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Command Query Responsibility Segregation
In Babysteps
@talboomerik
Founder of Software craftsmanship guild Belgium
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Thanks to
@yreynhout @alastairs
@sandromancuso @adibolb
And a special thanks to@StefanBilliet
ENTRY LEVEL INTRODUCTION
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WHERE DID IT COME FROM?A little history
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Command and Query Separation
It 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.
Wikipedia - Bertrand Meyer
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IF YOU HAVE A RETURN TYPE, YOU SHOULD NOT CHANGE STATE
Query
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IF YOU CHANGE STATE YOUR RETURN TYPE SHOULD BE VOID
Command
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BASICALLY CQRS IS CQS SPLIT IN DIFFERENT OBJECTS
The simple definition
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QUERY OBJECTS OVER QUERY SERVICES / REPOSITORIES
http://lostechies.com/jimmybogard/2012/10/08/favor-query-objects-over-repositories/@talboomerik
WHAT IS THIS ALL ABOUT?What’s the fuss
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YOUR TYPICAL N-TIER ARCHITECTURE
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REMEMBER WHAT UNCLE SANDRO SHOWED US?
FEELS FAMILIAR?
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WE CAN MAKE THIS SIMPLER
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REMOVING QUERIES FROM YOUR DOMAIN MODEL
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@sandromancuso
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LET’S TAKE IT ONE STEP FURTHERIf needed
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OR EVEN FURTHERIf applicable
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HOW TO SYNCHRONIZE READ/WRITE?
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DOMAIN EVENTSA cool pattern
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LET’S HAVE A LOOK AT SOME CODE
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TAKING IT TO ANOTHER LEVELEvent sourcing
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FOCUSING ON BEHAVIOR INSTEAD OF STATE
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SOME MISCONCEPTIONS ABOUT CQRS
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CQRS <> DDDIt’s not the same
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CQRS <> EVENT SOURCINGThey just go well together
http://lostechies.com/jimmybogard/2012/08/22/busting-some-cqrs-myths/ @talboomerik
CQRS CAN HAVE AN IMMEDIATE CONSISTENT READ STORE
http://lostechies.com/jimmybogard/2012/08/22/busting-some-cqrs-myths/ @talboomerik
CQRS DOES NOT FORCE YOU TO USE A BUS
http://lostechies.com/jimmybogard/2012/08/22/busting-some-cqrs-myths/ @talboomerik
COMMANDS DO NOT NEED TO BE FIRE-AND-FORGET
Accepting the request vs Fulfilling the request
http://lostechies.com/jimmybogard/2012/08/22/busting-some-cqrs-myths/ @talboomerik
WHY IS THIS A GOOD IDEA?Pros and cons
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SIMPLER DOMAIN MODELBy splitting
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BETTER SEPARATION OF CONCERNS AND RESPONSIBILITIES
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SCALABILITYTechnical and team
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COMPLETE REBUILD OF THE APPLICATION STATE THROUGH EVENT SOURCING
http://martinfowler.com/eaaDev/EventSourcing.html@talboomerik
TEMPORAL QUERYINGReplay history
http://martinfowler.com/eaaDev/EventSourcing.html@talboomerik
AUDIT LOGI see you
http://martinfowler.com/eaaDev/EventSourcing.html@talboomerik
IT DOES REQUIRE “SOME” WORK UPFRONT
Set up infrastructure
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IT WORKS BEST WITH TASK-BASED UI
Delivery mechanism and domain architecture need to collaborate
IT IS NOT EASYIt’s a big shift from what we are used to
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HOW CAN WE GET THIS DONE?Code, show us code
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THANK YOUThat’s all folks
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