System requirements

Functional:

• Allow customers to read and post comments
• Preserve hierarchical structure
• Show real-time updates
• No updates

Non-Functional:

• Consistency: eventual consistency
• Real time latency: within seconds
• Availability
• Scalability

Capacity estimation

• TPS
• Storage

API design

post(customer_id, content, parent_id)

get(post_id, level=5, customer_id, pagination_token, page_size=10)

Database design

NoSQL Database

post_id, children_ids, content

• optimistic lock during post
• BFS during get
  • if level=5, needs to call database 5 times

High-level design

Clients: customers

Load Balancer: distribute traffic

Server: host application services

• CreatePostService: component to handle create
• GetPostService: compnent to handle get

Database: NoSQL DB

Request flows

Create:

Client->LB -> CreatePostService -> Unique ID generator (UUID, or customized id genertor) -> Database

Get:

Client->LB -> GetPostService -> Database

Detailed component design

• (Optional) Add caching layer before Database
• (Optional) Add a messaging queue to handle creation requests

Trade offs/Tech choices

• Cannot return all replies to a comment as it can be huge

Failure scenarios/bottlenecks

DLQ

retry

Future improvements