CQRS – Eine EinführungPt. 2 – Event-Centric Architecture

.Net Online Usergroup – 20.02.2011

Presenter: Dennis Traub

Speaker, Trainer, Author, Software Developer since 1991

Twitter: @DTraub

CQRS Revisited

Domain Events

An Event-Centric Architecture

Some Code

What about Testing?

Summary

CQRS revisited

Data Storage

Thin Read LayerORM

Domain Model

Command Handlers

Client

Commands Queries

CQRS Revisited

Domain Events

An Event-Centric Architecture

Some Code

What about Testing?

Summary

Definition: Domain Event

• Martin Fowler: (PEAA, 2002)

▫ Captures the memory of something interesting which affects the domain

▫ The essence […] is to capture things that can trigger a change to the state

▫ These event objects are then processed to cause changes to the system, and stored to provide an audit log

State

Shopping Cart

What we dont know:

What led to this state?

Addedto

Cart

Addedto

Cart

Addedto

Cart

Addedto

Cart

Addedto

Cart

Addedto

Cart

Addedto

Cart

Addedto

Cart

Addedto

Cart

RemovedfromCart

Which model contains

more Information?

This one?

Shopping Cart

Or this one?

Addedto

Cart

Addedto

Cart

Addedto

Cart

RemovedfromCart

What if we capture every single event?

And reproduce state from

this stream of events?

An Event Stream …

Addedto

Cart

Addedto

Cart

Addedto

Cart

RemovedfromCart

Shopping Cart

An Event Stream …

Addedto

Cart

Addedto

Cart

Addedto

Cart

RemovedfromCart

Can be projected into:

Shopping Cart

An Event Stream …

Addedto

Cart

Addedto

Cart

Addedto

Cart

RemovedfromCart

Sales History

… or into this:

Shopping Cart

An Event Stream …

Addedto

Cart

Addedto

Cart

Addedto

Cart

RemovedfromCart

Sales History

… or into this:

Audit Trail

Shopping Cart

An Event Stream …

Addedto

Cart

Addedto

Cart

Addedto

Cart

RemovedfromCart

Sales HistoryCampaign Effectiveness

… or into this:

Campaign Effectiveness

CQRS Revisited

Domain Events

Towards an Event-Centric Architecture

Some Code

What about Testing?

Summary

Simple CQRS

Relational Data Storage

Thin Read LayerORM

Domain Model

Command Handlers

Client

Commands Queries

SQLViews

Optimized Write / Complex to Read

Relational Data Storage

Thin Read LayerORM

Domain Model

Command Handlers

Client

Commands Queries

SQLViews

INNER JOIN …LEFT OUTER JOIN …UNION …GROUP BY …

INNER JOIN …LEFT OUTER JOIN …UNION …GROUP BY …

Optimized Write / Complex to Read

Relational Data Storage

Thin Read LayerORM

Domain Model

Command Handlers

Client

Commands Queries

SQLViews

Replace Views with Table per Query

Relational Data Storage

Thin Read LayerORM

Domain Model

Command Handlers

Client

Commands Queries

Tableper

Query

SELECT * FROM …

Emit Events when they occur

Relational Data Storage

Thin Read LayerORM

Domain Model

Command Handlers

Client

Commands Queries

Tableper

View

Denormalize Events into Read Model

Relational Data Storage

Thin Read LayerORM

Domain Model

Command Handlers

Client

Commands Queries

Tableper

View

Den

orm

ali

zer

Relational Data Storage

Thin Read LayerORM

Domain Model

Command Handlers

Client

Commands Queries

Tableper

View

Den

orm

ali

zer

Event Log

Add an Event Log for future retrieval

Simplified:

Client

Domain Read Model

DTOsCommands

Events

CQRS Revisited

Domain Events

Towards an Event-Centric Architecture

Some Code

What about Testing?

Summary

Denormalized Store:If ISBN exists in ShoppingCartDetails

Increase AmountElse

Add New Row

Denormalized Store:If Shopping Cart ID exists in ShoppingCartSummary

Increase Number of ItemsAdd Offered Price to Total Amount

ElseAdd New Row

Denormalized Store:If Amount > 1 in ShoppingCartDetails

Decrease AmountElse

Delete Row

Denormalized Store:

Subtract Offered Price from Total AmountDecrease Number of Items

CQRS Revisited

Domain Events

Towards an Event-Centric Architecture

Some Code

What about Testing?

Summary

Testing the Read Model

Thin Read Layer

Tableper

View

Den

orm

ali

zer

Testing the Read Model

Thin Read Layer

Tableper

View

Den

orm

ali

zer

Given that certain Events have occurred

Testing the Read Model

Thin Read Layer

Tableper

View

Den

orm

ali

zer

Given that certain Events have occurred

When a specific Event has occurred

Testing the Read Model

Thin Read Layer

Tableper

View

Den

orm

ali

zer

Given that certain Events have occurred

Then each query should produce the expected results

When a specific Event has occurred

Testing the Read Model

Given that a book with an ISBN of xxx and a price of USD 24.00 was added to a new shopping cart

When a book with an ISBN of yyy and a price of USD 19.00 was added to that same shopping cart

Then the Shopping Cart Summary should contain 2 booksand have a total amount of USD 43.00

And the Shopping Cart Details should contain book xxx

And the Shopping Cart Details should contain book yyy

Testing the Write Model

Domain Model

Command Handlers

Testing the Write Model

Domain Model

Command Handlers

Given that certain Events have occurred

Testing the Write Model

Domain Model

Command Handlers

CommandsWhen a specific Commands is sent

Given that certain Events have occurred

Testing the Write Model

Domain Model

Command Handlers

CommandsWhen a specific Commands is sent

Then the expected Events (and only those) should be emitted

Given that certain Events have occurred

Testing the Write Model

Given that an Add Book To Shopping Cart event withan ISBN of xxx had been sent

When a Remove Book From Shopping Cart command issent with an ISBN of zzz

Then a Book Removal Failed event should be emitted

And NO Book Was Removed From Shopping Cart eventshould be emitted

CQRS Revisited

Domain Events

Towards an Event-Centric Architecture

Some Code

What about Testing?

Summary

Main Value:

• Additive only, i.e. we don‘t lose information

• Linearly scalable and distributable Read Model

• Every new view can be created from the beginning of time

• We can come up with new questions at any time

• Built-In Integration Model

Proven Technology:

Mature business models move away from Update/Delete and become purely transactional:

▫ Human Resources

▫ Medicine

▫ Bookkeeping

▫ …

▫ Banking

▫ Financial Trading

▫ Government

What we haven‘t touched yet:

▫ Messaging

▫ Event Sourcing

▫ Sagas

▫ Eventual Consistency

▫ Integration Scenarios

▫ …

Resources

• http:// cqrs.wordpress.com

• http:// groups.google.com/group/dddcqrs

• http:// distributedpodcast.com

• http:// abdullin.com/cqrs

• http://cqrs.wikidot.com

Vielen Dank!

Dennis Traub@Dtraub

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