Design live commenting

Design live commenting

Ref: Infoq - Streaming a Million Likes/Second: Real-Time Interactions on Live Video

Ref: FB - Building Real Time Infrastructure at Facebook

Ref: FB - Live Commenting: Behind the Scenes

Ref: 1p3a 讨论

Requirements

Functional requirements

• user can leave comment, real-time

• others can see comments/posts in real-time

• only focus on comments

Non-functional requirements

• high available

• low latency

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Traffic Estimates

• 100 M posts, any posts can receive comments

• our system should handle 100k/s comments delivery

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High-level Design

Single host pattern

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Detailed Design

scale challenge 1: hard to manage multiple connections

• Linkedin has built AKKA, which has multiple Actors

• each Actor is responsible for communicating via one connection

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scale challenge 2: watch multiple videos and comment

Pub/Sub pattern

• viewers subscribe to live videos

• each host maintain a in-memory subscription table

• publish using subscription

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scale challenge 3: 10 K Concurrent viewers

Add a Real-time Dispatcher Service

Subscribe Process

Publish(Fan-out) Process

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scale challenge 4: 100 k /s

Use a key-value store for subscription (e.g. Redis)

Subscribe Process

• user publish comments via dispatcher service

• dispatcher service looks up the nodes for those videos

• dispatch comments to frontend nodes

• nodes fanout to subscribed users

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scale challenge 5: 100 messages/s, 10 K viewers, distribution of 1M messages/s

Subscription flow

• user first subscribe the video in the frontend

• in the frontend, it maintains a subscription table, noting down the <topic-connectionId> entry

• frontend route subscribe requests to dispatcher service

• dispatcher service then register <topic-node> entry in Redis

Publish flow