Blob Storage & Presigned URLs
Keeping huge files out of your app servers.
Blob storage (S3, GCS) holds large unstructured files — videos, images, backups — cheaply and extremely durably. The rule: your database stores only the file's metadata and its URL; the bytes live in the blob store.
The animation shows the wrong way first: a huge file squeezes through a tiny app-server pipe that bulges and glows red. Routing a 50GB upload through your API servers ties up their memory and bandwidth twice (in and out), hits request timeouts, and is one of the most common mid-level interview mistakes.
Then the fix. The app server hands the client a golden ticket — a presigned URL, a temporary link that grants permission to upload one specific file directly to the blob store. Watch the file fly straight from the client to the S3 bucket in fat 5MB chunks, each with its own progress bar. Chunking is what makes uploads resumable: if the connection drops, the client re-sends only the missing chunks. Finally, a notification bell rings — an S3 event notification telling your backend the upload landed, so it can update the metadata and kick off processing.
Dropbox, YouTube, and Instagram questions all live or die on this pattern. Say "presigned URL" before the interviewer has to ask.