Practice

A comment system

Design a comment system, such as Disqus, is a feature integrated into websites that allows users to post comments on published content, interact with other users through replies, upvote or downvote comments, and sort or filter comments. This system must be robust, scalable, and efficient to handle a large volume of users and comments.

Functional requirement:

  • User Authentication: Users can create an account, log in, and log out. This requires a secure authentication system that can handle 100M Daily Active Users.
  • Comment Submission and Display: Users can submit comments, which are then stored in a database and displayed under the related content. Assuming each user posts 10 comments every day, and each comment is 100 bytes, the system needs to handle 1 billion comments daily.
  • Replying to Comments: Users can reply to other comments, with replies nested under the original comment. This requires a system that can handle complex data relationships.
  • Upvote or Downvote Comments: Users can upvote or downvote comments. This requires a system that can track and update the vote count for each comment in real-time.
  • Moderation Tools: Administrators can delete inappropriate comments or ban users. This requires a robust administration system that can handle a large volume of moderation actions.

Scale requirement:

  • 100M Daily Active Users and 1 billion comments daily.
  • Data retention for 10 years.
  • Assuming a read:write ratio of 100:1.
  • Assuming that each comment needs 100 Bytes for storage.
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1. Resource Estimation