Twitch (Live Streaming)

Phase 1 — Clarify & Scope

Interviewer: Design Twitch — a live video streaming platform. What do you want to clarify?

Candidate: Key questions:

1. Both live streaming (broadcast) and VOD (replay past streams)?
2. What's the scale — concurrent streamers and viewers?
3. What's the key latency requirement — broadcast delay from streamer to viewer?
4. Chat and interactive features (polls, subscriptions, donations)?
5. Adaptive bitrate streaming — do we transcode to multiple quality levels?

Interviewer: Both live and VOD. 8 million concurrent viewers, 100,000 concurrent streamers. Target latency: < 6 seconds (standard) with a low-latency mode < 3 seconds. Include chat. Yes to adaptive bitrate — transcode to 360p, 480p, 720p, 1080p. Focus on the video pipeline.

Candidate: Let me size this:

$\text{Ingest bandwidth} = 100{,}000 \text{ streamers} \times 6 \text{ Mbps avg} = 600 \text{ Gbps ingest}$

$\text{Delivery bandwidth} = 8\text{M viewers} \times 4 \text{ Mbps avg} = 32 \text{ Tbps delivery}$

$\text{Transcoding load} = 100{,}000 \text{ streams} \times 4 \text{ quality levels} = 400{,}000 \text{ transcoding jobs}$

The delivery-to-ingest ratio is 32 Tbps / 600 Gbps ≈ 53× — CDN is essential; we cannot serve all viewers from origin.

Interviewer: What are the core requirements?

Candidate:

• Accept RTMP stream from streamers, transcode to multiple quality levels (HLS/DASH adaptive)
• Distribute video to millions of concurrent viewers with < 6 second latency
• Low-latency mode: < 3 second end-to-end delay
• Live chat: messages visible to all viewers within 2 seconds
• VOD: past streams stored and searchable
• Stream health monitoring: alert streamer if their upload quality degrades

Non-functional:

• 99.99%…