System requirements

Functional:

List functional requirements for the system (Ask the chat bot for hints if stuck.)...

• Users can publish new comments.
• Users can reply to other users' comments
• The system guarantees the same comment order for every user
• The user can't delete the comments only add new ones
• The user gets a notification if another user replies to his comments
• Let's restrict the one comment thread, which may have 1000 items and only 4 nested depths.(It would ignore the comments if its depth exceeds 4).
• Users can't delete or change the comments; they can only append new ones.

Non-Functional:

List non-functional requirements for the system...

• This service should be highly available
• This service should be highly scalable
• When users publish comments, the new comments appear with a delay of 10 milliseconds.
• The service should be highly reliable
• Only registered users can leave comments.
• There are the mobile and web clients.
• The comments will be shown in chronological order.

Capacity estimation

Estimate the scale of the system you are going to design...

1 million daily active users.

Every user leaves 5 comments per 5 minutes.

Every user reads 10 comments per 5 minutes.

Every message takes 400 bytes on average.

Users write 17000 new comments per second or 7 MBytes per second.

Users read 34000 comments per second or 13.6 MBytes per second.

The system persists 442 Terabytes of comments for 1 year( store 1 duplication of every comment).

The system persists 2,2 Petabytes of comments for 5 years.

We are going to use RDMS to store users and comments information:

The table Users:

id string (1000 bytes)

API design

Define what APIs are expected from the system...

Database design

Defining the system data model early on will clarify how data will flow among different components of the system. Also you could draw an ER diagram using the diagramming tool to enhance your design...

High-level design

You should identify enough components that are needed to solve the actual problem from end to end. Also remember to draw a block diagram using the diagramming tool to augment your design. If you are unfamiliar with the tool, you can simply describe your design to the chat bot and ask it to generate a starter diagram for you to modify...

Request flows

Explain how the request flows from end to end in your high level design. Also you could draw a sequence diagram using the diagramming tool to enhance your explanation...

Detailed component design

Dig deeper into 2-3 components and explain in detail how they work. For example, how well does each component scale? Any relevant algorithm or data structure you like to use for a component? Also you could draw a diagram using the diagramming tool to enhance your design...

Trade offs/Tech choices

Explain any trade offs you have made and why you made certain tech choices...

Failure scenarios/bottlenecks

Try to discuss as many failure scenarios/bottlenecks as possible.

Future improvements

What are some future improvements you would make? How would you mitigate the failure scenario(s) you described above?