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🧬 System Design: Low-Level Design (LLD)

Low-Level Design (LLD) focuses on the internal structure of components defined in High-Level Design. It captures class diagrams, database schemas, API definitions, and detailed logic.

🎯 Purpose of LLD

🧩 Key Components of LLD

1. Class Design / Object Model

Define internal data structures and classes with responsibilities.

class Post {
    int id;
    int userId;
    string content;
    datetime createdAt;

    bool isVisible();
    List<Comment> getComments();
}

2. Database Schema Design

Tables, indexes, relationships, constraints.

Table: posts
- id (PK)
- user_id (FK)
- content TEXT
- created_at TIMESTAMP
- visibility BOOLEAN

3. API Contract

Detailed request and response structure.

POST /api/v1/posts

Request:
{
  "userId": 1,
  "content": "Hello world!"
}

Response:
{
  "id": 123,
  "createdAt": "2025-07-15T12:00:00Z"
}

4. Sequence Diagrams (Optional)

Defines the step-by-step interaction between components.

Client -> API Gateway -> Post Service -> DB
Post Service -> Notification Service (async)

5. Data Flow and Validation

Explicitly show:

- How data is validated at each step
- How errors are handled
- What business rules apply

6. Pseudocode or Logic Workflows

When logic is complex, include step-by-step logic.

function createPost(userId, content):
    if content.length == 0:
        throw ValidationError("Content required")
    
    save to DB
    publish event to Kafka

7. Error Handling

Define how to handle:

- Invalid inputs
- DB constraint violations
- Timeouts or downstream service errors

8. Test Cases and Edge Conditions

Identify key paths and edge cases to test.

- Content is empty (400 error)
- User doesn’t exist (404)
- System crash retry logic

9. Performance Constraints

If needed, include:

- Query optimizations (e.g., using proper indexes)
- Caching strategy (e.g., Redis for hot content)

🧠 Example: Comment Feature for Social App

Class Diagram

class Comment {
    int id;
    int postId;
    int userId;
    string message;
    datetime createdAt;

    bool isOffensive();
}

API Definition

GET /api/v1/posts/:postId/comments

Response:
[
  {
    "id": 1,
    "userId": 42,
    "message": "Nice post!",
    "createdAt": "2025-07-15T12:00:00Z"
  }
]

DB Schema

Table: comments
- id (PK)
- post_id (FK)
- user_id (FK)
- message TEXT
- created_at TIMESTAMP

✅ Best Practices

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