# Design an online presence service: system design interview question

> Presence across devices: sudden disconnects, large viewer audiences, and status freshness during partial failures.

Source: https://sysround.com/questions/design-online-presence-service
Updated: 2026-11-05


Difficulty: hard. Levels: Mid-level, Senior, Staff+.

## How the interviewer opens it

"Design a service that shows whether users are online, idle, or offline across multiple devices. Keep viewers informed as users connect and disconnect."

## What to clarify first

- Population and connections
- Status rules
- Heartbeats and disconnect detection
- Subscriptions and audience size
- Read and write mix
- Latency
- Consistency and freshness
- Durability
- Authorization
- Deployment and exclusions

## Scope of a strong answer

Core requirements:

- Accept device connections, heartbeats, activity reports, and disconnects
- Determine one user status from all current device sessions
- Read the current status of multiple users
- Send status changes and freshness information to authorized viewers

Usually out of scope:

- Chat messages and message delivery
- Typing indicators
- Presence history and last-seen time
- Mobile push notifications
- The contact permission service

## What interviewers probe

### Status across devices

A quiet device can remain connected. Delayed events from an old connection can corrupt a new connection's state.

### Status at subscription time

A viewer needs a snapshot and later changes without a gap. Retries and owner changes can deliver older states after newer states.

### Large viewer audiences

One status change can reach 100,000 viewers. Routine heartbeat and freshness traffic can exceed actual status traffic.

### Freshness during failures

An unchanged status produces no change event. A live viewer connection does not prove that the state owner or update path still works.

## Common mistakes

- A single user flag lets one device disconnect hide another active device.
- Heartbeats keep a user online forever, even without user input.
- Delayed disconnects or expiry tasks delete a newer device session.
- The traffic estimate omits subscription edges, recipient deliveries, or freshness renewals.
- Snapshot requests leave a gap before status updates begin.
- A shared failure creates false offline states or leaves old states fresh forever.

## The bar by level

- **Mid-level:** Defines device and user states. Builds a heartbeat and expiry flow. Supports multiple devices and authorized subscriptions. Explains when clients mark status stale.
- **Senior:** Handles stale session events, expiry races, snapshot gaps, and duplicate updates. Calculates delivery load and bounds queues. Defines freshness deadlines across source and path failures.
- **Staff+:** Defines quorum-backed ownership and complete freshness proofs across failover. Budgets subscription memory and recovery capacity. Limits failure scope and sets explicit overload behavior.

Practice this question with an AI interviewer: https://sysround.com/practice/new?problem=online-presence


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SysRound (https://sysround.com) is an AI practice tool for system design interviews. It has a voice or typed AI interviewer and a shared whiteboard. At the end, you get feedback on each skill and a model answer. The AI feedback can be wrong.