I was building a virtual attendance tracking system for a university. The client requested a feature we’ve all built a hundred times: "Allow admins to upload an Excel file to bulk-import students."
Easy, right? I set up multer, grabbed the file buffer, parsed it, looped over the rows, and inserted them into Postgres.
Locally, with a dummy file of 5 rows, it was blazingly fast. We shipped it to production.
The next day, an admin uploaded a real-world file containing 1,500 students.
The server choked. The API timed out. The database connection pool was entirely exhausted, and every other user trying to use the app got a spinning wheel of death. 💀
