Command/Query Responsibility Segregation

SESSION CODE: ARC302

Udi Dahan – The Software SimplistEnterprise Development Expert & SOA Specialist

Agenda

History Arch. UI Design

Domain Model

History

Paper ruled the world

And then came the computer

And with it data entry

And efficient UIs were designed for it

OrdersID Total Date Shipped Account etc etc

317 $37.87 Sep 1 2010 Yes A17T5

318 $99.99 July 3 2010 Yes A17T5

319 $100.11 Aug 4 2010 Yes P313Z

320 $69.47 Sep 9 2010 No P599Z

CancelSave

Source of truth

Gone…

But the UI lingers to this day

Since then…

Commands

QueriesUsers

Distributed Systems Architecture

Common Distributed Architecture

UI

Services

BL

DAL

DB

Collaboration is good – isn’t it ?

Get dataGet data

Change dataUser 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

?

SearchFirst think about what the user wants to find

Avoid Excel-like combo-boxes above each columnDesign some custom widgets/screens for that

Eg. Amazon recommendations

Separate UI talking to different data store

SQL Server Full Text SearchLucene.net

Why not be upfront about staleness?

Data correct as of 10 minutes ago

List of customers

Keep it simple - 2-Tier

Persistent View Model

UI

Query only

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

2 Layers == 2 Tiers

SELECT * FROM MyTable (WHERE ID = @ID)

Data Duplicated, No Relationships, Data Pre-Calculated

List of customers

Customer Service Rep view

ID Name Phone

List of customers

Supervisor view

ID Name Phone Lifetime value

Rep_Customers_TableID Name Phone

Supervisor_Customers_TableID Name Phone Lifetime Value

Deployment and SecurityDeploy the persistent view model DB to the web tier (only SELECT is permitted)

Don’t have to go through the firewall – faster

Role-based securityDifferent screens for different roles go to different tables – SELECT permissions per role

Just as secure as in-memory cachesIf not more so

Use for preliminary validationBefore going to submit data, check if it already exists in the persistent view model

UniquenessCan expose to user (Yahoo user signup)

Related Entity ExistenceAddress validation – existence of street name

Results in less commands being rejected

Commands – accepting user input

!

Validation and Business RulesValidation: 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

OrdersID Total Date Shipped Account 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

CancelSave

Capturing user intentWhy 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 intentIn 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 intentGroup 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 benefitsMore 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

You need a queue

MSMQOutgoing Incoming

Server

Client

MSMQOutgoing Incoming

Eg. Posting a comment to a blogAfter the browser submits the comment, immediately shows it on the page using AJAX

Eg. Withdrawing cash from an ATMUsing the account information retrieved(Account type, Max single withdrawal)

Decides to send the command (or not)

If so, dispenses cash not waiting for a response

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

Eg. When a new order is submitted, if the customer has ordered more than X in the past, give them a discount

Only persist what they needThe domain model isn’t responsible for persisting all data

only what it needs for its purposes

The rest of the world is using the data in the query store anyway

Putting it all together

For better scalability, add pub/sub

Queries

CommandsDB

WSInput

Validation Validation

Rules

QueriesView Model

View ModelUpdater

Publish

Client

Data from input immediately overlaid on queries

Free OSS .NET Framework

www.NServiceBus.com

In Closing

SummaryKeep queries simple – 2-Tier if possible

No 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

SESSION CODE: ARC302

email@UdiDahan.comwww.UdiDahan.com

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