Ruby CQRS with Event Sourcing

A Ruby implementation of Command-Query Responsibility Segregation (CQRS) with Event Sourcing, based upon the ideas of Greg Young.

Find out more about CQRS.

Getting Started

Dependencies are managed using Bundler.

$ sudo gem install bundler

Install all of the required gems for this application

$ sudo bundle install

Specs

Run the RSpec specifications as follows.

$ rspec spec/

Basic Design Overview

###UI

• display query results using read-only reporting datastore
• create commands - must be 'task focused'
• basic validation of command (e.g. required fields, uniqueness using queries against the reporting data store)

###Commands such as RegisterCompanyCommand

• capture users’ intent
• named in the imperative (e.g. create account, upgrade customer, complete checkout)
• can fail or be declined

###Command Bus

• validates command
• routes command to registered handler (there can be only one handler per command)

###Command Handler such as RegisterCompanyHandler

• loads corresponding aggregate root (using domain repository)
• executes action on aggregate root

###Aggregate Roots such as Company

• guard clause (raises exceptions when invalid commands applied)
• calculations (but no state changes)
• create & raise corresponding domain events
• subscribes to domain events to update internal state

###Domain Events such as CompanyRegisteredEvent

• inform something that has already happened
• must be in the past tense and cannot fail
• used to update internal state of the corresponding aggregate root

###Event Bus

• subscribes to domain events
• persist domain events to event store
• routes events to registered handler(s) (can have more than one handler per event)

###Event Handler such as CompanyRegisteredHandler

• update de-normalised reporting data store(s)
• email sending
• execute long running processes (e.g. 3rd party APIs, file upload)

###Event Store

• persists all domain events applied to each aggregate root (stored as JSON)
• reconstitutes aggregate roots from events (from serialised JSON)
• currently two adapters: ActiveRecord and in memory (for testing)
• adapter interface is 2 methods: find(guid) and save(aggregate_root)
• could be extended to use a NoSQL store