FB Likes (Live Updates)

What is being asked

Viewers of a post should see its like count tick upwards live. Design how changes are pushed to clients, and what consistency each viewer can expect.

Requirements

Functional

• Live updates of the count on a post being viewed.
• The viewer's own like appears immediately.

Non-functional

• Millions of posts on screen at once; a few are extremely hot.
• Counts may be seconds behind and need not match between two viewers at the same instant.
• Efficient use of connections and battery.

Design

1. A like is recorded and emitted as an event.
2. An aggregator batches events per post and, at most once a second or so, writes the new total and publishes "post X is now 12,345".
3. Clients subscribe through a realtime gateway to the posts currently on their screen and unsubscribe when they scroll away.
4. The gateway forwards the latest total to subscribers. It sends the value, not an increment.

Key decisions and trade-offs

• Eventual consistency is the right model. Each viewer converges to the true number; nobody needs to see every intermediate value.
• Send totals, not deltas. A lost message is then harmless, because the next one corrects it. Deltas would drift.
• Throttle per post. One update a second for a hot post is the same cost whether it received ten likes or ten thousand.
• Read-your-own-writes. The client adds its own like locally at once and reconciles when the next total arrives, so the user never sees their like vanish.
• Push or poll. For posts off screen or apps in the background, do nothing; refresh the count when the post is shown again.

Follow-up questions

• How do you avoid the count on screen going down briefly after a like?
• How would you handle a client that has 200 posts on screen?
• What happens when the gateway loses its subscription state?
• How does this differ from the design for live comments?