Design a real-time online user counter
PremiumReal-timeEasy
- Beginner
- System design
- Real time
Quick answer
This real-time interview question asks you to design a real-time online user counter. Aim for a design that delivers updates to many users with low delay, then explain connections, fan-out, ordering and trade-offs. Interviewers want to see how you keep latency low, handle reconnects and stay correct under load.
The problem
Design a system that tracks and displays the number of currently active users on an application in real time.
What must a real-time online user counter do?
- Track users when they come online.
- Detect when users disconnect or become inactive.
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How well must it do it?
- Online count updates should be low latency.
- The system should scale across multiple servers.
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What scale should you design for?
Use these numbers in your estimates.
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Key concepts
- Beginner
- System design
- Real time
- Websockets
- Presence
- Sessions
How you’ll practise
6 short steps · save and come back any time
- Requirements
- Estimate
- Connections
- Message flow
- Delivery guarantees
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any real-time design round.
- Clarifies latency, ordering and delivery guarantees
- Chooses a connection model with reasons
- Explains fan-out to many receivers
- Handles reconnects, missed messages and duplicates
- Plans for hot spots and sudden traffic spikes
- States trade-offs between consistency and speed
Questions people ask about this interview question
- Which design type is this question?
- It is a real-time question. You design a real-time online user counter and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Easy, so it suits engineers who are new to real-time interviews. It is also a good warm-up before harder questions.
- What should I cover first?
- Start with who sends and receives updates, how fast they must arrive and what order matters. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team