What Really Happens When You Upload a Large File and Generate a Temporary Download Link
Sending a 5 GB video file might seem simple. You select the file, click Upload, wait for the progress bar to finish, and share a link with someone. But a lot happens behind the scenes while that file is uploading.
The file has to travel across a network, reach a server, and be stored somewhere before another person can download it. The service also has to let the recipient access the file without giving them access to the sender's account. If the connection drops halfway through, the system needs a way to resume without resending the entire file.
File transfer services rely on HTTP requests, storage systems, databases, encryption, and background processes to manage these tasks. But how does a website actually handle a file that is several gigabytes in size? Does it upload everything in a single request? Where does the file go after the upload finishes? And what happens when its download link expires?
In this article, we'll examine how file transfer services handle large uploads, store files, generate download links, and manage interrupted connections and expired transfers. Then, once you understand the theory, we'll build a basic chunked file uploader with Node.js to see some of these processes in action. (By 2026, resumable, chunked uploads and short-lived pre-signed URLs have become standard expectations for any serious file-sharing product.)
What We'll Cover
- Why Email Attachments Fail: SMTP Wasn't Designed for Large Files
- SMTP and Attachment Size Limits
- The Base64 Problem
- The Core Architecture: What Happens Behind a File Transfer Service?
- The Client
- The Upload Server
- [...]
via FreeCodeCamp