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The Myth of the
Modular Monolith
Eileen M. Uchitelle
Rails World 2024
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Eileen M. Uchitelle
@eileencodes
eileencodes.com
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Ruby & Rails Infra
at Shopify
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Rails Core Team
since 2017
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2010
🧍
My 14 years of Rails
Small & large companies
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2010 2024
My 14 years of Rails
Small & large companies
🌎
🧍
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2010
My 14 years of Rails
New & legacy applications
👧
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2010 2024
👧
My 14 years of Rails
New & legacy applications
👵
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2010
👩💻
My 14 years of Rails
New developers & DHH
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2010 2024
My 14 years of Rails
New developers & DHH
🏎
👩💻
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2014
My 14 years of Rails
Applications I've worked on
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2014 2022
My 14 years of Rails
Applications I've worked on
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No content
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2014 2024
My 14 years of Rails
Applications I've worked on
2017
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💩💻
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The Myth of the
Modular Monolith
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Common problems
in large monoliths
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Common problems:
Architectural
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Architectural
‣ Lack of
organization
and structure
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Architectural
‣ Lack of
organization
and structure
‣ Tight coupling
& no boundaries
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Common problems:
Operational
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Operational
‣ Flaky tests and
slow CI
Architectural
‣ Lack of
organization
and structure
‣ Tight coupling
& no boundaries
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Operational
‣ Flaky tests and
slow CI
‣ Scalability of
deployments
Architectural
‣ Lack of
organization
and structure
‣ Tight coupling
& no boundaries
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Common problems:
Organizational
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Operational
‣ Flaky tests and
slow CI
‣ Scalability of
deployments
Organizational
‣ Dif
fi
cult to
assign and
fi
nd
owners
Architectural
‣ Lack of
organization
and structure
‣ Tight coupling
& no boundaries
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Operational
‣ Flaky tests and
slow CI
‣ Scalability of
deployments
Organizational
‣ Dif
fi
cult to
assign and
fi
nd
owners
‣ Onboarding
new hires takes
too long
Architectural
‣ Lack of
organization
and structure
‣ Tight coupling
& no boundaries
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"I can't hold the
whole app in
my head!"
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*slaps roof of head*
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*slaps roof of head*
This bad boy can
fi
t so many models.
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"I can't hold the
whole app in
my head!"
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Operational
‣ Flaky tests and
slow CI
‣ Scalability of
deployments
Organizational
‣ Dif
fi
cult to
assign and
fi
nd
owners
‣ Onboarding
new hires is
dif
fi
cult
Architectural
‣ Lack of
organization
and structure
‣ Tight coupling
& no boundaries
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What is a
modular monolith?
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Modular monolith:
Use the same
language.
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Modular monolith:
Use the same
infrastructure.
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Modular monolith:
Path to package
isolation.
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How to implement a
modular monolith
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📁 app
📁 models
📄 dog.rb
📄 cat.rb
📄 tree.rb
📄 flower.rb
📁 controllers
📄 dogs_controller.rb
📄 cats_controller.rb
Fur & Foliage Application
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📁 packages
📁 pets
📄 dog.rb
📄 cat.rb
📁 app
📁 models
📁 plants
📄 tree.rb
📄 flower.rb
📁 app
📁 models
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📁 packages
📁 pets
📄 dog.rb
📄 cat.rb
📁 app
📁 models
📁 plants
📄 tree.rb
📄 flower.rb
📁 app
📁 models
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📁 packages
📁 pets
📄 dog.rb
📄 cat.rb
📁 app
📁 models
📁 plants
📄 tree.rb
📄 flower.rb
📁 app
📁 models
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📁 packages
📁 pets
📄 dog.rb
📄 cat.rb
📁 app
📁 models
📁 plants
📄 tree.rb
📄 flower.rb
📁 app
📁 models
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How to enforce
monolith boundaries
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Packwerk:
github.com/Shopify/
packwerk
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# packages/pets/package.yml
metadata:
stewards:
- "@FurAndFoliage/pets"
slack_channels:
- "#help-pets"
- "#team-pets"
enforce_dependencies: strict
dependencies:
- packages/plants
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# packages/pets/package_todo.yml
packages/billing:
"::Billing::Subscriptions::TestHelper":
violations:
- dependency
files:
- packages/pets/test/helpers/dog_test_helper.rb
packages/stores:
"::Store::TimelineEvent":
violations:
- dependency
files:
- packages/pets/app/models/dog.rb
- packages/pets/app/controllers/dogs_controller.rb
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The problem with
full isolation
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The problem with
full isolation
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New Challenges:
Primitive
obsession.
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# passing primitives instead of the cat AR object
class Flower < ApplicationRecord
def cat_can_eat?(cat_id)
cat = Pets::CatGetter.find_cat(cat_id)
cat && self.safe_for_cat?(cat)
end
end
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New Challenges:
Primitive
obsession.
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New Challenges:
Ownership
obsession.
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Pets Plants
X
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New Challenges:
Package
obsession.
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Canine
Feline
Daisy
Birch
Maple
Aster
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New Challenges:
Code
duplication.
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New Challenges:
Circular
dependencies.
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Pets Plants
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How modularization
falls short
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Operational
‣ Flaky tests and
slow CI
‣ Scalability of
deployments
Organizational
‣ Dif
fi
cult to
assign and
fi
nd
owners
‣ Onboarding
new hires is
dif
fi
cult
Architectural
‣ Lack of
organization
and structure
‣ Tight coupling
& no boundaries
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Architectural
Cultural
‣ Lack of
organization
and structure
‣ Tight coupling
& no boundaries
Operational
Cultural
‣ Flaky tests and
slow CI
‣ Scalability of
deployments
Organizational
Cultural
‣ Dif
fi
cult to
assign and
fi
nd
owners
‣ Onboarding
new hires is
dif
fi
cult
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Improve structure
& organization?
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Improve structure
& organization?
No.
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Code is less tightly
coupled?
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Code is less tightly
coupled?
No.
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Architectural
Cultural
‣ Lack of
organization
and structure
‣ Tight coupling
& no boundaries
Operational
Cultural
‣ Flaky tests and
slow CI
‣ Scalability of
deployments
Organizational
Cultural
‣ Dif
fi
cult to
assign and
fi
nd
owners
‣ Onboarding
new hires is
dif
fi
cult
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Architectural
Cultural
‣ Lack of
organization
and structure
‣ Tight coupling
& no boundaries
Operational
Cultural
‣ Flaky tests and
slow CI
‣ Scalability of
deployments
Organizational
Cultural
‣ Dif
fi
cult to
assign and
fi
nd
owners
‣ Onboarding
new hires is
dif
fi
cult
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CI is faster with
less
fl
aky tests?
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CI is faster with
less
fl
aky tests?
No.
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Deploys are more
scalable?
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Deploys are more
scalable?
Not yet.
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Architectural
Cultural
‣ Lack of
organization
and structure
‣ Tight coupling
& no boundaries
Operational
Cultural
‣ Flaky tests and
slow CI
‣ Scalability of
deployments
Organizational
Cultural
‣ Dif
fi
cult to
assign and
fi
nd
owners
‣ Onboarding
new hires is
dif
fi
cult
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Architectural
Cultural
‣ Lack of
organization
and structure
‣ Tight coupling
& no boundaries
Operational
Cultural
‣ Flaky tests and
slow CI
‣ Scalability of
deployments
Organizational
Cultural
‣ Dif
fi
cult to
assign and
fi
nd
owners
‣ Onboarding
new hires is
dif
fi
cult
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Find & assigning
owners is easier?
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Find & assigning
owners is easier?
Somewhat.
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Onboarding new
hires is easier?
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Onboarding new
hires is easier?
No.
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Why do we have
these problems?
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You cannot solve
human problems
with modularity.
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What causes
these problems?
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Causes:
Pressure to
ship.
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Causes:
Allowing tech
debt to grow.
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Causes:
Hiring faster than
you can onboard.
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Causes:
Misaligned
incentives.
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Causes:
Blame-based
development.
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How do we
fi
x
these problems?
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Team culture:
Improve developer
education.
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Team culture:
Indoctrinate
non-Rubyists.
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Team culture:
Reprioritize
quality.
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Team culture:
Realign reward
incentives.
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When to implement
modularization
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Modularization:
Start with just a
few packages.
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Modularization:
Functional
isolation.
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Modularization:
Don't do it
prematurely.
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Don't fall for
the sunk cost
fallacy.
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Scaling large
applications is hard
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We act like
the ball of mud
Is inevitable.
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These problems
aren't caused by
Ruby on Rails.
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You can't engineer
yourself out of a
large org.
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We want someone
or something
to blame.
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Engineering culture
comes from
the top.
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You cannot solve
human problems
with modularity.
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Let's invest in
fi
xing our human
problems.
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Thank
You ❤