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Functional CQRSFunctional CQRS

The double engine badass

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Have you ever

considered Event Sourcing

architecture?

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CQRS: When and Why

Very Good for✔ Complex mutable state✔ Separation of logic✔ Auditing required✔ Orders, Trades, Editors

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CQRS: When and Why

Very Good for✔ Complex mutable state✔ Separation of logic✔ Auditing required✔ Orders, Trades, Editors

Not good for✗ Read only aggregations (e.g. Reports)✗ Stateless calculations (e.g. risk)✗ Very High performance (e.g. exchange)✗ Trivial CRUD (e.g. TODO lists)

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Implementation Gradient

Command Pattern

Event Source

CQRS Monolith

CQRS Microservices

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Command pattern

✔ Allow replay to recreate State✔ Separation of logic from infrastructure✗ No automatic persistence✗ Limited Scalability

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Event Source

✔ Allow replay✔ Separation of logic✔ Automatic persistence✗ Limited Scalability

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Separation of Query Model

✔ Allow replay✔ Separation of logic✔ Automatic persistence✔ High Scalability✗ Complexity

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Split to Microservices

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Domain Driven Design

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Domain Driven Design

DDD is a creative collaboration between technical and domain experts.Domain language must be reflected in the code.

Code must let the business intention show up

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Immutability

Purity

Higher order functions

No exceptions

Transformations which preserse properties

Functional Programming

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DDD + Event Sourcing + Functional Style

There is no silver bullet but...

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Protagonists

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What is a ADT?

Represent entities that can change state in an immutable world.

An algebraic data type is a kind of composite type, i.e. a type formed by combining other types. Wikipedia

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What is a Fold?

Specialized type of recursionIn functional programming, fold refers to a family of higher-

order functions that through use of a given combining operation, recombine the results of recursively processing its constituent

parts, building up a return value. Wikipedia

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Commands

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

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Events

Events are the “atoms” of System state change

Nothing can change without an event, every event can change only one transactional aggregateState + Event => State

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Values and Entities

They represent the state of the domain

Values have no identity, Entities are distinct

They are all immutables

To keep state changes we have Algebraic Data Types

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Transactional Aggregates

An aggregate is an aggregation of Entities

They are composed by Entities and Values

It has all the information for a transaction unit

Aggregates is what Events fold to

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Queries

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 only eventually consistent with the domain

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Actors

More powerful and easy to use concurrency model than Threads/Locks

They communicate asynchronously and potentially remotely

Useful for Bounded Contexts and ServicesRemote Actors works like Microservices

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Event Store

Store tuples (timestamp, type, uuid, event, version)

Query by index fast

Dynamic query

In memory implementation can be enough

@Ramtopgithub.com/uberto/anticapizzeria

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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.

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Events Tree

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Commands

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Events

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Queries

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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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Commands emit Events

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Logic is called by Commands

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Actors(where we can put all this stuff)

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Application

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Tests can process C&Q

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Add a new State to represent Dispatchwhen a pizza left theshop.Keep track of delivery person.

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Live Code

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Will you consider Event Sourcing in

the future?

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QAUberto Barbini

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github.com/uberto/anticapizzeria

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