ddd dirty harry style

Dirty Harry’sStyle

@ziobrando

DDD

giovedì 23 maggio 13

Goal #1

Domain-Driven Design NOT starting from scratch


Goal #2

Arrive to DDD in a non prescriptive fashion


Do you really need me?giovedì 23 maggio 13

Checkpoint

When to apply DDD?- Complex problem- High expectations


Anti-pattern

I want to do DDDI don’t know DDD well...

...so I apply DDD to a simple problem

I don’t see any advantage...DDD is useless.


Wrong


We don’t learn in flatlandsgiovedì 23 maggio 13

That’s not our job


Call a Bomb Squadgiovedì 23 maggio 13

Our Safety Net

✓Environments

✓Build Scripts

✓Test suite

✓Private Database

✓Fixtures

✓Continuous Integration

✓...giovedì 23 maggio 13

Toolsgiovedì 23 maggio 13

Professionalsgiovedì 23 maggio 13

Proper Tools


Even in the worst situationsgiovedì 23 maggio 13

Could you please pass me

the ...thing?

The s5ngy one?

Yes, the one that looks

like ... that


Tools


Precision


Contraceptive

Aspirin


Language


Siete


avanscoperta

Checklist

Tools? --> Tool. (always the same)Precision? --> “When you are in state 12, the field MRKT_NFLD can be only B or K”Language? --> :-(


Application


Application

Application


Application

ApplicationApplication


Application


Application


Application


Application

Application


Application


Application

Application

Database


Application


Application

Application

Database

Application


Application


Application

Application

Database



Application


Application

Application

Database


Database


Application


Application

Application

Database


Database

Data-based integrationgiovedì 23 maggio 13

avanscoperta

Data-‐based integra5on checklist

Can we tell...

Which application is responsible for the data structure?

Which requirement influenced data definition?

Is it still valid?

It it valid in my context?

Which applications use that data?

Which consequences if we alter the structure?


Goal #1

Arrive to DDD in a brownfield scenario


Solution #1

Put the mess aside and start form a greenfield scenario


Which problem do we want to

solve?


Onegiovedì 23 maggio 13

Our

problemgiovedì 23 maggio 13

85%

15%

Our PROBLEM domain

past SOLUTIONS

Complexity


Our problem

Il nostro contesto

I nostri dati

I dati da condividere

Altro contesto

Altro contesto

Il nostro database

Language!!


TRESPASSERS WILL BE SHOT

SURVIVORS WILL BE SHOT AGAIN

THIS DATABASE IS MY DATABASE


“Put in this way, it’s simple...”

Dev:


DDD helps to solve complex problems,


DDD helps to solve complex problems, NOT to make simple problems complicated


Zombie management

strategies


A fence...giovedì 23 maggio 13

Learnt from the Romans


Romulus, let me run this query,

please...



please...

Nope.



please...

Nope.Please,

Romulus ... it’s a read-‐only one


Bounded Context


Anti-Corruption Layer

getContattiUnici(…):List<Contatto>

<<Service>>ContactService

ContactService

context boundary

AttributoAttributo

<<Value Object>>Contatto

AttributoAttributo

<<Entity>>MyEntity

Our Context ACLcontext boundary

Componente

Other Context





ContactService

context boundary

AttributoAttributo


AttributoAttributo

<<Entity>>MyEntity


Componente

Other Context

Model as it should be





ContactService

context boundary

AttributoAttributo


AttributoAttributo

<<Entity>>MyEntity


Componente

Other Context


The legacy component with all its troubles





ContactService

context boundary

AttributoAttributo


AttributoAttributo

<<Entity>>MyEntity


Componente

Other Context


The legacy component with all its troubles

Here we solve the mess, by any possible means.


...and within our Bounded Context...


The ideal modelgiovedì 23 maggio 13

It’s not that important...


Compliance


There’s no medal


But maybe...

http://www.youtube.com/watch?v=zDZFcDGpL4Ugiovedì 23 maggio 13

http://www.youtube.com/watch?v=zDZFcDGpL4U

http://www.youtube.com/watch?v=zDZFcDGpL4U

DDD as a learning process


DDD patterns support frequent

rewritingsgiovedì 23 maggio 13

DDD as an architecture that

supports evolution


...better provide us the right support

tool for design


Whiteboards


CRC Cards


“Can we really do this?”

P.O.:


“We need to put a valida9on layer to avoid incorrect data to enter into the system”

Dev:


Really?


Complex

validation is a

smell


(almost) same data

Different Behaviour

submitregisterPayment

amountcustomerstate

<<Entity>>Fattura

editvalidate

amountcustomer

<<Entity>>InvoicePreview


Multiple models


Frequent

Archetypes


3 archetypes?

Collaborative constructionTopic+Conversation

Facebook, Basecamp, Github

ExecutionState machines, Commands

TrackingLogging, Auditing, Datawarehouse, Events


Haven’t put “why do all my DDD

application looks the same” online, yet


What does the DBA say?


“I was expec9nd the DB structure to be defined once for all...”

DBA:


...


My first idea...


She doesn’t exist in reality


Can a software component designed

before one application be adequate to N

following applications?giovedì 23 maggio 13

Obstaclesgiovedì 23 maggio 13

You have to watch your back

before interacting with system X


There are some f***ing stored

procedures that run every 10 minutes


Nobody knows exactly how do

they workgiovedì 23 maggio 13

Nobody knows exactly how do

they work

Nobody ever changed them and

came back alive


Hrmpf


Risk...giovedì 23 maggio 13

Dress up properly before going

out! Are you wearing your T-‐

shirt?Lock the door when you get out!

Drive slowly!


Anxiety


Cognitive load

How many

things should

I know, before

touching the

code?


can we be ignorant and

productive at the same time?


Transaction management?

Presentation Layer Application Layer Domain Layer Infrastructure Layer

DDD

99% of ex

ample

sgiovedì 23 maggio 13

Repository & SRP

delivery()

pricecustomer

<<Entity>>Order

createOrder(…)delivery(order_ID, …)

<<Facade>>Application Facade

+saveChanges()

<<ORM>>Context

save(Order, Context)findById(id, Context)

<<Repository>>OrderRepository

createEmpty()

<<factory>>OrderFactory

opens / closes

uses

creates

Infrastructure layer

Application LayerDomain Layer

Riceve il contesto transazionale dall'esternoIl Repository effettua le

operazioni di aggiunta e/o rimozione dal contesto, ma è l'application layer ad invocare saveChanges()

Crea il contesto e lo passa al repository


Do not

accept

candies

from

strangers


...why don’t we

set up a shared

database table?...


Ooops...


Do not

accept

candies

from

strangers


Do not accept candies


Motivationsgiovedì 23 maggio 13

Difficult childhood

namesurnamephoneNumber

Person

studentCardNumberStudent

subjectstatus

Professor


Show me you’re a

person...giovedì 23 maggio 13

...print yourself!


Difficult Childhood

namesurnamephoneNumber

Person

studentCardNumberStudent

subjectstatus

Professor


The two things do not match

...and it’s not an ORM problemgiovedì 23 maggio 13

Every architectural rule that starts with “every” is wrong


Consequencesgiovedì 23 maggio 13

You see, the worls is divided into two

categories: the one in a Bounded Context, and

the one who digs...


You ...dig!


Estimations


The tile layer


Accounting it right


Accounting it right

Implementing amazing functionalities € 10.000


Accounting it right

Implementing amazing functionalities € 10.000Learning new techniques and tools € 4.000


Accounting it right


Establish Safety Conditions € 12.000


Accounting it right


Establish Safety Conditions € 12.000Cleaning up somebody else’s mess € 8.000


That will cost you an €xtra, sir...


Value


Value

Don’t forget to count the extra value also.


Estimations


Estimations“Explorative” evolutions


Estimations“Explorative” evolutions

Legacy “Pandora’s box”giovedì 23 maggio 13

Ignorance based planning

100 1 2 3 4 5 6 7 8 9

100%

0

10%

20%

30%

40%

50%

60%

70%

80%

90%

Time

Igno

ranc

e

Ignorance

Breakthrough

Ignorance


Gold plating?


Product Owner

Team


Product Owner

Team

A simple requirement


Product Owner

Team

A simple requirement A simple requirement


Product Owner

Team


Done as DDD prescribes


Product Owner

Team



In a clean and elegant fashion


Product Owner

Team




Soon


Product Owner

Team




Soon ....ooops


Yes, but how can I understand if I am

doing DDD?


avanscoperta

Are we really doing DDD?

Result is better than expectationsNo fear in rewriting piecesWe’re having fun


Questions?


Questions?

Really, you can...


Questions?


Questions?

Really, you can...


Thank you

@[email protected]


mailto:[email protected]

mailto:[email protected]

ddd dirty harry style

Technology

bethe legacy

solve complex

accept candies

simple requirementdone

corruption

apply ddd

ddd prescribesin

bounded context