@cyriux
Cyrille Martraire
DDDPatterns
Beyond the Blue Book

Event Storming Event Sourcing
CQRS
DDD

What was before?

Is it useful?

Can I get better at DDD by looking
at things outside of DDD?

This talk is the answer.

Passionate developer
Deliberate Designer
PARIS
Since 1999
@cyriux
Cyrille Martraire

Paris Software Craftsmanship
Community since 2011
http://www.meetup.com/paris-software-craftsmanship/

arolla.fr @arollafr

arolla.fr @arollafr
TDDBDDDDD

Software Development

Its history

Software Engineering

Tools, notations, MDA

19
Vendor-sponsored conferences

Fortunately

Eric Evans - Domain-Driven Design

Should it be the
only book to read
to learn Domain-
Driven Design?
THE ONE & ONLY BOOK?

Any headline that ends
in a question mark can
be answered by no.

Eric Evans advocates in
his book to look outside
for more patterns &
inspirations.

I did.

For this talk I have
selected 43 patterns and
I'm going to explain them
one by one now.

<Facepalm>

OK.

"The best way to get the
right answer on the Internet
is not to ask a question, it's
to post the wrong answer.”
— Cunningham’s Law

Ward Cunningham
WIKI
XP
CRC Cards

Ward Cunningham
Patterns
in software!

http://c2.com/ppr/checks.html
DDD Inspiration

CHECKS Patterns
1. Whole Value
2. Exceptional cases (absence, error…)
3. Meaningless Behavior
4. Echo Back
5. Visible Implication
6. Deferred Validation
7. Instant Projection
8. Hypothetical Publication
9. Forecast Confirmation
10. Diagnostic Query Analysis Patterns

25m + 10m = 35m
(Custom Arithmetics)
Whole Object with unit.
Whole Object

Whole Object
#NoPrimitive
”Because bits, strings and numbers can be
used to represent almost anything, any one
in isolation means almost nothing.”

Exceptional Value
NaN
Unde:ined
NotApplicable
25m + NaN = NaN
Don’t clutter your code
with special case handing

going further…

Cashflow Sequence
+
Cashflow Sequence
=
Cashflow Sequence
Monoid

Could we say
that?
HotelBooking
Jan26to27 HotelBooking
Jan27to29
HotelBooking
Jan26to29
+
=
Monoid

Monoids
Vector Spaces
Cyclic Groups

Monoids
advertisement
brought to you by
@cyriux!

CHECKS Patterns
Traceability
Normal display: 652 MM USD EQV
Diagnostic display: 622,456,325.07 USD
+ 3,624,878,450 JPY
+ 23,549.54 FRF

CHECKS Patterns
1. Whole Value
2. Exceptional cases (absence, error…)
3. Meaningless Behavior
4. Echo Back
5. Visible Implication
6. Deferred Validation
7. Instant Projection
8. Hypothetical Publication
9. Forecast Confirmation
10. Diagnostic Query Analysis Patterns

THE
WORLD’s
FIRST
WIKI

PLoP98

Mind the biblio

In most domain
models, most design
patterns are
technical noise.
observer
Singleton
Factory
Command
VISITOR
MEDIATOR
Manager
observer
PROXY
FACADE
MEMENTO

But some design
patterns also
describe business
domain concepts
and relations.

Composite
Interpreter
Flyweight
Strategy

observer
Singleton
Factory
Command
VISITOR
MEDIATOR
Manager
observer
PROXY
FACADE
MEMENTO
Good-Old hint:
”Use pattern
names in type
names.”

In a domain model,
avoid pattern names
in types names.

DateAdjustmentStrategy
Strategy

DateAdjustment
DayShiftMethod
EndOfMonthRule
RoundingConvention
CalculationMode
Strategy

DateAdjustment
DayShiftMethod
EndOfMonthRule
RoundingConvention
CalculationMode
Strategy
Belongs to the
Ubiquitous
Language

ReimbursementPolicy
EligibilityCriterion
AcceptanceSpecification
Strategy
Belongs to the
Ubiquitous Language

AccountingValuation
NetPresentValueValuation
LinearValuation
BenchmarkBasedValuation
AssetBasedValuation
Strategy

AccountingValuations
NetPresentValueValuation
LinearValuation
BenchmarkBasedValuation
AssetBasedValuation
Strategy
Belongs to the
Ubiquitous
Language

AccountingValuations
NetPresentValueValuation
LinearValuation
BenchmarkBasedValuation
AssetBasedValuation
Strategy
No Noise

ValuationImpl
Strategy

Strategy Pattern
VAT
VAT on net price
Not Applicable
Calculation
+ Null Object

Null Object.
No, you should not
replace every
Null Object with
Optional<T>

When to apply a
pattern? (or not)

IF Statements vs. Strategy pattern?
Naming-Driven
Whichever reveals the
business domain best

AccountingValuation
NetPresentValueValuation
LinearValuation
BenchmarkBasedValuation
AssetBasedValuation
Strategy

DateAdjustmentStrategy
Domain Patterns
#NotInMyName

Annotate classes with custom annotations
Stereotypes Annotations

Stereotypes Annotations
Annotate classes with custom annotations

Domain type names are
precious. They are for
domain naming.
Use annotations to
document the
stereotypes instead.

Stereotypes Annotations
Annotate classes with custom annotations
Embedded Learning

Stereotypes Annotations
Annotate with custom annotations

Stereotypes Annotations
Annotate with custom annotations

Stereotypes Annotations
Annotate domain classes with custom annotations

Stereotypes Annotations
Annotate domain classes with custom annotations

And learn on the job!

https://leanpub.com/livingdocumentation
BUY MY BOOK!
LIVING DOCUMENTATION

And now for more Object-
Oriented Patterns

But Cyrille,
Nobody does
Object-
Oriented
anymore!

NOW WE DO
FP!
#NoPAttern

Evergreen
SKILLS

Evergreen skills
are worth
learning

So now…
Introducing the Flyweight
pattern!

Flyweight
Intent: Use sharing to support large
numbers of [ine-grained objects ef[iciently

”Hahaha.
Nobody uses the
Flyweight”

A matter of
Compression
= 3x

503010 …
Flyweight
BUY SELL

503010 …
Flyweight
We always want to make
multiplicity manageable

503010 …
-503010 …
Flyweight

-503010 …
{10, 30, -50 …}
Flyweight

{10, 30, -50 …}
∑ = -10
Flyweight

∑ = -10
manageable
= aggregable,
comparable…
Flyweight

Equity Trading:
Standard Stock
Multiple Trades of the same
$10M
-
$30M
- -
$50M
x

$10M
IBM
$30M
IBM AXA
$50M
Derivative Trading:
Multiple Contracts with
a common structure

$10M
IBM
$30M
IBM AXA
$50M
Derivative Trading:
x

$10M
IBM
$30M
IBM AXA
$50M
The essence of Flyweight
x
Many small, calculable elements One Blackbox
IntrinsicExtrinsic

#WIN
Flyweight

#WIN

This never works.

I see what you mean…
BUT no way, IT DOES NOT
WORK from a legal
perspective!

FATAL
OBJECTION

FATAL
OBJECTION
as hint of bounded
Context!

Tip: Focus on one context
=
≠
≠

Works in this context only
=
≠
≠
Legal
Money
Management

It works in my context!
=
≠
Transport
Regulation
Cargo
Shipping

In what
PERSPECTIVE could
we say that?
Bounded
Context

Different Bounded
Contexts,
Different Patterns.

Different Patterns,
Different Bounded
Contexts.

#WorksInMyContext

HEY
CYRILLE

WHY U SO
FASCINATED
with patterns?

Would you build
Photoshop only
with IF statements
and FOR loops?

for (...)
if (...)
if (..)
{
for...
}
else
else
while

for (...)
if (...)
if (..)
{
for...
}
else
else
while

You should
have a look at
design patterns!

Strategy
+
State
=
EASY AGAIN!

Patterns
ABUSE
(Like every
beginner)

Then I discovered Martin Fowler

Event Sourcing-ish

Retail Banking
Medical Care

Traceability

Patterns illustrate
deeper principles.

Traceability always
points to the Past.
(aka. Consequences depend on the Cause)

ORDER
BASKET
PAYMENT
RETURN
time
refers torefers torefers to
causality
e-commerce

Study the patterns.
Digest the principles.
Apply to new domains.

CLAIM
INSURANCE
POLICY
LOSS
$
PAYMENT
time
refers torefers torefers to
causality
Insurance

Evergreen
SKILLS

Different domains,
same principle

Supple Domain Models
Bespoke Code

Bespoke, Custom.
OK.
Reinventing the Wheel.
NOT OK.

https://martinfowler.com/eaaDev/timeNarrative.html
Temporal Property

https://martinfowler.com/eaaDev/timeNarrative.html
Snapshot

https://martinfowler.com/eaaDev/timeNarrative.html
Temporal Object

Effectivity Range
Agreement v1 - effectivity: [01/2016, 01/2017[
Agreement v2 - effectivity: [01/2017, 01/2018[
Agreement v3 - effectivity: [01/2018, - [
Invariant: ranges are continuous & non-
overlapping

Effectivity Range
Agreement v1 - effectivity: [01/2016, 01/2017[
Agreement v2 - effectivity: [01/2017, 01/2018[
Agreement v3 - effectivity: [01/2018, - [
06/2017
Invariant: ranges are continuous & non-
overlappingWhat version was
effective on this date?

https://martinfowler.com/eaaDev/timeNarrative.html
Bi-temporal Time
Time has two
dimensions in software!

[…] you may have only additive
changes. An additive change
always goes onto the end of the
record.
https://martinfowler.com/eaaDev/timeNarrative.html
Append-only
Compensation

[…] you may have only additive
changes. An additive change
always goes onto the end of the
record.
https://martinfowler.com/eaaDev/timeNarrative.html
Event-Sourcing-ish

Excellent Design
Teaching Material

Learn the
history, not just
the end state.
Learning Steps
(à la Event Sourcing)

Know the Holy Family
:)

Illustrious
Ancestors

Rebecca J. Wirfs-Brock
First RDD
paper in
1989!

http://tonyprusac.blogspot.fr/2015/11/deco2300-week-6.html
CRC Cards

Event Storming Ancestor
@mathiasverraes@ziobrando

MOAR
ANALYSIS
PATTERNS

TO DESIGN
DOMAIN
MODELS

Not just Code
Also Ideas!
Reuse all the things!
And past insights

164
A Business Transaction
A quantity of
something
(e.g. cash)
A quantity of
something
else

165
A Business Transaction
A quantity of
something
(e.g. cash)
A quantity of
something
else
Law: one side has to be cash

166
A Business Transaction
A quantity of
something
(e.g. cash)
A quantity of
something
else
Received
Date
Paid
Date
Trade
Date

Congratulations!
You can now reuse
this domain insight!

WHY NOT WRITE
YOUR OWN
PATTERNS?

SO MANY
ANALYSIS
PATTERNS

TO DESIGN
DOMAIN
MODELS

Most developers
have never seen a
domain model.
They've only seen
data models.

WHAT’S A
DOMAIN MODEL?

Let’s take an
example

Loan Approval
Loan
Application
System
Credit
Bureau
Signatures
Requests
Approval
100k$
Approved/
Rejected

Loan Approval
Loan
Application
System
Credit
Bureau
Signatures
Requests
Approval
100k$
change
to
90k$
Accept with
in-place mutation :(

Do you
approve
this 100k$
loan?

I approved
this 90k$
loan!
Do you
approve
this 100k$
loan?

… What
do I do
with that
now?!!

Answering a
different question is
rude!
(plus sync back the
mutation is not so easy)

What if we just
reject with a
suggestion to change
the amount before
re-submit?

In other words:
”Single Path to
Approval”
Jez Humble and David Farley, “Continuous Delivery: Reliable Software Releases through
Build, Test, and Deployment Automation”, Addison Wesley Professional, 2010

In other words:
”Single Path to
Approval”Production
Jez Humble and David Farley, “Continuous Delivery: Reliable Software Releases through
Build, Test, and Deployment Automation”, Addison Wesley Professional, 2010
Continuous Delivery

Loan Approval
Loan
Application
System
Credit
Bureau
Signatures
Reject + suggest
reduced amount
Requests
Approval
100k$
OK for
90k$
x
Approved/
Rejected

No patch here!
Fix the source then commit as usual

No way! It would
delay the process!
Plus we don’t want to
ask approvers to sign
again!
Business

OK.
Still, perhaps we
could do it anyway…
Dev Team

OK for 100k$
➜
a fortiori OK for 90k$
➜
”A fortiori principle”
No need to sign again!

Now we need to codify
accurately what
a fortiori means.
That’s modeling the
domain.

Not that hard:
Lower risk -> a fortiori OK.

Domain Modeling
means forming an
opinion on how it
behaves.

Programming as Theory-
Building
— Peter Naur (1985)

Anything can be an
inspiration.
Even Continuous Delivery
patterns!

Let’s take another
example

Only rebuild what needs to
be rebuilt after a change
Given a set of source code
files

http://maemo.org/maemo_release_documentation/maemo4.1.x/node5.html

http://maemo.org/maemo_release_documentation/maemo4.1.x/node5.html

http://maemo.org/maemo_release_documentation/maemo4.1.x/node5.html
Imagine doing that
with a workflow
engine?

GNU Make has an
opinion on how to
consider the
problem.

Declare the dependencies.
Let the tool find out the
workflow.

http://maemo.org/maemo_release_documentation/maemo4.1.x/node5.html

http://maemo.org/maemo_release_documentation/maemo4.1.x/node5.html
ReviewRisk
check
Legal
check
Management
Review
Management
Review
Product
Launch
R&D
Done
Sales
Ready
Marketing
Campaign
Product Specs
Product Design

A breakthrough in domain
modeling
Why insist on using
workflow engines
everywhere?

Do you have an
opinion on how to
consider the
business domain?

(Hint: ”We need Event
Sourcing” is probably not the
best answer)

Which pattern?
”Propose several models, choose one.”
Design Spike.
Try them.

Which pattern?
from simple to less simple.

Teaching Patterns
to your team mates

Mob-Programming
@woodyzuill

https://www.industriallogic.com/blog/design-patterns-playing-cards/
Design Patterns Poker cards

Design Patterns Poker cards

Show the benefits.
Don’t preach.

Own Internal Branding
You don’t have to talk
Flyweight or Monoid

The best code sample for training is
your own code base.
Embedded Learning

YOU DIDN’T SEE
ALL THESE PATTERNS
BEFORE?

You probably
procrastinate on
tech stuff.

No, you probably
don’t need this
Event Sourcing
here.

You probably should
spend more time
Domain Model
Technical Infrastructure
HERE

Design is hard?
Do it more often!

From Scratch?
Lack of domain
maturity makes
everything look
CRUD.

Mature Legacy
Systems FTW!

NEW HORIZONS

DCI by Jim Coplien #DDDEU

https://blog.acolyer.org/ http://paperswelove.org/

In closing

Many patterns authors.
Many patterns.
They converge, mostly.
No need to learn them all.

Many patterns authors.
Many patterns.
Still, learn more than one!

So many patterns outside
the Blue Book
But do you really know all
the patterns in the Blue
Book?

Merci !
h$p://cathy313.centerblog.net/539-bisounours

Follow me @cyriux
Slides: slideshare.net/cyriux
Blog: cyrille.martraire.com
We offer training, coaching & consulting on DDD, BDD, TDD