Live comments for millions of viewers: SSE streams, a Redis channel per video with co-located viewers, sampling and CDN snapshots for hot videos.
Backend and product engineers designing software behavior.
Your approach: Use diagrams, tables or prose for customer contracts, state transitions and data ownership; include APIs only to the depth the question requests.
Design Live Comments like Facebook Live. Live comments for millions of viewers: SSE streams, a Redis channel per video with co-located viewers, sampling and CDN snapshots for hot videos. Work from the scoping questions below. State assumptions for any unspecified load, guarantee or target, then trace your design end to end. Explain one difficult case and a credible alternative; the worked example is a reference, not a required implementation.
Resolve the scoping questions for Live Comments like Facebook Live. Separate stated behavior from assumptions, and identify what is outside your design.
Declare relevant volume, latency, freshness, quality or cost targets with units. Show calculations or an evaluation plan that can test them; unspecified targets are your assumptions, not hidden pass criteria.
Explain how your guarantees hold in a difficult case relevant to this subject. Address: Moderation? After the live ends?
Identify users, required behavior and exclusions. Answer: How many videos and viewers? How fast are comments?
Define the information owned by the system and the inputs, outputs and errors at its boundaries. Resolve: Must every viewer see every comment? How fresh?
Estimate the dominant workload and resource demand with units and explicit assumptions. For a learned system, also state how quality is measured and what data is available.
Draw or describe the responsibilities needed for Live Comments like Facebook Live. Trace a representative request, event or job from its input to a visible result; identify durable state owners.
Walk through a difficult case step by step, including detection and recovery. Consider: What after a network blip? Consistency? Moderation? After the live ends?
Compare a credible alternative using your chosen workload and guarantees. Explain a remaining risk, a signal to watch and when you would change the design.