Slides of my talk at DDD eXchange 2018 London.

1. @Ramtop
CQRS with Event
Source
in Functional sauce
served by Kotlin
Uberto Barbinigithub.com/uberto/anticapizzeria

2. @Ramtop
CQRS + Event Source
Very powerful
but a bit
intimidating…
Can we do
better?

3. @Ramtop
Functional Programming
● Immutability
● Purity
● Higher order functions
● Transformations and preserving
properties

4. @Ramtop
Functional vs OO Programming

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Command
●
A Command is a request for changing the
internal state of the system
●
A Command can “fail” if is not congruent with
the current state of the System
●
Each Command is executed in an atomic
context

6. @Ramtop
Query
●
A query ask for a snapshot of the state
●
Queries typically need different data and
denormalization from domain model. So we separate
the models in CQRS.
●
Queries are eventually consistent with the domain

7. @Ramtop
Values-Entities
●
They represent the state of the domain
●
In functional programming everything is
immutable, so there is no identity
●
To keep state changes we have Algebraic Data
Types

8. @Ramtop
Aggregates
●
An aggregate has all the information for a
transaction unit
●
Aggregates is what Events fold to
●
They are composed by Entities

9. @Ramtop
Event
●
Events are the “atoms” of System state change
●
Nothing can change without an event, every
event can change only one entity
● Entity + Event => Entity

10. @Ramtop
Actors
●
More powerful and easy to use concurrency
model than Threads/Locks
●
They communicate asynchronously and
potentially remotely
●
Useful for Bounded Contexts and Services

11. @Ramtop

12. @Ramtop
Antica Pizzeria
●
Real application is about finance products
booking but a pizzeria has a surprising similar
domain
●
We want to do implement the backend for a
ChatBot that will assist booking and enquires
about orders.

13. @Ramtop
Kotlin Syntax
●
No semicolon!
●
String? Is a different type from String
●
myFun{println(it)} is equivalent to
myFun(x -> println(x))
●
val xy is declaring xy immutable
var xy is declaring xy mutable
●
User(“Joe”) is equivalent to Java
new User(“Joe”)

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Commands

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Commands

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Queries

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Actors
(where we can put all this stuff)

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Commands emit Events
(it’s the only way to change)

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Events

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Queries listen to Events
(to keep up with the changes)

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Let's fold Events
(to create Entities)

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Entities as EventComposable

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Execute Commands

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Logic inside Commands

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Application

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We can now process C&Q
(Fun with Kotlin ExtFun)

27. @Ramtop
Real
Code
Now!

28. @Ramtop
QAUberto Barbini
@Ramtop
github.com/uberto/anticapizzeria

29. @Ramtop

30. @Ramtop