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Messenger is a chat application.
User is able to:
If I have time, I will design additional features:
Support web clients and mobile clients.
Scalable - 500M DAU
Low response time - message appears in everyone's chat window within 200 - 300ms after being typed
Availability
Reasonably strong consistency. We would like to minimize of inconsistent communication, e.g., messages arriving out of order. But it does not need to be a strong consistency like ACID properties. Eventual consistency with reasonable limit (e.g. messages become ordered within 1 second) would suffice.
Error scenarios to note:
Network failure preventing user from sending messages and receiving messages. Both cases should be considered and guarded against.
500M DAU
Each person spends 10 minutes on chat, on average.
Each person generates 2KB of text every day.
500M * 2KB = 1TB data each day.
To store the conversations up to 1 year, data size would be 1TB.
Assuming data is backed up in 3 ways, that would be 3TB of data.
At peak, let's say 20% of users - 100M users - can be chatting in one second.
Define what APIs are expected from the system...
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...
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...
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...
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...
Explain any trade offs you have made and why you made certain tech choices...
Try to discuss as many failure scenarios/bottlenecks as possible.
What are some future improvements you would make? How would you mitigate the failure scenario(s) you described above?