Netflix Screen Limiting

What is being asked

A subscription plan allows a fixed number of simultaneous streams. Design the service that counts active streams per account and refuses a new one beyond the limit.

Requirements

Functional

• Start, continue and stop a stream; refuse the start when the plan's limit is reached.
• A stream that ends without notice (the app is killed) must free its slot soon.

Non-functional

• Millions of concurrent streams.
• The check must be fast and must not be a single point of failure for playback.
• Devices and data centres are spread across the world.

Design

1. On play, the session service counts the account's active sessions and compares with the plan's limit. If there is room it records a session with a time to live of about a minute.
2. The player sends a heartbeat every 30 seconds, which renews the session's expiry.
3. On stop the session is deleted. If the app dies, the session simply expires.
4. The count and the insert happen as one atomic operation on the account's key, so two devices starting together cannot both take the last slot.

Key decisions and trade-offs

• Expiry instead of explicit stop. Clients disappear without saying goodbye; a short lease that must be renewed is the reliable signal.
• Heartbeat interval. Shorter frees slots sooner but multiplies load; longer leaves a crashed device holding a slot.
• Fail open. If the session store is unreachable, allow playback. A brief period of over-sharing is cheaper than customers who cannot watch.
• Regions. Keep an account's sessions in one home region to get an exact count, or count per region and tolerate a small overshoot during network partitions.

Follow-up questions

• How would you let the user choose which device to stop?
• How do you prevent a modified client from skipping heartbeats to keep playing?
• What changes if downloads for offline viewing also count?
• How do you move an account's sessions when its home region fails?