Design a Video Streaming Service

Design a Video Streaming Service (YouTube / Netflix)

Two very different problems in one product: ingesting/processing huge uploads, and delivering video to millions with smooth playback.

1. Requirements

• Functional: upload video; transcode to multiple qualities; stream/playback; search & recommend.
• Non-functional: massive storage, global low-latency delivery, adaptive quality, high availability. Extremely read-heavy.

2. The upload & processing pipeline

Upload ─▶ Raw store (S3) ─▶ Queue ─▶ Transcoding workers
                                        │
                              ┌─────────┼──────────┐
                            240p      720p       1080p (+ HLS/DASH chunks)
                                        ▼
                                  CDN-backed store

• Store the raw upload in object storage.
• A queue feeds a fleet of transcoding workers that produce multiple resolutions/bitrates, split into small segments (~2–10 s each) with a manifest file.

3. The delivery problem: adaptive bitrate streaming

Video is served via HLS or DASH: the client downloads a manifest listing segments at several bitrates, then picks the quality that matches current bandwidth — switching mid-playback to avoid buffering.

• Segments and manifests are cached on a CDN at the edge — the origin barely sees playback traffic.
• The player buffers a few segments ahead for smoothness.

4. Data model & metadata

videos:  id | uploader_id | title | description | status | duration | created_at
assets:  video_id | resolution | bitrate | cdn_url | manifest_url

Metadata in SQL/NoSQL; the actual bytes live in object storage + CDN, never in the DB.

5. Deep dives

• Storage tiering: hot (popular) videos on fast storage/CDN; cold archives on cheap storage.
• Thumbnails: generate several at upload; serve via CDN.
• Recommendations: a separate ML pipeline over watch history.
• Resumable uploads: chunk large uploads so a dropped connection doesn't restart from zero.

Takeaway: separate the write path (upload → queue → transcode → store) from the read path (manifest → adaptive segments → CDN). The CDN is what makes global streaming affordable.