Social Microservice (Posts, Likes, Comments)

Simple TypeScript microservice with in-memory storage:

• Create text-only posts
• Like/unlike posts
• Comment on posts

Tech

• Node.js + TypeScript
• Express
• Zod for schema validation for entities
• Mocha, Sinon, and Chai for tests

Architecture & Design

The project follows a layered architecture with clear separation of concerns:

• Web Layer (routes/middleware) - handles HTTP requests/responses
• Application Layer (services) - contains business logic
• Entity Layer (models/validation) - defines entities and validation rules
• Infrastructure Layer (repositories) - abstracts data access

This structure makes the code testable and maintainable. Each layer can be tested independently, and changes in one layer don't affect others.

Design Patterns Used

Repository Pattern

• Data access is abstracted behind interfaces (PostsRepository, CommentsRepository, LikesRepository)
• Currently using in-memory storage, but can easily swap to a database later

Factory Pattern

• Using createPostsRouter() and createServer() functions initialising different repositories

Service Pattern

• Our PostsService contains the entire business logic instead of residing directly in routes
• Routes -> services -> repositories (separation of concerns between layers)

Dependency Injection

• Dependencies are injected through constructors

Middleware Pattern

• Using ErrorHandler and validateUUIDParam() for validating schema via middleware pattern

Chain Of Responsibility Pattern

• Express.JS beautifully implements the COR pattern via the app.use() & next() call inside the routes.
• Multiple handlers/routers can be 'chained' together to achieve modular functionality.

Code Quality

Type Safety

• Using TypeScript in strict mode
• Zod schemas for runtime validation
• Using UUIDv4 from node's crypto package (minimal collisions between generated IDs)

Error Handling

• Declared custom error classes (NotFoundError, ValidationError, ConflictError) for better error handling
• Centralized error handler maps errors to HTTP status codes

Testing

The project has comprehensive test coverage (34 tests in total):

Unit Tests (posts.service.utc.test.ts)

• Test pure business logic in isolation

Integration Tests (posts.api.itc.test.ts)

• Test API behavior end-to-end

Technical Decisions

In-Memory Storage

• Chose in-memory data atructures for simplicity and fast development
• Repository pattern makes it easy to swap to a database later
• Good for the current purpose of Assignment/MVP/demo

Zod for Validation

• Provides type-safe validation

Factory Functions

• Better for testing (can inject dependencies)
• Avoids singleton patterns
• More flexible approach

Custom Error Classes

• Easy to map to HTTP status codes

Performance

• Using Map data structures for O(1) constant time access
• Idempotent operations (safe to retry)

Getting Started

1. Install dependencies

npm install

2. Build and start

npm run build
npm start

Service listens on https://bp-social-network-app.onrender.com

Health

curl https://bp-social-network-app.onrender.com/health

I've used UptimeRobot to perform health monitoring: https://stats.uptimerobot.com/5KtXbR6uN5

API

I've uploaded the API collection for both Bruno and Postman API client:

• Bruno: ./API Collection_Bruno.json
• Postman: ./API Collection_Postman.json

Just download any of the two and import to start using the below mentioned APIs!

Base Hosted URL: https://bp-social-network-app.onrender.com/api/v1/posts

All postId and userId path parameters must be valid UUIDs. Invalid UUIDs will return a 400 Bad Request error.

Create Post

Creates a new text post.

Request:

POST /posts
Content-Type: application/json

Request Body:

{
  "userId": "550e8400-e29b-41d4-a716-446655440000",
  "description": "This is my first post!"
}

Validation Rules:

• userId: Required, must be a valid UUID
• description: Required, 1-1000 characters

Response: 201 Created

{
  "id": "123e4567-e89b-12d3-a456-426614174000",
  "userId": "550e8400-e29b-41d4-a716-446655440000",
  "description": "This is my first post!",
  "createdAt": 1704067200000,
  "updatedAt": 1704067200000,
  "likeCount": 0,
  "commentCount": 0
}

Error Responses:

• 400 Bad Request: Invalid request payload or validation errors

Example:

curl -X POST https://bp-social-network-app.onrender.com/api/v1/posts \
  -H 'Content-Type: application/json' \
  -d '{"userId":"550e8400-e29b-41d4-a716-446655440000","description":"Hello world!"}'

---

List Posts

Retrieves all posts, sorted by creation date (newest first).

Request:

GET /posts

Response: 200 OK

[
  {
    "id": "123e4567-e89b-12d3-a456-426614174000",
    "userId": "550e8400-e29b-41d4-a716-446655440000",
    "description": "Newest post",
    "createdAt": 1704067200000,
    "updatedAt": 1704067200000,
    "likeCount": 5,
    "commentCount": 2
  },
  {
    "id": "223e4567-e89b-12d3-a456-426614174001",
    "userId": "650e8400-e29b-41d4-a716-446655440001",
    "description": "Older post",
    "createdAt": 1703980800000,
    "updatedAt": 1703980800000,
    "likeCount": 10,
    "commentCount": 3
  }
]

Example:

curl https://bp-social-network-app.onrender.com/api/v1/posts

---

Get Post by ID

Retrieves a specific post by its ID.

Request:

GET /posts/:postId

Path Parameters:

• postId: UUID of the post

Response: 200 OK

{
  "id": "7982bfb7-1780-4828-8ed6-278d468e08c6",
  "userId": "550e8400-e29b-41d4-a716-446655440000",
  "description": "This is my first post!",
  "createdAt": 1704067200000,
  "updatedAt": 1704067200000,
  "likeCount": 5,
  "commentCount": 2
}

Error Responses:

• 400 Bad Request: Invalid UUID format for postId
• 404 Not Found: Post not found

Example:

curl https://bp-social-network-app.onrender.com/api/v1/posts/5f5ff573-c092-4360-a629-d03bf2d583e8

---

Delete Post

Deletes a post by its ID. Only the post owner can delete their post.

Request:

DELETE /posts/:postId
Content-Type: application/json

Path Parameters:

• postId: UUID of the post to delete

Request Body:

{
  "userId": "550e8400-e29b-41d4-a716-446655440000"
}

Validation Rules:

• userId: Required, must be a valid UUID. Must match the owner of the post.

Response: 204 No Content

Error Responses:

• 400 Bad Request: Invalid UUID format for postId, missing or invalid userId, or user is not the post owner
• 404 Not Found: Post not found

Example:

curl -X DELETE https://bp-social-network-app.onrender.com/api/v1/posts/5f5ff573-c092-4360-a629-d03bf2d583e8 \
  -H 'Content-Type: application/json' \
  -d '{"userId":"550e8400-e29b-41d4-a716-446655440000"}'

---

Like Post

Adds a like to a post. Users can only like a post once. Attempting to like the same post again will return a conflict error.

Request:

POST /posts/:postId/like
Content-Type: application/json

Path Parameters:

• postId: UUID of the post to like

Request Body:

{
  "userId": "660e8400-e29b-41d4-a716-446655440002"
}

Validation Rules:

• userId: Required, must be a valid UUID

Response: 201 Created

{
  "postId": "7982bfb7-1780-4828-8ed6-278d468e08c6",
  "userId": "660e8400-e29b-41d4-a716-446655440002",
  "createdAt": 1704067300000
}

Error Responses:

• 400 Bad Request: Invalid UUID format or validation errors
• 404 Not Found: Post not found
• 409 Conflict: User has already liked this post

Example:

curl -X POST https://bp-social-network-app.onrender.com/api/v1/posts/5f5ff573-c092-4360-a629-d03bf2d583e8/like \
  -H 'Content-Type: application/json' \
  -d '{"userId":"660e8400-e29b-41d4-a716-446655440002"}'

---

Unlike Post

Removes a like from a post. This operation is idempotent - removing a non-existent like will succeed silently.

Request:

DELETE /posts/:postId/like/:userId

Path Parameters:

• postId: UUID of the post
• userId: UUID of the user who liked the post

Response: 204 No Content

Error Responses:

• 400 Bad Request: Invalid UUID format for postId or userId
• 404 Not Found: Post not found

Example:

curl -X DELETE https://bp-social-network-app.onrender.com/api/v1/posts/5f5ff573-c092-4360-a629-d03bf2d583e8/like/660e8400-e29b-41d4-a716-446655440002

---

Add Comment

Adds a comment to a post.

Request:

POST /posts/:postId/comment
Content-Type: application/json

Path Parameters:

• postId: UUID of the post to comment on

Request Body:

{
  "userId": "660e8400-e29b-41d4-a716-446655440002",
  "text": "Great post! Thanks for sharing."
}

Validation Rules:

• userId: Required, must be a valid UUID
• text: Required, 1-500 characters

Response: 201 Created

{
  "id": "789e4567-e89b-12d3-a456-426614174003",
  "postId": "7982bfb7-1780-4828-8ed6-278d468e08c6",
  "userId": "660e8400-e29b-41d4-a716-446655440002",
  "text": "Great post! Thanks for sharing.",
  "createdAt": 1704067400000
}

Error Responses:

• 400 Bad Request: Invalid UUID format or validation errors
• 404 Not Found: Post not found

Example:

curl -X POST https://bp-social-network-app.onrender.com/api/v1/posts/5f5ff573-c092-4360-a629-d03bf2d583e8/comment \
  -H 'Content-Type: application/json' \
  -d '{"userId":"660e8400-e29b-41d4-a716-446655440002","text":"Great post!"}'

---

List Comments

Retrieves all comments for a specific post.

Request:

GET /posts/:postId/comments

Path Parameters:

• postId: UUID of the post

Response: 200 OK

[
  {
    "id": "789e4567-e89b-12d3-a456-426614174003",
    "postId": "7982bfb7-1780-4828-8ed6-278d468e08c6",
    "userId": "660e8400-e29b-41d4-a716-446655440002",
    "text": "Great post! Thanks for sharing.",
    "createdAt": 1704067400000
  },
  {
    "id": "889e4567-e89b-12d3-a456-426614174004",
    "postId": "7982bfb7-1780-4828-8ed6-278d468e08c6",
    "userId": "770e8400-e29b-41d4-a716-446655440003",
    "text": "I totally agree!",
    "createdAt": 1704067500000
  }
]

Error Responses:

• 400 Bad Request: Invalid UUID format for postId
• 404 Not Found: Post not found

Example:

curl https://bp-social-network-app.onrender.com/api/v1/posts/5f5ff573-c092-4360-a629-d03bf2d583e8/comments

---

Error Response Format

All error responses follow this format:

{
  "error": "Error message describing what went wrong"
}

HTTP Status Codes:

• 400 Bad Request: Validation errors or invalid UUID format
• 404 Not Found: Resource not found (post, comment, etc.)
• 409 Conflict: Business rule violation (e.g., duplicate like)
• 500 Internal Server Error: Unexpected server errors

Tests

Run tests with:

npm test

Repository Structure

src/
  application/       # services (business logic)
  domain/            # models and validation
  infrastructure/    # repos + errors
    memory/          # in-memory repo implementations
  server/            # express app
  web/               # routes + middleware