Case Study 3 — Design Netflix

Problem: Design a video streaming service like Netflix supporting 250M+ subscribers, streaming movies/TV shows globally with adaptive bitrate, personalization, and high availability.

Requirements

Type	Requirement
Functional	Browse catalog, search, play video (adaptive quality), resume from last position, personalized recommendations
Non-Functional	< 2s startup time, 99.99% availability, 250M+ users, global distribution, 100+ Tbps peak bandwidth

Architecture Overview

flowchart TB
    subgraph Client["Client Devices"]
        Web["Web Browser"]
        SmartTV["Smart TV"]
        Mobile["Mobile App"]
    end

    CDN["Open Connect CDN<br/>Custom appliances in ISP"]

    subgraph AWS["AWS Cloud"]
        CF["CloudFront CDN"]
        API["API Gateway"]
        Services["Microservices<br/>User, Catalog, Playback,<br/>Recommendation"]
        DB["AWS + Cassandra<br/>S3 for master files"]
        Pipeline["Transcoding Pipeline"]
    end

    Client --> CDN
    Client --> CF
    CF --> API --> Services --> DB
    DB --> Pipeline --> CDN

    style Client fill:#f59e0b,color:#fff
    style CDN fill:#7c3aed,color:#fff
    style AWS fill:#3b82f6,color:#fff
    style Pipeline fill:#059669,color:#fff

Video Upload & Transcoding Flow

sequenceDiagram
    participant Studio as Content Studio
    participant S3 as S3 (Master Files)
    participant Pipeline as Transcoding Pipeline
    participant CDN as Open Connect CDN

    Studio->>S3: Upload master file (large, 4K)
    Note over S3: Master file stored in S3
    S3->>Pipeline: Trigger transcoding

    par Transcode to formats
        Pipeline->>Pipeline: 4K → 2160p (HDR/SDR)
        Pipeline->>Pipeline: 1080p (various bitrates)
        Pipeline->>Pipeline: 720p, 480p, 360p (lower)
        Pipeline->>Pipeline: Audio tracks (5.1, stereo)
        Pipeline->>Pipeline: Subtitles (VTT)
    end

    Pipeline->>Pipeline: Package into segments<br/>(3-5 second chunks)
    Pipeline->>CDN: Push to Open Connect appliances
    Note over CDN: Distributed to ISPs worldwide

Key Design Decisions

1. Open Connect CDN

Netflix’s custom CDN — dedicated caching appliances deployed inside ISP data centers. This handles > 95% of traffic, reducing AWS bandwidth costs dramatically.

2. Adaptive Bitrate Streaming

Quality	Resolution	Bitrate	When Used
4K UHD	3840×2160	15-25 Mbps	Fast connection, big screen
1080p	1920×1080	5-8 Mbps	Standard HD
720p	1280×720	2-4 Mbps	Medium connection
480p	854×480	1-2 Mbps	Mobile, slow connection
360p	640×360	400-800 kbps	Poor connection

Client requests video in chunks (~3-5 seconds). Each chunk can be at a different quality level — the player switches automatically based on network conditions.

3. Recommendation Engine

Component	Technology
Collaborative filtering	Matrix factorization (user-item)
Content-based filtering	Tag/feature similarity
Deep learning	Neural networks for feature extraction
A/B testing	Online experimentation platform

Scaling & Trade-offs

Aspect	Approach	Trade-off
CDN	Custom Open Connect appliances	Capital expenditure, ISP partnerships
Microservices	700+ microservices	Coordination, deployment complexity
Data storage	Cassandra for viewing history, S3 for masters	Multi-region data sync
Chaos engineering	Simian Army (Chaos Monkey)	Accepts that failures will happen

In Simple Words

CDN is everything — Netflix built its own (Open Connect) to serve 95%+ of traffic
Adaptive bitrate = video split into 3-second chunks at multiple qualities, client chooses per chunk
Transcoding pipeline converts one master file into hundreds of format/quality/audio combinations
Recommendation drives 80%+ of content discovery — it’s the core of the product
Microservices architecture with 700+ services managed via chaos engineering
Global users get content from local ISP caches — not from a central server