Why data disappears on restart

Find out why the guestbook forgets messages when the server stops – and what we'll do about it.

What’s happening under the hood

In Level 3, we stored messages in an array in memory:

const vzkazy = [
  { jmeno: "Martin", text: "First message!" },
];

That works great – as long as the server keeps running. But when you stop it (Ctrl+C) and start it again, the array is empty. The messages are gone. Why?

Because RAM (the computer’s memory) is like a whiteboard in a classroom: when you leave, someone wipes it clean. For permanent storage, you need a disk – a file or a database.

💡

Disk vs. memory: a file on disk survives a restart. An array in server.js doesn’t. That’s why big websites use databases – MySQL, PostgreSQL, SQLite. We’ll start with SQLite: a database in a single file, no extra server needed.

Show me: what happens after a restart

Start the server from the guestbook lesson (node server.js).
Send a new message through React.
Stop the server (Ctrl+C) and start it again.
Refresh the page in your browser – the message is gone.

This isn’t a bug in the code. It’s a property of memory. And now you know why databases exist.

Your turn 💪

Repeat the experiment above and write down (maybe in notes in your project) exactly what happened.
Open server.js and find the line with const vzkazy = [...]. Add a comment: // WARNING: this disappears after a server restart!
Think about your own project: what shouldn’t disappear on your school/club/journal website? (schedule? photos? messages?) – list 2–3 things.

✅

Check: You understand the difference between memory (temporary) and disk (permanent). In the next lesson you’ll learn Git – to back up code on disk and online. Then SQLite – to store messages forever.

What to take away from this lesson

Data in an array in server.js only lives until the server restarts.
Permanent storage = a file or database on disk.
Level 4 will lead you to Git (code backups) and SQLite (a permanent guestbook).