All of us surely have come across bunch of systems that supports live commenting. For example, facebook live commenting, twitch/youtube live stream commenting, reddit live stream commenting etc. Lets deep dive in the system that support the live commenting feature. Requirements: User should be able to see active real time comments on the post/video/stream across the globe. System should be highly available, fault tolerant. Due to CAP theorem, we will need to trade the consistency. Consider our system to be eventually consistent. If the comment is made, its okay for us if it takes few seconds to appear everywhere else. Goal: to build a system to sync live comments across the demographies & data centers to build a system that supports the real time pushing of comments to the web/mobile clients. Estimation: Consider 100M Daily Active Users (DAU), 400M daily posts/videos/streams on the system and daily 10B comments being made on different streams/videos/posts. To support such high sc
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