Requirements

Functional Requirements:

• post a comment in a discussion
• reply to a comment
• edit a comment
• delete a comment
• content filtering: filter words
• support anonymous comments
• pagination

Non-Functional Requirements:

• Consistency: Aim for Causal consistency model - maintains the hierarchy of comments, with some leniency to allow good user experience by making comments available immediately.
• Availability: 99.9% uptime
• Latency: low latency, near real-time updates
  • Read: < 50 ms
  • Write: < 500 ms
• Scalability: Horizontal
  • DAU: 100 million users
  • Reads: About 500 million requests
  • 20% comment about 5 comments per day
  • Writes: About 100 million requests

Back of the envelope:

• Load:
  • Write: 100 million comments ~ 1000 RPS
  • Read: 5000 RPS
• Storage:
  • 500 bytes per comments - 50 GB per day
  • Growth: 18 TB per year
  • No files

API Design

• POST /comments: to create a comment with fields such as postId, parentId, and content.
• GET /posts/{postId}/comments?page=0&pageSize=50: to retrieve comments for a specified post, with pagination.
• DELETE /comments/{commentId}: to remove a comment (considering moderation).

Database Design

• For Causal consistency, we can use MongoDB, low latency writes and maintained hierarchy.
• Posts collection: postId, List<commentId>, title, description
• Comments collection: commentId, postId, parentId, content

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.

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.