Design an Image Resizing Service

Design an image service that accepts uploads and serves resized and reformatted variants through URLs. Scale: 20K image requests/s at the edge with a cache hit rate above 98%, so about 400 renders/s reach origin. Cover validating uploads, constraining the transform space (presets or signed URLs) so it cannot be abused, cache keys and CDN behaviour, request collapsing, eager versus lazy generation, format negotiation, and isolation of the render workers.