Command / Query Responsibility Segregation

Udi Dahan – The Software SimplistSession Code: ARC 304

What will you see here?

Distributed Systems ArchitectureUI DesignDomain Model

What won’t you see here?

Code

Distributed Systems Architecture

Common Distributed Architecture

UI

Services

BL

DAL

DB

Let’s deal with the obvious…

Collaboration is good – isn’t it ?

Get dataGet data

Change data

User is looking at stale data

So why go through all the layers?

UI

Services

BL

DAL

DB

Cache

Why transform between tiers?

Use EF to map from tables to domain objects

Map from DTOs and WS to domain object

Map from DTOs & WS to view model

DBWS

UI

Cache

What was the point of all this again?

Queries – showing data to the user

Why not be upfront about it?

Data correct as of 10 minutes ago

List of customers

Only data, no behavior: not an object

Keep it simple - 2-Tier

Persistent View Model

UI

Query only

For each view in the UI, have a view in the DB

Incrementally add from there

Commands – accepting user input

Can we fully trust users decisions?

Validation and Business Rules

Validation: Is the input potentially good?Structured correctly?Ranges, lengths, etc

Rules: Should we do this?Based on the current system

stateWhat the user saw is irrelevant

Command Processing Layers

Input from User

Validation

Rules

Persistence

DB

Transaction

Command Processing Tiers

DBWSInput

Validation Validation

Command

Get current state

Rules

Persist

Should we do what the user asks?

User Interface Design

Traditional User Interfaces

ID Total Date Shipped Account etc etc etc

317 $37.87 1/9/09 Yes A17T5

318 $99.99 3/7/09 Yes A17T5

319 $100.11 4/8/09 Yes P313Z

320 $69.47 9/9/09 No P599Z

Orders

CancelSave

Capturing user intent

Why is it important? Differentiating between:

Correcting a mistyped shipping addressThe user moved – reroute shipments underway

Sometimes users accidentally modify fields when tabbing between columns

Reservation systems

Reservation systems

Not capturing user intent

In a traditional UI – what we have is the checkbox

Why do users select multiple seats?Because they’re reserving for a family / friends

But then, concurrency happensSomebody else got in first on one of the seats

Try to find a block of seats somewhere else

Capturing user intent

Group reservation:Small group – sitting togetherLarge group – several small groups

Enter number of peopleEnter preferred seat type – indicates cost

System emails back when reservation can be filledInclude waiting list functionality

Usability benefits

More users get what they want with less clicks

Scalability benefitsThousands of seats, hundreds of thousands of requests

No need to show actual status

Revisiting the command

What’s a good command?

The kind you can reply with:

“Thank you. Your confirmation email will arrive shortly”

Inherently asynchronousNot really related to an entity

Commands versus Entities

It’s easier to validate the commandLess dataMore specific

Is this potentially good

Validating large entities is complex

Domain Models

What aren’t they for?

Validation

Commands are validated before the domain model is called

Queries

Entity relationships for reading are unnecessary

What are they for?

Answering the question:

Should I do what this valid command is asking?

If the answer is yes, change its state

Putting it all together

Queries

Commands

DBWSInput

Validation Validation

Rules

View ModelQueries

Queries

CommandsDB

WSInput

Validation Validation

Rules

Queries

For better scalability, add pub/sub

View Model

CacheUpdater

Publish

Summary

Summary

Keep queries simple – 2-Tier if possibleNo data transfer objects

Have commands reflect user intentAdjust UI design to capture intent

Get the benefits of asynchronous programmingSimple validation, focused business rules

Thank youUdi Dahan – The Software SimplistEnterprise Development Expert & SOA Specialist

www.UdiDahan.com

email@UdiDahan.com

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