Explain Presence Service Design.

Example

When Alice opens a chat app, her client sends a heartbeat marking her online; if no update arrives in X seconds, the system marks her away automatically.

Want to go deeper?

Explore Grokking System Design Fundamentals, Grokking the Coding Interview, or practice Mock Interviews with ex-FAANG engineers.

Why Is It Important

Presence enhances real-time experience, avoids wasted actions (calling offline users), and supports moderation, routing, and analytics in modern systems.

Interview Tips

• Define scope: user/device/session presence
• Discuss data model: userId → status, lastSeen, ttl
• Explain transport: WebSockets/SSE + pub/sub
• Cover Redis caching, sharding, expirations, and privacy
• Mention failure handling: retries, idempotency, cold starts

Trade-offs

• Freshness vs cost: shorter heartbeats = higher traffic
• Latency vs consistency: edge caches reduce lag but risk staleness
• Precision vs privacy: exact status vs protecting users

Pitfalls

• Relying only on client pings (miss app kills)
• No expirations → stale “ghost online” users
• Broadcasting full rosters (scales poorly)
• Ignoring backpressure, blocked-user privacy, or metrics