Requirements

Functional Requirements:

• Hierarchical comments: user is able to reply to any comment. The reply will appear under the parent comment
• Thread integrity: the comment is ordered chronologically.
• Comment display: user is able to view any comment and their reply.

Non-Functional Requirements:

• Consistency: Thread order and comment is consistent across every device
• Latency: Low, < 50ms for commenting
• Availability: The system up time is greater than > 99.99%

API Design

create_comment(thread_id: str, comment: str, parent_id: str | None):

REST API: POST

create comment in specific thread if parent_id is not None the comment is reply

get_comments(thread_id: str, parent_id: str | None, limit: int = 25):

REST API: GET

get {limit} comments with {parent_id} from thread {thread_id} if parent_id is None return only main comment

High-Level Design

Detailed Component Design

Database

Database choice: relational database like MySQL

Data schema:

Comment Table:

comment_id: primary key

thread_id: foreign key

comment: context of the comment

parent_id: parent comment's comment_id

Comment Rendering Algorithm

Pre-order depth first search:

To render, nested comments we need to render the parent comment first then we will render child comment sorted by created datetime.