DDD patterns that were not in the book

@cyriuxCyrille Martraire

DDDPatternsBeyond the Blue Book

Event Storming Event Sourcing

CQRSDDD

Whatwasbefore?

Isituseful?

CanIgetbetteratDDDbylookingatthingsoutsideofDDD?

Thistalkistheanswer.

Passionate developer Deliberate Designer

PARIS

Since 1999

@cyriuxCyrille Martraire

Paris Software Craftsmanship Community since 2011

http://www.meetup.com/paris-software-craftsmanship/

http://www.meetup.com/paris-software-craftsmanship/

arolla.fr @arollafr

http://arolla.fr

arolla.fr @arollafr

TDDBDDDDD

http://arolla.fr

Software Development

Its history

Software Engineering

Tools, notations, MDA

19

Vendor-sponsored conferences

Fortunately

Eric Evans - Domain-Driven Design

ShoulditbetheonlybooktoreadtolearnDomain-DrivenDesign?

THE ONE & ONLY BOOK?

Anyheadlinethatendsinaquestionmarkcanbeansweredbyno.

EricEvansadvocatesinhisbooktolookoutsideformorepatterns&

inspirations.

Idid.

ForthistalkIhaveselected43patternsandI'mgoingtoexplainthem

onebyonenow.

<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

”Becausebits,stringsandnumberscanbeusedtorepresentalmostanything,anyone

inisolationmeansalmostnothing.”

Exceptional Value

NaNUnde:ined

NotApplicable

25m+NaN=NaN

Don’tclutteryourcodewithspecialcasehanding

goingfurther…

Cashflow Sequence +

Cashflow Sequence =

Cashflow Sequence

Monoid

Could we say that?

Hotel Booking Jan 26 to 27 Hotel Booking

Jan 27 to 29

Hotel Booking Jan 26 to 29

+=

Monoid

Monoids Vector Spaces Cyclic Groups

Monoidsadvertisementbroughttoyouby

@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.

observerSingleton

FactoryCommand

VISITOR

MEDIATOR

Manager

observer

PROXY

FACADE

MEMENTO

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

Composite Interpreter Flyweight Strategy

observerSingleton

FactoryCommand

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 name s.

DateAdjustmentStrategy

Strategy

DateAdjustment DayShiftMethod EndOfMonthRule RoundingConvention CalculationMode

Strategy

DateAdjustment DayShiftMethod EndOfMonthRule RoundingConvention CalculationMode

Strategy

Belongs to the Ubiquitous Language

ReimbursementPolicy EligibilityCriterion AcceptanceSpecification

Strategy


AccountingValuation NetPresentValueValuation LinearValuation BenchmarkBasedValuation AssetBasedValuation

Strategy

AccountingValuations NetPresentValueValuation LinearValuation BenchmarkBasedValuation AssetBasedValuation

Strategy


AccountingValuations NetPresentValueValuation LinearValuation BenchmarkBasedValuation AssetBasedValuation

Strategy

No Noise

ValuationImpl

Strategy

Strategy PatternVAT

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 annotationsStereotypes Annotations

Stereotypes AnnotationsAnnotate classes with custom annotations

Domain type names are precious. They are for

domain naming.

Use annotations to document the

stereotypes instead.

Stereotypes AnnotationsAnnotate classes with custom annotations

Embedded Learning

Stereotypes AnnotationsAnnotate with custom annotations

Stereotypes AnnotationsAnnotate with custom annotations

Stereotypes AnnotationsAnnotate domain classes with custom annotations

Stereotypes AnnotationsAnnotate domain classes with custom annotations

And learn on the job!

https://leanpub.com/livingdocumentation

BUY MY BOOK!

LIVING DOCUMENTATION

AndnowformoreObject-OrientedPatterns

But Cyrille, Nobody does

Object-Oriented anymore!

NOW WE DO FP!

#NoPAttern

EvergreenSKILLS

Evergreen skills are worth learning

Sonow…IntroducingtheFlyweight

pattern!

Flyweight

Intent:Usesharingtosupportlargenumbersof[ine-grainedobjectsef[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- -

$50Mx

$10MIBM

$30MIBM AXA

$50M

Derivative Trading:

Multiple Contracts with a common structure

$10MIBM

$30MIBM AXA

$50M

Derivative Trading:

x

$10MIBM

$30MIBM AXA

$50M

The essence of Flyweight

x

Many small, calculable elements One BlackboxIntrinsicExtrinsic

#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 SOFASCINATEDwith 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)

ThenIdiscoveredMartinFowler

Event Sourcing-ish

Retail Banking

Medical Care

Traceability

Patterns illustrate deeper principles.

Traceability always points to the Past.

(aka. Consequences depend on the Cause)

ORDERBASKET

PAYMENTRETURN

timerefers torefers torefers to

causality

e-commerce

Study the patterns. Digest the principles.

Apply to new domains.

CLAIMINSURANCE

POLICYLOSS

$ PAYMENT

timerefers torefers torefers to

causality

Insurance

EvergreenSKILLS

Different domains, same principle

SuppleDomainModels

BespokeCode

Bespoke, Custom. OK.

Reinventing the Wheel. NOT OK.

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

Temporal Property


Snapshot


Temporal Object

Effectivity Range

Agreement v1 - effectivity: [01/2016, 01/2017[ Agreement v2 - effectivity: [01/2017, 01/2018[

Agreement v3 - effectivity: [01/2018, - [

Invariant:rangesarecontinuous&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:rangesarecontinuous&non-overlappingWhat version was

effective on this date?


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.


Append-onlyCompensation

[…] you may have only additive changes. An additive change

always goes onto the end of the record.


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 ANALYSISPATTERNS

TO DESIGNDOMAINMODELS

Not just CodeAlso 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




else

Law: one side has to be cash

166




else

Received Date

Paid Date

Trade Date

Congratulations! You can now reuse

this domain insight!

WHY NOT WRITE YOUR OWN PATTERNS?

SO MANY ANALYSISPATTERNS

TO DESIGNDOMAINMODELS

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

to90k$

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



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.



ReviewRisk checkLegal

check

Management Review

Management Review

Product Launch

R&D Done Sales

ReadyMarketing Campaign

Product SpecsProduct 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 talkFlyweight 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

http://cyrille.martraire.com