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Hacker news-inspired forum app built with TypeScript & DDD from
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A SOLID hackernews-inspired forum site built with TypeScript using the clean architecture and DDD best practices.

About is the application that we build in - The Software Design and Architecture Handbook.


You can visit the site here.

Note: It's currently deployed on free tier Heroku, which has some undesirable side-effects like shutting off the server during periods of inactivity. So if it's down for you, refresh a couple of times. Thinking about migrating this to a serverless architecture later on.

Built with


  • Sequelize - The ORM for Node.js
  • Express.js - Lightweight webserver
  • Redis - For holding onto JWT tokens and refresh tokens



We built this based on the Clean Architecture, SOLID principles, and Domain-Driven Design best practices using TypeScript.

Clean architecture

There's obviously a lot that went into building this from front to back.

The Clean Architecture is a way to reason about where different types of application logic belongs.

Frame 8 (1)

There's a lot more to learn about the clean architecture, but for now- just know that it's a way to really separate the concerns of everything that goes into building complex enterprise applications. You'll never see any infrastructure-related code alongside domain layer code.

The clean architecture, when combined with Domain-Driven Design, is very powerful :)

In DDD, we build applications on top of a number of subdomains.


A subdomain is a cohesive unit of code that represents exactly one core concept and is responsible for a specific set of concerns in an application architecture. For example, every appliciation has a users subdomain. That's responsible for users, identity & access management, authentication, authorization, etc. Sometimes you don't want to build that yourself. Sometimes you can go with an off-the-shelf solution like Auth0. But there are subdomains in your application that you cannot simply outsource. These are the family jewels; the thing that's actually novel about your app. This is the subdomain that no one (except you) can code. Know why? Because only you have the domain knowledge to build it exactly the way that it should be built. You understand the domain.

In DDDForum, we have 2 subdomains: The users subdomain and the forum subdomain.

Frame 3 (1)

Each subdomain has a:

  • domain layer: where the highest-level policy, domain objects, and domain rules belong (user, email, etc)
  • application layer: where the use cases / features that utilize domain objects belong (createUser, login, etc)
  • adapter layer: where we define abstractions so that application layer code can interact with infrastructure layer concepts, without actually requiring on infrastructure (because that would break the dependency rule). Here we write things like IUserRepo - repository adapter, IJWTTokenService - an abstraction of a cache (redis) that manages tokens, etc.
  • infrastructure layer: where we create concrete implementations of the abstractions from the adapter layer so that they can be spun up at runtime thanks to the power of polymorhpism :) (more on this later).

If you haven't already, I recommend you read this article on use cases and subdomains.

Let's identify some of the actual concepts that exist in each subdomain.

users subdomain

In the users subdomain, we're only concerned with concepts that are related to authentication, roles, etc. Here are a few examples of classes and concepts that exist at each layer.

forum subdomain

In the forum subdomain, we're only concerned with concepts that have to do with building a forum. You won't see any domain concepts from the user in forum. In the forum subdomain, the concept most equivalent to a user, is a member.

Here are a few examples of concepts from the forum subdomain.

  • domain layer: member, comment, post, postVote, commentVote, commentVotesChanged
  • application layer: replyToComment, getMemberByUserName, upvotePost, downvotePost
  • adapter layer: ICommentRepo, IPostRepo, IMemberRepo
  • infrastructure layer: SequelizeCommentRepo, SequelizePostRepo, SequelizeMemberRepo


This project is licensed under the ISC License - see the file for details

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