…WHAT’S THAT CQRS THING?

introduction for adopters

effect

free!

COMMAND QUERY RESPONSIBILITY

SEGREGATION PATTERN

ORIGINAL SIN

CREATE

READ

UPDATE

DELETE

is an architectural pattern proposed by Greg Young that segregates reads and writes of a system into two separate subsystems.

CQRS

writes are usually asynchronous and stored in a transactional storage while reads are eventually consistent and retrieved from de-normalised views

CQRS (details)

BERTRAND MAYERDesigner of Eiffel

programming language

Coined “open/closed principle” and “design

by contract”

Introduced command query separation

(CQS)

Use of commands and queries separates write and read concerns

COMMANDTell to do something

COMMAND• Command is the message to change state

• Message handled by a CommandHandler

• Returns void (so nothing)

• Describes business intent

• Immutable

QUERYFetch state from data store

QUERY

• Read state from data store

• Does not change state

• Has a return value

WHY SHOULD I USE CQRS?

• Scalability

• Reduced complexity

• Flexibility

• Focus on the business

• Fear of change

• Mental leap

• All-Or-Nothing approach

BE AWARE OF

CQRS MYTHS

• is easy

• CQRS = Event Sourcing and vice versa

• requires an eventual consistent read store

• requires a bus/queues/asynchronous messaging

– Udi Dahan

“Most people using CQRS (and event sourcing too) shouldn't have done so”

– Greg Young

“It's important to note though, that these are things you can do, not necessarily things you should do.

Separating the read and write models can be quite costly”

WHEN SHOULD I AVOID IT?

WHATS NEXT?

• Event sourcing (ES)

• Eventual consistency

• Task based UI

• …

Thank you.

@realfuzzyMichele Franzin

CREDITSslide10 - www.defenceimagery.mod.uk - 45155579slide 12 - https://www.flickr.com/photos/broeckxsven/slides 4,9 - http://martinfowler.com/bliki/

http://martinfowler.com/bliki/CQRS.htmlhttp://udidahan.com/2009/12/09/clarified-cqrs/https://en.wikipedia.org/wiki/Command–query_separationhttps://cqrs.files.wordpress.com/2010/11/cqrs_documents.pdf

RESOURCES