Ruby CQRS example with rom, dry-system, kafka and different services

Feedback policy

We can implement one pattern or idea in absolutely different ways and it's really cool. If you find some mistakes in implementation let's discuss it in github issues or personal.

F.A.Q.

Why did you create this repository?

Because it was interesting to build something like this for me and I got fun doing it.

Why did you choose <technology_name>?

Because I know this technology better, or I just wanted to show how to use it in the "real" code, or I just wanted to play with technology in this project.

Motivation

CQRS as a pattern is not super popular in the ruby world. This repository can show you how you can implement commands and queries in your services system and use CQRS for sharing data between separate services and DBs.

Important: The main goal of this repository just shows one more way how to implement CQRS pattern in ruby. it's not a production-ready example. And you can use other libraries or DBs without any problems.

Project details

It's a simple blog platform with creating new users, posts with authors and comments for each post (with the author too). Now this repository implements only creation logic. If you want to add update/delete logic - feel free to send PR.

Architecture

List of services

Users service

CQRS: one relation DB.
Kafka: no consumers, producer for user_created event
Sync communications: zero

Posts service

CQRS: two relation DB (one for write, one for read)
Kafka: consumers for users, posts and comments topics, producer for post_created event
Sync communications: zero

Comments service

CQRS: two relation DB (one for write, one for read)
Kafka: consumer for comments topic, producer for comment_created event
Sync communications: one http call for getting user full name for read model

Service implementation

Each service use rom for persistence, sitantra as a HTTP transport, dry-system as a dependency management system

In each service you can find same structure:

web_app.rb - sintatra HTTP application with list of endpoints
karafka.rb - karafka consumer application
config/boot.rb - entripoint for booting dry-container and .env files
system/container.rb - dry-system container
system/types.rb - base dry-types module
system/boot/kafka_producer.rb - simple wrapper around waterdrop for producing events to kafka
system/boot/*_rom.rb - ROM instances for read and write models

lib/commands/*.rb - list of struct objects with existing commands for specific service
lib/commands_handler/base.rb - general class for execut specific command struct object
lib/commands_handler/*.rb - separated classes with logic of each command
lib/queries/*.rb - query classes which we use for getting agretate or data
lib/read_model/*.rb - repositories and entities for read model
lib/write_model/*.rb - repositories and entities for write model
lib/event_handlers/*.rb - list of event execution logic which I call from karafka consumers
lib/events/*.rb - list of struct objects which I use for produce events to kafka

Useful links

Really good blog post wich explain CQRS and small details around this pattern
CQRS example from arkency. They use rails and rails event store
Russian speakers only (sorry for this): Record from the stream where I worked on this repository

How to setup project locally

SOON

davydovanton/cqrs-ruby-example