Facebook Live Comments

What is being asked

Design live comments. Viewers post comments that everyone else watching the same video sees within a second or two. Some videos have a dozen viewers, a few have millions.

Requirements

Functional

• Post a comment; receive new comments for the video being watched.
• Load recent comments when joining.

Non-functional

• Millions of concurrent connections.
• Low latency; strict ordering is not required.
• A very popular video must not overwhelm viewers or servers.

Design

1. A viewer holds one long-lived connection to an edge server and tells it which video they are watching.
2. Posting goes through the comment API, which stores the comment and publishes it to the video's channel.
3. Each edge server subscribes to the channels of the videos its own viewers are watching, and forwards messages to those connections.
4. For a hot video, a dispatcher limits what each viewer receives to a readable rate, a few comments per second.

Key decisions and trade-offs

• Fan-out through pub/sub, not per-viewer queues. A comment is published once per video; each edge server delivers it to its local subscribers. Cost grows with servers, not with viewers.
• Nobody can read 5,000 comments a second. For hot streams, sample: show each viewer a subset, preferring friends and high-quality comments. This is a product decision that also saves the system.
• Server-sent events or WebSockets. Comments flow mostly one way, so SSE over HTTP is enough and simpler to operate; WebSockets are needed only for two-way features.
• Best effort. A lost comment is acceptable. This removes the need for acknowledgements and per-viewer state.

Follow-up questions

• How do you reconnect five million viewers when an edge server tier restarts?
• How would you remove abusive comments already delivered?
• How do you show the commenter their own comment instantly?
• How would you scale the pub/sub layer itself for one enormous channel?