Introduction

In this repository, I will demonstrate how to implement CQRS (Command Query Responsibility Segregation), DDD (Domain-Driven Design), and Event Sourcing patterns using the NestJS framework. These patterns can significantly help in managing complex business logic, improving scalability, and maintaining a clean architecture in your applications.

While there are many high-quality articles on the theories behind these patterns and strategies available online, this guide aims to provide a practical, hands-on example.

Join me as I delve into the intricacies of CQRS, DDD, and Event Sourcing with NestJS, offering valuable insights and practical examples to help you implement these patterns effectively in your projects.

CQRS

CQRS (Command Query Responsibility Segregation) is a pattern that separates the models used to update data (commands) from those used to read data (queries). This separation can be useful in complex domains and high-performance applications, allowing for independent scaling and optimization of read and write operations. However, CQRS adds significant complexity and is only suitable for specific parts of a system. Most systems that fit a CRUD model may not benefit from CQRS. Properly evaluating the domain and application needs is crucial before implementation.

For more details, visit Martin Fowler's article on CQRS.

• Separating the read model, its database, and even read-only servers will significantly increase system complexity.
• I recommend using a highly segregated CQRS system only when absolutely necessary.
• Don't apply it everywhere. It's enough to use it where needed. Let's avoid the trap of overengineering.

Event Sourcing

Event Sourcing is a way to persist state changes as a sequence of events rather than storing the current state. Each event represents a state change and is stored in an append-only event log. This pattern allows rebuilding the current state by replaying events, provides a natural audit log, and facilitates debugging by allowing developers to see exactly what happened in the system. It can also support complex scenarios like temporal queries and retroactive corrections.

For more details, visit Martin Fowler's article on EventSourcing

• There's a big difference between simply streaming and consuming events and storing events for later use.
• You can reassemble data from a specific point in time or create different models from a single source as needed. It’s also useful for monitoring and debugging.
• If you're already streaming and consuming events, consider storing them as well. It can make solving complex problems easier.

DDD

Domain-Driven Design (DDD) is an approach to software development that emphasizes the importance of a domain model, which is a rich understanding of the business processes and rules. Originating from Eric Evans's 2003 book, DDD involves creating a Ubiquitous Language that integrates domain terminology into software systems, evolving models over time, and categorizing objects into Entities, Value Objects, and Service Objects. Strategic Design in DDD includes organizing large domains into Bounded Contexts, providing a structured way to handle complex domains.

For more details, visit Martin Fowler's article on DDD.

• If the project’s scale or complexity isn't significant, this highly structured pattern might be unnecessary.
• Applying this pattern to an immature project is risky because once it's implemented, it becomes very difficult to dismantle and reassemble if the domain scope and boundaries change significantly.
• The best time to apply this pattern is during a major refactoring of an established project with messy code, introducing it gradually.

Strategies for managing data

RDBMS (master - slave)

• This structure is very simple, and some may not consider it as CQRS.
• But does it really matter? This structure is likely the most suitable for most projects and it ensures the safest and most reliable improvement in read performance.

Event stream + Event handler + Key-value store(projection model)

• This structure is very effective when the read model is complex and requires a lot of data joins and processing.
• The key-value store will have eventual consistency
• The complexity of the system and the difficulty of monitoring and handling failures will increase.

Event store + Event sourcing

• All state changes in the system are stored in the event store, and the system operates based on this event data.
• Various forms of model assembly are possible. In particular, it allows for the flexible assembly of data at specific points in time or under specific conditions.

Event store + Event sourcing + Key-value store(snapshot)

• Assembling all events from the beginning to the present each time to calculate the current state may be unnecessary.
• This issue can be resolved by storing snapshots, and for more detailed explanations, it is recommended to refer to this article(CQRS with Amazon DynamoDB).

Everything

• There is no one right answer! No single approach needs to be applied!
• Consider and apply the strategy that best suits your service (or a specific part of it)!

Getting started

Prerequisites

• Install node.js: https://nodejs.org/en/download/
• Install docker desktop for mac: https://docs.docker.com/docker-for-mac/install/
• Install docker desktop for windows: https://docs.docker.com/docker-for-windows/install/
• Install docker compose: https://docs.docker.com/compose/install/

Script

Fetch codes

git clone https://github.com/kimyh03/nestjs-cqrs-ddd-example

Install packages

npm install

Run databases

docker compose up -d

Run

npm run start

Links

• NestJS: https://nestjs.com/
• EventStore DB: https://www.eventstore.com/
• Martin Fowler
  • CQRS: https://martinfowler.com/bliki/CQRS.html
  • DDD: https://martinfowler.com/bliki/DomainDrivenDesign.html
  • Event sourcing: https://martinfowler.com/eaaDev/EventSourcing.html
  • Event driven: https://martinfowler.com/articles/201701-event-driven.html
• Microsoft
  • CQRS: https://learn.microsoft.com/en-us/azure/architecture/patterns/cqrs
  • Event sourcing: https://learn.microsoft.com/en-us/azure/architecture/patterns/event-sourcing
• AWS
  • CQRS with Amazon DynamoDB: https://aws.amazon.com/ko/blogs/database/build-a-cqrs-event-store-with-amazon-dynamodb/