Design an Image Upload System

Problem Design a service that lets clients upload, retrieve and delete images, with well-defined REST APIs, correct HTTP semantics, and efficient retrieval at scale.

Functional requirements

• Upload an image (single and, for large files, resumable/multipart).
• Retrieve an image by id; list images for a user or album with pagination.
• Delete an image; enforce ownership.
• Serve derived sizes (thumbnail, medium, original).
• Store and query metadata: owner, dimensions, content type, upload time.

Non-functional requirements

• ~5M uploads/day (~60/sec average, ~300/sec peak), average object 2 MB -> ~10 TB ingested/day.
• Read:write ~50:1 -> ~3k image GETs/sec at peak, mostly served from CDN with a >90% hit rate.
• Upload p95 < 2 s for a 2 MB file; cached read p95 < 100 ms.
• 11-nines object durability; metadata store 99.99% available.
• Max object size 25 MB; reject anything larger at the edge.

Areas to go deep

• The upload path: proxying bytes through the API vs pre-signed URLs direct to the object store.
• Where the bytes live (object store vs DB) and how metadata points at them.
• Listing/pagination at scale, thumbnail generation strategy, and idempotency of retried uploads.