System Design

Image Hosting

Design an image hosting and delivery service like Imgur or Instagram.

Functional requirements

• Users can upload an image (JPEG, PNG, GIF, or WebP, up to 20 MB) and get back a permanent public URL for it (e.g., img.host/a8Kd2Qx).
• Each image is served in several sizes, at least a thumbnail, a feed-sized medium variant, and the full-resolution original, each with its own stable URL.
• Viewers anywhere can load any variant through its URL, whether embedded in a web page, opened directly, or fetched by a mobile app.
• Uploaders can list their own images with basic metadata (dimensions, byte size, upload time) and delete any of them; a deleted image stops resolving within a bounded time.
• Uploading bytes identical to an image already stored (e.g., the same meme reposted by thousands of accounts) reuses the existing copy rather than storing it again.

Non-functional requirements

• Delivery is the critical path: p99 latency to first byte under 50ms for a variant served from the edge, and under 400ms on a miss that goes back to origin storage.
• The upload call returns a usable URL within 2s at p99 for a 5 MB file; a variant that is not ready yet must still resolve to something usable, never a 404.
• Availability over consistency for reads: a new upload may take seconds to appear worldwide and a deleted one may linger at edges for up to a minute, but a URL must never serve a different image than before.
• Once the upload call returns a URL, the original bytes must never be lost (99.999999999% durability across at least three zones); derived variants are disposable and regenerable from the original.
• Uploads are validated (real image bytes, size and dimension limits, location EXIF stripped) and URLs are not enumerable, so private images cannot be found by guessing.

Scaling & constraints

• 10M image uploads per day (~115/s average, ~600/s at the daily peak); the average original is 2 MB, the maximum 20 MB.
• Read:write ratio is roughly 150:1: ~1.5B image views per day (~17K/s average, 3x at peak), about 85% of them thumbnails or medium variants and 15% full resolution.
• Metadata per image is ~1 KB; originals are retained indefinitely, adding ~20 TB of new originals per day, and upload volume grows ~30% per year.
• Roughly 1 in 12 uploads is byte-identical to an image already stored, and a viral meme can be re-uploaded 50,000 times in a single day.
• Views are highly skewed: the top 1% of images receive ~80% of views, and a new image gets most of its lifetime views in its first 24 hours.
• Resizing a 2 MB original into the full variant set takes ~300-800ms of CPU on one core; a blob storage put is ~50-100ms at p50 and an in-region origin read is ~30ms at p50.

Out of scope

• User accounts, authentication, and billing.
• Content moderation, NSFW detection, and copyright matching.
• Albums, comments, likes, and other social features.
• On-the-fly transformations requested via URL parameters (arbitrary crops, filters).