Case Study 4 — Design YouTube

flowchart TB
    subgraph Upload["Upload Path"]
        UserUpload["📤 User Upload"]
        UploadSrv["Upload Service"]
        S3["S3 / Blob Store<br/>Master files"]
    end

    subgraph Process["Processing Path"]
        Queue["Processing Queue"]
        Transcoder["Transcoder Farm"]
        Thumbnail["Thumbnail Generator"]
        QA["Quality Analysis"]
    end

    subgraph Serve["Serving Path"]
        CDN["Google Global Cache CDN"]
        Edge["Edge Servers"]
        API["API Servers"]
    end

    subgraph Storage["Storage"]
        BigTable["BigTable<br/>Video metadata"]
        Index["Search Index<br/>Elasticsearch"]
        Cache["Cache Layer<br/>Popular videos"]
    end

    UserUpload --> UploadSrv --> S3
    S3 --> Queue --> Transcoder
    Transcoder --> CDN
    API --> Cache --> BigTable
    API --> Index

    style Upload fill:#3b82f6,color:#fff
    style Process fill:#7c3aed,color:#fff
    style Serve fill:#059669,color:#fff
    style Storage fill:#f59e0b,color:#fff

sequenceDiagram
    participant User as YouTuber
    participant UpSrv as Upload Service
    participant S3 as Blob Store
    participant Queue as Processing Queue

    User->>UpSrv: Start upload (resumable)
    UpSrv->>UpSrv: Assign video ID
    UpSrv->>S3: Store in chunks
    S3-->>UpSrv: Chunk saved
    UpSrv-->>User: ✅ Chunk received

    Note over User,S3: Resumable upload — can pause/resume

    User->>UpSrv: Finalize upload
    UpSrv->>S3: Assemble chunks → single file
    UpSrv->>Queue: New video ready for processing
    UpSrv-->>User: 201 Created (processing in background)