System requirements
Functional:
User can tweet (send) up to 140 character message.
User can follow other users.
User can like other users' tweets.
User's home feed will show tweets from the users they are following.
The home feed will show top K popular tweets, based on the number of likes they receive, and the number of the followers the author has.
If I have time, I will add retweeting.
Non-Functional:
Scalability. It will have to serve a very large population, e.g., 500M DAU.
Response time. User has to see tweets quickly. When user opens home feed, the first 10 tweets should show up within 500ms.
Availability.
Consistency requirement can be a little bit relaxed. It does not require strong consistency like banking transactions. Eventual consistency would suffice. For example, if a user tweets something. If a user in the same geographic region sees the tweet in 1 second, and another user on the other side of the earth sees it after 30 seconds, that would be acceptable.
Security, content moderation, and anti abuse protection are all important, but I will not focus on them in this exercise due to lack of time. I will come back to them if I have time.
Capacity estimation
Estimate the scale of the system you are going to design...
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?