Cqrs in babysteps

Command Query Responsibility Segregation

In Babysteps

@talboomerik

Founder of Software craftsmanship guild Belgium

@talboomerik

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

@sandromancuso

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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