Requirements

Functional Requirements:

Text: the users can comment a text of 500 words.

Answers: The user can reply to a comment another user left there.

Comments: The comments come before the first comment.

Edits: the user can edit or delete the comments

real-time notifications: the user must see the real time notifcations

Non-Functional Requirements:

1. Scalability — one line: "Must handle millions of comments/threads; read-heavy workload; horizontal scaling via Lambda."
2. Low latency — one line: "Sub-second thread loads; posting feels instant."

Capacity Estimation

Estimate the scale of the system. Consider daily active users, read/write ratio, storage requirements, bandwidth, and any relevant QPS calculations...

API Design

API Design

Write API — REST

POST /v1/posts/{postId}/comments

Create a new root comment.

POST /v1/comments/{commentId}/replies

Create a reply to an existing comment.

PATCH /v1/comments/{commentId}

Update an existing comment.

DELETE /v1/comments/{commentId}

Delete an existing comment.

Read API — GraphQL

Query:

post(id: ID!) {

comments(first: Int, after: String) {

id

text

createdAt

author {

id

name

}

replies(first: Int, after: String) {

id

text

createdAt

author {

id

name

}

}

}

}

High-Level Design

Describe the overall system architecture. Identify the main components needed to solve the problem end-to-end. Use the diagramming tool to create a block diagram.

Database Design

Define the data model. Identify the main entities, their attributes, and relationships. Consider the choice of database type (SQL vs NoSQL) and justify your decision based on access patterns...

Detailed Component Design

Deep dive into 2-3 key components. Explain how they work, how they scale, discuss tradeoffs, capacity, and any relevant algorithms or data structures.