Greenfield Development with CQRS and Windows Azure

David HoersterOctober 18, 2012

C# MVP (since April 2011)Co-Founder of BrainCredits (braincredits.com)Principal at AgileWaysPresident of the Pittsburgh .NET User Group (meetup.com/pghdotnet) and organizer of PGH TechFestTwitter - @DavidHoersterBlog – http://geekswithblogs.net/DavidHoersterEmail – david@braincredits.com

About Me

What is CQRS?Why would you use it?BenefitsSome examplesUsing CQRS in Azure

Goals

Command

Query

Responsibility

Segregation

Is it a pattern or an architecture?

What is CQRS?

Command Query Responsibility Segregation

Based upon Bertrand Meyer’s Command Query Separation (CQS) principle:

“every method should either be a command that performs an action, or a query that returns data to the caller, but not both. In other words, asking a question should not change the answer” (Meyer)

In essence, commands return acknowledgement; queries return data.

What is CQRS?

Backstory

Goals for BrainCredits.com ProductFast readsScalableHandle, potentially, large volumes of requestsBe able to change quicklyAbility to track user in system and events that occurEasy to deploy, manageInexpensive (FREE????)

Backstory

Decided to jump into cloud with AzureScalabilityDeployment via VS2010 (Familiarity)Pretty inexpensive – no HW to manage

BizSpark company (at the time)– Currently 1500 compute hours of small instance– 5 GB of SQL Azure– Small cache instance– 2 million transactions/mo– and more…

But how to store data and design system?

Backstory

I would recommend to start looking along the CQRS style of architectures- Rinat Abdullin

Articles by Greg Young, Udi Dahan, J. Oliver, R. Abdullin

Common Architecture IssuesBusiness logic bleeds through

layersValidation logic ends up

everywhereCopy and Paste developmentApps that cater to modification

opsLeads to general complexity within

tiersCQRS simplifies this with classes performing specific duties (SRP)

What is CQRS?

Simple App (tightly coupled)

Evolution of a System to CQRS

ApplicationDatabase

Breaking out business logic

WriteData(string, string, int, bool, …) { }

Evolution of a System to CQRS

Application

DatabaseBusiness Logic

Introduce messaging

Evolution of a System to CQRS

ApplicationDatabase

Business ServiceMSG

Evolution of a System to CQRS

Client

Read Model StoreEvent Store

Repository Command Service

Domain (ARs)

Denormalizer

Event Handler

Command Issued – ack/nackData Requested/Returned

Command SideQuery Side

Optional

Encapsulates business logic in the domainAllows you to model tasks instead of CRUDAllows for scalability very easilyCan provide for fast reads at the ‘expense’ of more expensive writesOnce the data is read by the client, it’s already old

A system that is eventually consistent is usually acceptableEVENTUAL CONSISTENCY!

– The read model for the system eventually is consistent with the system events

What is CQRS?

Taking this further, CQRS describes a pattern where

Commands are messages (possibly asynchronous)Queries go against a read model

Since commands are returning void, it allows for the message to be transformed into the read modelThis flips the conventional wisdom of fast writes and slow reads

What is CQRS?

Demo

Simple Console-based System

CommandA message sent to tell the system to do something

public class CreateReservation : CommandBase { public Guid ReservationId { get; set; } public String Name { get; set; } public Int32 NumberOfSeats { get; set; } public String DiscountCode { get; set; } }

High-Level Code Walk-through of CQRS

DomainYour system’s business logic and raises eventspublic class Reservation : AggregateRootBase { public void CreateNewReservation(CreateReservation cmd) { //do some logic to figure out TotalCost ApplyEvent(new ReservationCreated() { ReservationId = cmd.ReservationId, ReservationMade = DateTime.UtcNow, NumberOfSeats = cmd.NumberOfSeats, SeatsReservedFor = cmd.Name, TotalCost = totalCost }); }

High-Level Code Walk-through of CQRS

EventsMessages raised by the domain stating that something has been done

public class ReservationCreated : EventBase { public Guid ReservationId { get; set; } public DateTime ReservationMade { get; set; } public Int32 NumberOfSeats { get; set; } public String SeatsReservedFor { get; set; } public Decimal TotalCost { get; set; }}

High-Level Code Walk-through of CQRS

Event Handlers (Denormalizers)Persist event information to the read modelpublic void Handle(ReservationCreated theEvent) { var reservation = new Reservation() {

IsDeleted = false,NumberOfSeats = theEvent.NumberOfSeats,ReservationId = theEvent.ReservationId,ReservationMade = theEvent.ReservationMade,SeatsReservedFor = theEvent.SeatsReservedFor,TotalCost = theEvent.TotalCost };

var repo = new ReservationRepository(); repo.CreateReservation(reservation);}

High-Level Code Walk-through of CQRS

Get all of the orders for user “David” in last 30 days

Benefits

Customer (SalesLT)CustomerID

NameStyle

Title

FirstName

MiddleName

LastName

Suffix

CompanyName

SalesPerson

EmailAddress

Product (SalesLT)ProductID

Name

ProductNumber

Color

StandardCost

ListPrice

Size

Weight

ProductCategoryID

ProductModelID

SellStartDate

SellEndDate

ProductCategory (SalesLT)ProductCategoryID

ParentProductCategoryID

Name

rowguid

ModifiedDate

ProductModel (SalesLT)ProductModelID

Name

CatalogDescription

rowguid

ModifiedDate

SalesOrderDetail (SalesLT)SalesOrderID

SalesOrderDetailID

OrderQty

ProductID

UnitPrice

UnitPriceDiscount

LineTotal

rowguid

ModifiedDate

SalesOrderHeader (SalesLT)SalesOrderID

RevisionNumber

OrderDate

DueDate

ShipDate

Status

OnlineOrderFlag

SalesOrderNumber

PurchaseOrderNumber

AccountNumber

CustomerID

ShipToAddressID

BillToAddressID

ShipMethod

CreditCardApprovalCode

SubTotal

TaxAmt

Freight

Get all the orders for user ‘David’ in last 30 days

Benefits

SELECT c.FirstName, c.MiddleName, c.LastName, soh.SalesOrderID, soh.OrderDate, sod.UnitPrice, sod.OrderQty, sod.LineTotal, p.Name as 'ProductName', p.Color, p.ProductNumber, pm.Name as 'ProductModel', pc.Name as 'ProductCategory', pcParent.Name as 'ProductParentCategory'FROM SalesLT.Customer c INNER JOIN SalesLT.SalesOrderHeader soh

ON c.CustomerID = soh.CustomerIDINNER JOIN SalesLT.SalesOrderDetail sod ON soh.SalesOrderID = sod.SalesOrderIDINNER JOIN SalesLT.Product p ON sod.ProductID = p.ProductIDINNER JOIN SalesLT.ProductModel pm ON p.ProductModelID = pm.ProductModelIDINNER JOIN SalesLT.ProductCategory pc ON p.ProductCategoryID = pc.ProductCategoryIDINNER JOIN SalesLT.ProductCategory pcParent ON pc.ParentProductCategoryID =

pcParent.ProductCategoryIDWHERE c.FirstName = 'David'

AND soh.OrderDate > (GETDATE()-30)

Wouldn’t it be great if it was something like?

Benefits

SELECT FirstName, MiddleName, LastName, SalesOrderID, OrderDate, UnitPrice, OrderQty, LineTotal, ProductName, Color, ProductNumber, ProductModel, ProductCategory, ProductParentCategoryFROM CustomerSalesWHERE FirstName = 'David'

AND OrderDate > (GETDATE()-30)

Command Side Flow

Command Domain

Event 1

Event 2

Event N

Handler 1

Handler 2

Handler N

ReadModel

Read Side Flow

ReadModel

Query Provider(Repo/

WCFDS/etc)

Client

Request for Data

DTO

Query

Results

Demo

Moving CQRS to Azure

Hosting CQRS in AzureTypically, you’d want 2 roles

Web Role for the UI, which also performs queryingWorker Role for the Command handling

Both roles can scale out as much as needed

Multiple web roles with a single worker role, or vice versa or both!

CQRS in the Cloud

CQRS in Azure

Client

Read Model StoreEvent Store

Repository

Command Service

Domain (ARs)

Denormalizer

Event Handler

Worker Role

Web Role Command Side

SQL Azureor other

Repository

Table Storage?

Query Side

How do I issue commands from my web role?Some typical solutions

WCF service on the worker role side3rd party service bus toolUse Azure storage features (blobs, queues, tables)

All have advantages and disadvantages

CQRS in the Cloud

General flow is thisWeb role issues a command by dropping it in a blob container (name is GUID).Command blob name is then dropped in a queueWorker role monitors queue for workWhen it finds a message in the queue, it grabs blob from storage based on id in queue messageDeserializes command and then sends it through normal CQRS pipeline

CQRS in the Cloud

CQRS in the Cloud

ControllerQueue

Blob Storage

Command Handler

Azure Table Storage

Domain

Event(s)

Denormalizer

Read Model

Repository

Web Role Worker Role

Azure Sub-system

Web and Worker Role SeparationShare knowledge of queues and interface

If worker were to go down, web would still run

Commands would queue upProvides some level of fault tolerance

CQRS in the Cloud

Worker role can spawn multiple threads to monitor queues

BrainCredits has several threadsCommandQueue and LogQueueDifferent polling intervalsDepending on importance of information

CQRS in the Cloud

Using Azure Table Storage for logging user actions…

Different than events. Want to know user behavior.Storage is huge – just watch transaction costs

Consider batching log reads

But you can create replays of user interaction

Similar to event sourcing’s replay of eventsEvent sourcing doesn’t capture “Cancels” or “Reads”, which may be useful from a marketing perspective

CQRS in the Cloud

Claim Credit for this Session

BrainCreditshttp://www.braincredits.com/Lesson/17210

Resources

Cerebrata Cloud Storage Studio – http://cerebrata.comParaleap AzureWatch – http://paraleap.com

Conference Reservation Example on GitHubhttps://github.com/DavidHoerster/Conference

CQRS Journey – https://github.com/mspnp/cqrs-journey-docCQRS Info Site – http://cqrs.wordpress.com/DDD/CQRS Google Group - http://groups.google.com/group/dddcqrsUdi Dahan’s CQRS article “Clarified CQRS” – http://www.udidahan.com/2009/12/09/clarified-cqrs /Rinat Abdullin’s CQRS information – http://abdullin.com/cqrsDistributed Podcast - http://distributedpodcast.com/