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

ServiceResponsibility
API Gateway	Provides the public API entry point and routes read and write requests to the appropriate backend service.
Read Service	Handles GraphQL queries and retrieves comment data from the cache, falling back to the database when necessary.
Write Service	Handles REST commands such as creating, updating, and deleting comments and persists changes to the database.
Cache Layer	Stores frequently requested comment data to reduce database reads and improve response latency.
NoSQL Database	Acts as the source of truth for comments, replies, relationships, and persistent comment state.
Event Bus	Publishes comment-domain events such as CommentCreated or ReplyCreated to independent asynchronous consumers.
Cache Queue	Buffers cache-update events so temporary processing spikes do not overload the cache worker.
Cache Worker	Consumes cache events and invalidates or refreshes stale cached comment data.
Notification Queue	Buffers notification events independently so slow notification processing does not affect cache synchronization.
Notification Worker	Consumes comment/reply events and determines which notification operations must be executed.
Notification Service	Delivers notifications to affected users through the configured notification channels.
Dead Letter Queue	Stores messages that repeatedly fail processing so they do not block normal queue consumption.

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.