Requirements

Functional Requirements:

• Allow users to post top-level comments and replies in a nested hierarchy.
• Enable users to edit or update their own comments and delete them; deletion may be restricted if the comment has replies.
• Provide real-time updates or notifications when someone replies to their comment or posts in a discussion they follow.
• Allow users to upvote or downvote comments to indicate quality or popularity; vote counts influence sorting.
• Enable sorting comments chronologically or by popularity.
• Enforce a maximum depth for nested replies.

Non-Functional Requirements:

1. Availability: The System should be highly available
2. Scalable: System should be highly Scalable for 100M users and be able to tolerate request spikes.
3. Latency: The Read and write request should have very low latency.
4. Durability
   1. Once comment is created, needs to ensure that the data never lost
5. Reliability
   1. The System should behave correctly and deliver the functional requirement even in case of failure, request spikes and other outage.  

API Design

• Add Comment
  1. HTTP POST
  2. API Endpoint: /api/v1/post-comment
  3. Request: {

"userid":"",

  "commentMessage":""

}

Response: {

"postid":"",

"parentpostid":"",

"userid":"",

"userName":"",

"commentDateTime":"",

"commentmessage":"",

"path":""

}

• Add Reply: User Can add a reply to the comment
  1. HTTP POST
  2. API Endpoint: /api/v1/post-reply
  3. Request: {

"parentid":"",

"userid":"",

"commenttMessage":"",

"path":""

}

Response: {

"postid":"",

"parentpostid":"",

"userid":"",

"userName":"",

"commentDateTime":"",

"commentMessage":"",

"path":""

}

• Upload Comment: User Can update the comment only if it doesn’t have a replies or Unvote or downvote
  1. HTTP PATCH
  2. API Endpoint: /api/v1/update-comment
  3. Request:

{

"postid":"",

"userid":"",

"commentMessage":""

}

Response:

{

"postid":"",

"parentpostid":"",

"userid":"",

"userName":"",

"commentDateTime":"",

"commentmessage":""

}

• Delete Comment: User Can delete the comment only if it doesn’t have a replies or Unvote or downvote
  1. HTTP DELETE
  2. API Endpoint: /api/v1/delete-comment/{postid}
  3. Request Path Variable: postid to be deleted
  4. Response:

{

"postid":"",

"parentpostid":"",

"userid":"",

"userName":"",

"commentDateTime":"",

"commentmessage":""

}

• Up/Down Vote Comment
  1. HTTP POST
  2. API Endpoint:
  3. Request: {

"postid":"",

"voteType":"",

"userId":"",

"currentCount":""

}

Response:{

"postid":"",

"voteType":"",

"updatedCount":""

}

• Get Comment with Sorting: Get a Post comments
  1. HTTP GET: api/v1/post-comments/{post-id}/{page-id}/{sort}
  2. Response: {

"postid":"",

"comments":[{

"postid":"",

"parentpostid":"",

"userid":"",

"userName":"",

"commentDateTime":"",

"commentmessage":"",

"path":"",

"replayCount":""

}]

}

High-Level Design

1. Comment Write Service
   1. When User Add a comment the request along with details will be first hit to the API gateway where it gets routed to the service and a specific instance of the service.
   2. Once the comment details reach to the backend Service it gets store in the relational database table. And the parent connect count gets updated to +1.
   3. While inserting the data into database table it also verify the depth of the comment if depth of the comment id greater than configured allowed depth then set the depth of the new comment as the highest one.
   4. The Recently added comment will be added/updated to the Radis Cache and the message will be sent to the messaging queue for the notification.
2. Comment Read Service
   1. When lookup for a comment for a specific post the request goes to the Backend Service through the API gateway where it routed to the appropriate instance of the read service.
   2. The comments will be loaded in a pagination fashion where first it will load only the first 20 parent(whose parentid will be null) comments using a path field.
   3. Once your click on see reply of option of the parent comment the next request will be generated and get all the first level comment of the parent comment using path column (path like "parentid/%")
   4. This is how can load the comments of every hierarchy of the parent comment.
3. Notification Service
   1. Notification service listen to the queue where consumer write service push the message after successfully insert into database.
   2. And it will be push to the user if WebSocket is open or will be sent as a notification to the user.

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.