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An intro into CQRS I gave for the Belgian Visual studio User group in March 2012
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CQRS 101All your BASE belong to us
First dive into CQRS by Tom Janssens
March 6th, 2012
Hosted for VISUG.be by AE NV - Leuven, Belgium
Tom Janssens ● [email protected]● ToJans@twitter● http://www.corebvba.be/blog● http://github.com/ToJans
● Fun Fact: 10 startups in one year
Freelance problem solver - contact me !
Y U NO CQRS ?
● Occasionally disconnected client app● Google Groups Forum DDD/CQRS● CQRS articles on blog● OSS CQRS frameworks/libs:
Scritchy & MinimalisticCQRS● Member of MS P & P CQRS advisory board
A.K.A. my CQRS background and - experience
Overview Talk the talk● What● Why● When
Walk the walkDemo BankAccount● MinimalisticCQRS● Uniqueness ● Account transfer
WHAT: complexity evolutionSpaghetti approach
UI Data layer
WHAT: complexity evolutionLasagna approach *:
* Can contain additional layers
UI App layer Data layerDomain layer
Command: change state - push data (state changes)
Query: query state - pull data
WHAT: complexity evolution
* Command-Query separation - Bertrand Meyer"Asking a question should not change the answer"
CQS approach *:
UI App layer Data layer
Advantages: Scaling, risk, impact
UI App layer Data layerDomain layer
WHAT: complexity evolutionCQRS: Command Query Responsibility Segregation@Kellabyte on twitter
This is #CQRS http://bit.ly/zR2VuV That's it, that's all. What you put in behind those is entirely up to you.
WHAT: complexity evolution// CQS (Command-Query Separation)// Bertrand Meyer devised the CQS principle// [deleted for brevity]
public class CustomerService{ // Commands void MakeCustomerPreferred(CustomerId) void ChangeCustomerLocale(CustomerId, NewLocale) void CreateCustomer(Customer) void EditCustomerDetails(CustomerDetails)
// Queries Customer GetCustomer(CustomerId) CustomerSet GetCustomersWithName(Name) CustomerSet GetPreferredCustomers()}
WHAT: complexity evolution// CQRS (Command-Query Responsibility Segregation)// Defined by Greg Young.// [deleted for brevity]
public class CustomerWriteService{ // Commands void MakeCustomerPreferred(CustomerId) void ChangeCustomerLocale(CustomerId, NewLocale) void CreateCustomer(Customer) void EditCustomerDetails(CustomerDetails)}
public class CustomerReadService{ // Queries Customer GetCustomer(CustomerId) CustomerSet GetCustomersWithName(Name) CustomerSet GetPreferredCustomers()}
That's all folks !!! The End !... Or not ?
Command: change state - push data (state changes)
Query: build/query state - pull data
WHAT: complexity evolution
* Command-Query Responsibility Segregation - Greg Young, Udi Dahan
CQRS approach *:
"The creation of 2 objects where there previously was one"
EventsUI Data layer
UI Domain layerCommandsEvents
buildquery
What: messagesCommand● DoStuff !● Imperative
void DoStuff( ... ){ if (somethingtrue) apply(StuffDone);}
Event● StuffDone● Past tense
void Handle(StuffDone){ SomeState = true;}
IMPORTANT!!An event handler should ALWAYS have an immutable execution path,i.e. no logic !!
Command: change state - push data (state changes)
Query: build/query state - pull data
WHAT: complexity evolutionCQRS approach *:
Example code : Scritchy example
EventsUI Data layer
UI Domain layerCommandsEvents
buildquery
Why ?Clear separation of concerns
Domain
Query side
UICommands
Events
update view
models
"Circular architecture" Evolution of Alister Cockburn's hexagonal architecture; no more adapters needed
WhyClear separation of concerns● Requires different knowledge/other devs● Testability● Ubiquitous language● New business requirements
WhyEvent-driven architecture and the cloud: CAP theorem: pick 2
Consistency
Availability Partitionability
Why: events & the cloudACID : Consistency & Availability● Atomicity
All operations complete, or none● Consistency
(During a transaction)● Isolation
Every operation behaves as it is the only one● Durability
Finished tx will not be reversed
Why: events & the cloudBASE : Availability & Partitionability● Basically
Available● Soft state● Eventually consistent=> CLOUD !!!
=> Solution for the consistency boundariesDDD: AR's & sagas
Why: events & the cloud
Why: events & the cloud
Why: events & the cloud
Why: events & the cloud
When to use CQRS ?Only apply it to the parts of your problem domain that offer you a competitive advantage.
Indicator for Complexity/uniqueness:=> T-shirt categories (Greg Young)
CRUD - tell what can't be doneDDD - tell what can be done
ref: free eBook "DDD quickly","What is DDD?"
Works really well with DDD/ES=> When your problem domains are not fixed
BreakUp next - demo live coding
Conclusion
What:
Two objects where there previously was one
Remove layers and abstractions
Circular architecture
=> Fire and forget
Conclusion
Why:
Separation of concerns
Event driven architecture
Flexibility, Scalability
Testability
Conclusion
When:
Complexity
Uniqueness
Competitive advantage
Parts of your problem domain
Thanks
CQRS folks - for being such an avid community
@YReynhout and @Abdullin for review
VISUG.be and AE NV for hosting the event
