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Arnaud Langlade
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Software-Architecture Design-Patterns


WHAT IS THE DIFFERENCE BETWEEN CQS AND CQRS PATTERNS?

Published at Feb 6, 2023

Image by @mihaistrompl

I recently found out that I did not grasp those design patterns. There are a lot
of resources on the Internet about them but they are not always accurate. That’s
a shame because they are pretty simple. I will share my understanding of them
with you.


WHAT IS COMMAND QUERY SEGREGATION (CQS)?

> The fundamental idea is that we should divide an object’s methods into two
> sharply separated categories:
> 
>  * Queries: Return a result and do not change the observable state of the
>    system (are free of side effects).
>  * Commands: Change the state of a system but do not return a value.
> 
> Martin Fowler

This concept is not specific to Object Oriented Programming but improves the
object’s design. The object methods only have a single purpose: reading or
changing the object state. We can see an object as a living entity. We can ask a
question to someone because we need information. For example, we can ask someone
what time it is. This is a query. We can ask someone to do something, we don’t
expect an answer but we want to get the job done. For example, we can ask a
child to finish his/her spinach. This is a command.

We can apply this pattern to any object: like an aggregate. Let’s take an
example! I would like to add markers on a map and then I would like to find
which markers are the closest to a specific location (GPS Coordinates). k

class Map {
    addMarker(label: string, latitude: number, longitude: number): void {
        // ...
    }

    findClosestMarkers(location: Location): Marker[] {
        // ...
    }
}


The addMarker method is in charge of mutating the object state without returning
any result, while the ‘findClosestMarkers’ method finds the right markers
without changing the object’s state. This object follows the CQS definition.

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Let’s go further. If we design our aggregates following the CQS pattern, we
should apply it to the classes that handle use cases.

interface MapService {
    addMarkerToTheMap(label: string, latitude: number, longitude: number); void
    findAllMarkersCloseToLocation(): Marker[]
}


This ensures there is no inconsistency in the codebase. The business services
use and manipulate the aggregates. For example, if the
MapService.addMarkerToTheMap method returns a result, it might mean that the
Map.addMarker method will need to return the expected result.


WHAT IS COMMAND QUERY RESPONSIBILITY SEGREGATION (CQRS)?

> Starting with CQRS, CQRS is simply the creation of two objects where there was
> previously only one. The separation occurs based upon whether the methods are
> a command or a query (the same definition that is used by Meyer in Command and
> Query Separation, a command is any method that mutates state and a query is
> any method that returns a value).
> 
> Greg Young

Note: Greg Young’s blog does not exist anymore but his blog posts are still
available thanks to archived.org.

CQRS is the separation of command and query into two different objects instead
of only one. MapService does not follow the CQRS pattern because it has a query
and a command. We need to cut this object in half.

interface MapReadService {
    addMarkerToTheMap(label: string, latitude: number, longitude: number); void
}

interface MapWriteService {
    findAllMarkersCloseToLocation(): Marker[]
}


That’s pretty simple, right? Anyway, we don’t need to introduce complicated
things in our application to use this tactical design pattern. We don’t need a
write and a read model, a command and query bus, an event sourcing architecture
or multiple databases. Greg Young published this blog post in 2012 to explain
what CQRS was not about.

> CQRS is not a silver bullet
> 
> CQRS is not a top level architecture
> 
> CQRS is not new
> 
> CQRS is not shiny
> 
> CQRS will not make your jump shot any better
> 
> CQRS is not intrinsically linked to DDD
> 
> CQRS is not Event Sourcing
> 
> CQRS does not require a message bus
> 
> CQRS is not a guiding principle / CQS is
> 
> CQRS is not a good wife
> 
> CQRS is learnable in 5 minutes
> 
> CQRS is a small tactical pattern
> 
> CQRS can open many doors.

Note: This blog does not exist anymore but it has been archived by archived.org.
The post is available here

Depending on the number of use cases the service classes can become really huge.
CQRS helps to decrease their size but I am a big fan of them. I like to separate
each use case into a dedicated class.

I’ve written a blog post to explain what is a commands and we can apply it to
query too:

Command and command handler design pattern

Thanks to my proofreader @LaureBrosseau .


HELLO, I’AM ARNAUD!

I’m a technical coach and software architect. I aim to make complex software
concepts accessible and interesting for all.

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