Command/ Query Responsibility

Segregation

• How we started?• Distributed systems architecture• UI design• Domain model

What are we going to see

What not

And then came

And with it

Ultimate source of truth

• Paper was gone.• The source of truth changed

And then…

But

And

Distributed Architecture

Change

Data

Get D

ata

Get Data

User Collaboration

Users looking

into stale data

Cache

So why all the layers?

Cache

Relational data to DTO’s using ORMs

Map DTO’s to Domain Objects/ WS

Map from Domain Objects/ WS to View Models

Why transform between layers?

List of customers------------------------------------------------------------Data corrected as of 10 minutes

Be upfront about it

Only data, no behavior

No objects so why ORM’s

Query only

Persistent View Model

For each view in the UI have a table/ view in the DB

KISS

Search

• Find what the users want• Separate UI talking to a different store

No relations between Views/ Tables

Key

Value

Key

Value

Key

Value

Key

Value

Key

Value

Key

Value

Key

Value

Key

Value

Key

Value

Key

Value

Key

Value

Vs.

• Data duplication.• Avoid calculations on data.• Deploy to web tier (No need to hop

through firewalls, only SELECT is allowed)• Role-based security not needed.• Use for preliminary validation results in

less command rejection.

Data duplication

Commands

Can we fully trust our user decisions?

• Validations– Is the input potentially good– Structured correctly– Ranges, lengths etc.

• Rules– Should we do this?– Based on the current system state– What the user saw is irrelevant

Validations and Business Rules

• Easy versioning• Deployment

Autonomous components

Command Processing tiers

Capturing user intent

• Differentiating between– Correcting a shipping address – User has moved, rerouting shipment location

• Sometimes users accidently modify fields by tabbing.

Reservation systems

Reservation systems

• Checkbox based UI• Why user’s need bulk booking?• But then concurrency happens• Then book seats somewhere else

Not capturing user intent

Capturing user intent

• Group reservation• Enter number of people and preferred

seating location• System emails back when reservation is

confirmed

• On submit from browser, show comment using AJAX on page.

• Why wait for response from server.• Does the user calls all his friends and tell

them about the comment he posted?

Posting a comment on Facebook

DOMAIN MODEL

• Validations:– Commands are validated before the domain

model is called• Queries:

– Entity relations for reading are unnecessary

What aren’t they for

• In addition to doing what the command said, doing other things as well.

What are they for?

• The domain model is not responsible for persisting all the data. Only persist what is needed.

• The rest of the world is using the data from the query store anyway

Only persist what is needed

CQRS

Domain

Commands

Queries

PUTTING IT ALL TOGETHER

For better scalability, add pub/ sub