Previously... in Software Development

Source: https://cqrs.wordpress.com/2010/09/13/cqrs-and-event-sourcing-an-alternative-for-ddd/

Read / Write

• Forced into 3rd Normal Form
• Fast Writes
• Complex and slow reads

The Light...

Source: https://cqrs.wordpress.com/2010/09/13/cqrs-and-event-sourcing-an-alternative-for-ddd/

Command Query Responsibility Segregation

• Close friend of Command Query Separation
• For systems, not just methods

Example: Profiles

DTO Flow

• User loads a profile from 
• User updates the URL field
• User tells server to Update the DTO
• Server performs validation
• Server saves the DTO
• Browser reloads the profile

CQRS Flow

• User loads a profile via query system
• User updates the URL
• UpdatePhotoUrl command sent to command system
• Handler loads the profile
• Handler sets the photo URL
• Handler saves the profile
• Browser reloads the profile via query system

CQRS: Complete

But let's look at some code anyway

The Simple Profile

Old

DTOs

DDD / CQRS

Light query objects

Commands

Transfer

Old

Get

Update

DDD / CQRS

Get (?)

SetPhotoURL

Verbs

Old

As expected

DDD / CQRS

Double

Code Size

Old

Command handler?

DTO?

DDD / CQRS

Domain Object

Business Rules

Old

Fast Reads

Fast Writes

DDD / CQRS

Fast Reads

Fast Writes

Speed

Winner

DTOs

Complex Profiles

Friends complicate everything

Old

Get

Update

DDD / CQRS

Get

SetPhotoUrl

Befriend

Verbs

Old

Getting bigger...

DDD / CQRS

Smaller growth

Code Size

Old

Still unclear...

DDD / CQRS

Domain Object

Business Rules

Old

Slow validation

Fast Writes

Slow Reads

DDD / CQRS

Fast Validation

Fast Writes

? Reads (hint: Fast)

Speed

Old

Difficult

DDD / CQRS

It Just Works

Scale Out

About them reads...

Eventual Consistency

Tailored for Speed

But it's behind...

ORM?

Reconstruction

Testing!

Commands

Events

Exceptions

Given a profile

• When its URL is set to a shock site
  • An exception is thrown
• When its URL is set to something normal
  • PhotoURLChanged event is published
• When a friend is added
  • FriendAdded event is published
  • When the same friend added again
    • Exception
  • When a different friend added
    • Event published

Internal State

Internal State

There are NO getters!

Behaviour Driven

The Read Model

Concurrency

Partitioning

Next Week(s)

Event Sourcing - Attempt 2

Aggregates

Bounded COntext

?