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Short summary

A practical walkthrough of designing a user online/offline presence system, progressing from a naive boolean column through heartbeat polling to a Redis TTL-based approach. The key insight is that presence is ephemeral state that doesn't belong in a primary transactional database. The author provides working Python/FastAPI code using Redis SET with EX for automatic expiry, eliminating timestamp math and self-healing on restart.

• •Naive is_online column fails because users don't explicitly log out
• •Heartbeat polling works but hammers the primary DB with constant writes
• •Redis TTL keys treat presence as ephemeral state: SET key EX 30, then EXISTS to check online status

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