(trip_id: trip.id)
end
end
end
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Decoupling domains now will ease your
architectural transitions later
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You can do this all in small
steps!
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Toward a distributed
architecture
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Namespaced, Isolated Modules to
Rails Engines
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Component-Based Rails
Engines (Stephan Hagemann)
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Rails engines to a remote (micro-)service
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Good architecture goes a very long way.
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To recap
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Drew things on a wall
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Came up with a language
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Moved code around
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Team is on the same page
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Big idea: Your software speaks the
same language as your domain
experts
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Big idea: Bounded contexts are separators
for linguistic drivers. Keep your language
consistent in one and only one system
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Big idea: Domain code
should live together
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Big idea: Adapters between
domains enforce domain purity
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Big idea: Incremental
changes
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Thanks!
Sample code: https://github.com/andrewhao/delorean
Slides: https://github.com/andrewhao/dddrail-talk
Twitter @andrewhao
Github @andrewhao
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