Design a Video Streaming Service

// Client-side chunked concurrent uploader using S3 Multipart
async function uploadPartWithRetry(file: File, uploadId: string, parts: PartUrl[]) {
  const CHUNK_SIZE = 5 * 1024 * 1024; // 5MB standard blocks
  const etags: { partNumber: number; etag: string }[] = [];
  const CONCURRENCY_LIMIT = 4;

  for (let i = 0; i < parts.length; i += CONCURRENCY_LIMIT) {
    const batch = parts.slice(i, i + CONCURRENCY_LIMIT);
    const results = await Promise.all(
      batch.map(async (part) => {
        const start = (part.partNumber - 1) * CHUNK_SIZE;
        const chunk = file.slice(start, start + CHUNK_SIZE);
        
        // Direct-to-S3 PUT write
        const res = await fetch(part.url, {
          method: "PUT",
          body: chunk,
          headers: { "Content-Type": file.type }
        });
        
        return { partNumber: part.partNumber, etag: res.headers.get("ETag")! };
      })
    );
    etags.push(...results);
  }
  
  // Confirms state completion and triggers transcoding queue
  await fetch(`/api/v1/videos/${uploadId}/complete`, {
    method: "POST",
    body: JSON.stringify({ parts: etags })
  });
}

# FFmpeg segment scale command targeting visual VMAF perceptual scores
ffmpeg -y -hwaccel cuda -hwaccel_output_format cuda -i input_raw.mov \
  -vf "scale_cuda=1920:1080" \
  -c:v h264_nvenc -preset p4 -b:v 5000k -maxrate 5500k -bufsize 11000k \
  -c:a aac -b:a 192k \
  -f hls \
  -hls_time 6 \
  -hls_playlist_type vod \
  -hls_segment_type fmp4 \
  -hls_segment_filename "s3://prod-media/video-12/1080p/seg_%03d.m4s" \
  "s3://prod-media/video-12/1080p/index.m3u8"

// Simplified client-side quality decider matching buffer capacities
class ABRController {
  private currentQuality = "720p";
  private bufferLevelSeconds = 30; // Active player buffer
  private estimatedBandwidthBps = 4500000;

  public getTargetQuality(): string {
    // Safety drop-down boundaries
    if (this.bufferLevelSeconds < 5) {
      return "360p"; // Emergency fallback
    }
    if (this.bufferLevelSeconds > 40 && this.estimatedBandwidthBps > 8000000) {
      return "1080p"; // Upshift
    }
    return this.currentQuality;
  }
}