Imagine you are building a chat application in which a user can chat with any other user, provided they are both connected. For a user to initiate the chat, it is always helpful if we show who all are online.
In this assignment, let's design a system that indicates who all are online at the moment. The micro-problem statement is as simple as answering the question - Given a user, return if he/she is online or not.
The problem statement is something to start with, be creative and dive into the product details and add constraints and features you think would be important.
- should update online status of a user within 10 seconds of user coming online
- can be lineant in marking a user offline
- should scale for 5 million active users at any given moment
- a user should see accurate status of any other user, eventually
- make your high-level components operate with high availability
- ensure that the data in your system is durable, not matter what happens
- define how your system would behave while scaling-up and scaling-down
- make your system cost-effective and provide a justification for the same
- describe how capacity planning helped you made a good design decision
- think about how other services will interact with your service
- ensure the data in your system is never going in an inconsistent state
- ensure your system is free of deadlocks (if applicable)
- ensure that the throughput of your system is not affected by locking, if it does, state how it would affect
Create a design document of this system/feature stating all critical design decisions, tradeoffs, components, services, and communications. Also specify how your system handles at scale, and what will eventually become a chokepoint.
Do not create unnecessary components, just to make design look complicated. A good design is always simple and elegant. A good way to think about it is if you were to create a spearate process/machine/infra for each component and you will have to code it yourself, would you still do it?
To understand the nuances and internals of this system, build a prototype that
- shows a list of users
- shows if a particular user is online or offline
- the user list is grouped such that the online users are shown first and then the offline ones
- if the user is offline, also show "was online X mins ago"
This is a recommended tech-stack for building this prototype
| Which | Options |
|---|---|
| Language | NodeJS, or any language that supports SocketIO |
| Database | MySQL, MongoDB, or any one that you like |
| Library | SocketIO, for realtime status update |
These are the common pitfalls that you should keep in mind while you are building this prototype
- IO calls in NodeJS are asynchronous
- if you are broadcasting the status update, do not do it to "all" the users
- identify when a user is "online"
- finding when a user goes "offline"
- database schema design and deciding what data to store to make this efficient
- building realtime user experiences using SocketIO
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This assignment is part of Arpit's System Design Masterclass - A masterclass that helps you become great at designing scalable, fault-tolerant, and highly available systems.