CQRS:

Crack for Architecture Addicts…-or-

…An Idea Who’s Time Has Come?

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CQRS:

Crack for Architecture Addicts…-or-

…An Idea Who’s Time Has Come?

Do I Suck?

http://speakerrate.com/talks/9298

Who am I?• …and why should you care?• Steve Bohlen• I Read Books + Write Software• vs. “Read Software + Write Books”

• Blog, Screencast, Speak, Share, Learn

Nearly 20 years developing softwareLISP, Delphi, C/C++, VB, VB.NET, C#, RubySenior Engineer Springsource/VMwareCo-Founder, NYC Alt.Net User Group

http://nyalt.netCo-Organizer, NYC DDD User Group

http://dddnyc.orgContributor: various OSS projects

Nhibernate http://www.nhforge.orgNDbUnit http://www.googlecode.com/ndbunitSpring.NET http://www.springframework.net

blog: http://blog.unhandled-exceptions.come-mail: sbohlen@gmail.comtwitter: @sbohlen

CYND D D

Beta Presentation

Opinions Ahead

Agenda• What’s wrong with Traditional Application Architecture?• What is CQRS?• Commands and Events• Persistence• Understanding Event Sourcing• Exploring a simple CQRS sample app• Exploring a comprehensive CQRS sample app (?)• Resources to Learn more• Q+A

What’s wrong withTraditional Application Architecture?

Traditional Architecture

LAN/WAN

Traditional Architecture

Application Services

Domain Object

Domain Object

Domain Object

Domain Object

Client

Remote Services

Request DTO

Retu

rn D

TO

Comm

and ACK

Retu

rned

RDBMS

Application Design Pain Points• Scalability of System is by

larger and larger RDBMS• Surprise, surprise!

• Persistence means ‘save the Current State’ of the domain model

• Query concerns are at odds with Transactional concerns• 1NF vs. 3NF

• more…

Domain Modeling Pain Points• Business Rules only exist in

user’s heads• Converting Domain Objects

to DTOs (and back again!)• Domain Object Persistence• Lazy vs. Eager Fetching• Optimizing Domain for

efficient Queries• Setter/Getter Anti-Pattern• Tests are (often) brittle• more…

Business Pain Points

• Must know what business questions we want to ask up front

• Optimizing for Queries and Optimzing for Transactions are tied together (and expensive)

• Evolving Requirements is hard/expensive

• more…

Self-Delusion• One model can be made

to efficiently serve two (or more!) different masters

• Competing optimization needs of Queries and the Transactions can be efficiently balanced

• Getters/Setters are a necessary evil

• Persisting State is the only way to think about the problem

What is CQRS?

Command-Query SeparationCommand-Query Separation (CQS) states that 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.More formally, methods should return a value only if they are referentially transparent and hence possess no side effects.

-Bertrand Meyer, designing the Eiffel language

Interface Segregation Principle

Command-QueryResponsibility Segregation

• The CQS principles applied to your domain modeling and system architecture

• Enable each element of your architecture to be optimized for a single responsibility

• Decomposition for your architecture rather than just for your objects

LAN/WAN

Slicing and Dicing our Architecture

Application Services

Domain Object

Domain Object

Domain Object

Domain Object

Client

Remote Services

Request DTO

Retu

rn D

TO

Comm

and ACK

Retu

rned

RDBMS

Client

Server

WriteRead

LAN/WAN

Refactoring our Architecture

Application Services

Domain Object

Domain Object

Client

Remote Services

Request DTO

Retu

rn D

TO

Application Services

Comm

and ACK

Retu

rned

WriteRead

Light-Weight Query Services

RDBMS

Domain Object

Domain Object

Persistence Persistence

Remote ServicesRemote Services

Eventually

C PACAP Theorem: onsistency, vailability, artition-Tolerance; pick any two!

Write

Read

Scaling our Systems

C A P

About “Eventual Consistency”• Domain Experts deal in ‘stale’ data all the time

– Edit screens, Reports, Analysis, OLAP• ‘Staleness’ is often already implicitly in our systems

– Caching, Optimistic Concurrency, and more• Eventual Consistency is making that ‘staleness’ an explicit

(and acknowledged) fact in the system• Language matters: say stale and older not out-of-date or

inconsistent– Opportunity to actually discuss ‘how stale is OK?’– Often we make data stale without this discussion at all

Commands and Events in CQRS

Refresher: the Aggregate Root Pattern

Aggregate Boundary

Entity1

Entity2

Entity3

ValueObject1

ValueObject2

Aggregate Boundary

Entity1

Entity2

ValueObject1

Aggregate Boundary

Entity1

ValueObject1

ValueObject2

The BIG Idea

Command Aggregate Root

Event

Event

PlaceOrderCommand

Order Domain Object

OrderPlacedEvent

*or* OrderNotPlacedEvent!

Application Domain

The even BIGGER Idea

Command Command Handler

Aggregate Root Event

Event Handler

Event Handler

Aggregate Root

Event Event Handler

Event

Event Handler

Event Handler

OutputInput

Domain Testing (conceptual)

Command Command Handler

Aggregate Root Event

Event Handler

Event Handler

Aggregate Root

Event Event Handler

Event

Event Handler

Event Handler

Understanding Commands• Commands are directives to the domain to perform some

action• Commands may be declined/rejected by the domain for

various reasons• Processing a Command will result in 0:n Events raised• Commands are always in the imperative:

– PlaceOrder, not OrderPlaced– Recall: in DDD, lexical differences matter

• Commands are a component of the Ubiquitous Language we use to describe our domain

Demystifying Commandspublic class PlaceOrderCommand {         

//properties public readonly Guid OrderId;public readonly string Comment;

//ctorpublic PlaceOrderCommand(Guid id, string comment){OrderId = id;Comment = comment; }

}

Understanding Events• Events are the result of some action having already

happened in the Domain• Events may never be declined/rejected; they are

evidence that something already happened• Events are always in the past-tense:– OrderPlaced, not PlaceOrder– Recall: in DDD, lexical differences matter

• Events are a component of the Ubiquitous Language we use to describe our domain

Demystifying Eventspublic class OrderPlacedEvent {         

//properties public readonly Guid OrderId;public readonly Guid ConfirmationId;public readonly string Comment;

//ctorpublic OrderPlacedEvent(Guid orderId, Guid confirmationId, string comment){OrderId = id;ConfirmationId = id;Comment = comment; }

}

Role of Commands and Events

Persistence in CQRS

Compromises inPersistence Needs

• Query (read) optimization and Transaction (write) Optimization needs are in opposition– 1NF for queries, 3NF for transactions– Scalability needs (imbalanced Reads and Writes)

• Even RDBMS vendors acknowledge this– OLTP vs OLAP

• One set of optimizations shared between Read and Write will by definition be suboptimal for each

Choose the Proper Persistence• Optimize the Read store for Queries• Optimize the Write store for Transactions• Let each side be designed for its single-purpose role• Don’t try to twist a single persistence store into

doing “double-duty”– It will do neither well

• RDBMS is no longer our only choice for persistence– NoSQL choices offer different pros/cons

Event Sourcing in CQRS

Thinking More About Events• Events are past-tense• An Audit-Log of the entire Domain for the life

of the system• A history of what happened over time– not a record of the state of the domain at a point

in time

Event Sequencing vs. State

Event Sequencing vs. State

Search for Product ID1

Add Product ID1

to Cart

Search for Product ID2

Add Product ID2

to Cart

Remove Product ID1

from Cart

Final State: Cart with Product ID2 in it

Typical Application w Audit Log

The System

Audit Log

• Audit Log *might* be right• Have to remember to write

to it “when important”• Side-effect of the system

Typical Application w Event Log

The System

Audit Log

• Audit Log *must* be right• Entries are guaranteed to be

recorded when needed• Core to the system’s function

Interesting Event PossibilitiesIf Events are a record of what happened in The Domain…• We can (re)create the current state of the Domain by

“replaying” events from the log• If we want a point-in-time view of the system, we can

(re)create the Domain at that time by “replaying” events just to that point– Historical State for “free”– Bug Reproduction

Event Store Concepts10

9

8

7

6

5

4

3

2

1

Repl

ay F

orw

ard

Read

to B

egin

ning

10

9

8

snap

7

6

5

4

3

2

1

Repl

ay

Read

Events and Domain Persistence• Saving our Aggregate is just serializing events• Loading our Aggregate becomes:/* load the snapshot for this aggregate or if no snapshot, begin w/ empty ‘default’ */var customer = customerRepo.LoadById(customerId);

//replay the events to make its state ‘current’customer.ReplayAllEvents();

//customer ready to accept commands nowConsider: Why do we need an ORM now???

Exploring a Simple CQRS Sample

Exploring a Comprehensive CQRS Sample

Should I do CQRS?

Absolutely Not!…or at least “Probably Not”

Additional ResourcesCentral Hub for All Things CQRS

http://www.cqrsinfo.comUdi Dahan’s on CQRS (et. al.)

http://www.udidahan.com/2009/12/09/clarified-cqrs/Simple CQRS Sample Application

https://github.com/gregoryyoung/m-rComprehensive CQRS Sample Application

https://github.com/MarkNijhof/FohjinJonathan Oliver’s Post on his Event Store project

http://blog.jonathanoliver.com/2010/07/cqrs-event-store/Rinat Abdulluin on Getting Started with CQRS

http://abdullin.com/cqrs/Greg Young on Testing Using Events (video)

http://skillsmatter.com/podcast/design-architecture/talk-from-greg-young

Rate Me!

http://speakerrate.com/talks/9298