candid-cqrs

A simple an accessible CQS/CQRS set of libraries.

Goals :

1. Illustrate the CQS/CQRS usages in a simple and clear manner, even for the beginner
2. Provide a "ready to use" seed that people can use as it is or really easily modify for their own usage
3. Show how CQRS can allow for total framework independence. AKA : Moving you domain logic from Spring to dropwizard IS simple.
4. Illustrate and insist on the fact that different flavors of CQS/CQRS exist. AKA: You probably do not need event sourcing right now and even a really simple CQS split will greatly help you in your unit testing. (Refer to the link provided for more infos on that).

Rationale :

During my research on the subject, I could not find a simple enough Java example that would help me fully understand the CQS/CQRS concepts. Not many implementation exist or, really often, they take a highly generalized, highly functional and/or straight to event sourcing approach. My experience is that those are terrible to read for the beginner and confuse more than illustrate the underlying concepts.

Guiding principles:

1. No hidden magic, no weird annotations, no over-engineered generic abstract classes, no dark voodoo.
2. "Repetition is better than the wrong abstraction" - Sandi Metz.
3. "Get simple or die trying" - 50 cents, the untold quotes.
4. "You use libraries. But frameworks use you"
5. There might be better ways to do it. Still, nobody will be fired for doing it this way.

What do I have to gain?

• Say goodbye to service layers and lasagna code
• An external world proof code base
• Finally! freedom on the read side, a word without the dictatorship of Mappers (You might never come back)
• Testability by use case
• Amazing decoupling
• Better traceability of what happens on your system
• Easy handling of specifications change, especially when it come to side effects

What are the trade offs?

More classes really, but they should be way simpler to deal with

That's is cute but is it reliable/fast/worth the pain?

I use this architecture in production, not on a "let's code twitter in 30 seconds" toy project but in a payroll app people rely on to receive their money. They are pretty unforgiving audience and did not complain so far. Once understood, I find this type of architecture to be a delight to work with and to offer a flexibility I was herdly finding in my projects previously.

Do not to take my word on it. Take it for a spin.

How to start

If you are a newbie (Some of this stuff is in progress):

1. Looks at the linked presentations / slides.
2. Checkout the cqrs-api project. As an exercise, add unit tests and a decorator for each of the command/query buses. That should give you an idea of what happens.
3. Looks at the happy-neighbourhood domain code.
4. Look at the various frameworks integrations (only Spring is available so far).
5. Add your own spices to it.

Sources of inspiration :

Those great presentations :

https://speakerdeck.com/lilobase/ddd-and-cqrs-php-tour-2018

https://www.slideshare.net/rosstuck/command-bus-to-awesome-town

https://speakerdeck.com/lilobase/cqrs-fonctionnel-event-sourcing-and-domain-driven-design-breizhcamp-2017

Alistair in the hexagone: https://www.youtube.com/watch?v=th4AgBcrEHA

Models & Service Layers; Hemoglobin & Hobgoblins: https://www.youtube.com/watch?v=ajhqScWECMo

Recommended reads

To be done but in the meantime

• hexagonal architecture
• Implementing Domain Driven Design

More to come...

Final word

If you are a beginner, you should definitely stay away from frameworks. Don't even get me started on Lagom.