🔄 What Happens When 50 Users Hit Your Counter at the Same Time? Here’s How to Handle It in Node.js

“50 users hit an endpoint that increments a counter. How do you make sure the value is correct in the database?”

That’s the kind of question that makes developers sweat in interviews — and burn hours in production.

At first glance, it sounds simple. But as you probably know by now, anything involving simultaneous updates is a concurrency puzzle waiting to go wrong.

Let me take you into a story — part bug hunt, part architecture lesson — that every backend developer needs to understand.

🧩 The Day My Counter Lied

A while ago, I built a feature for a dashboard. It had a simple goal: track how many times a button was clicked.

// In theory...
counter += 1;

You already see the red flag, right?

In testing, everything worked fine. But once it hit production and real users started hammering the button, the counter started acting weird.

Clicks were missed. Totals didn’t add up.

Welcome to the wonderful world of race conditions.

🎯 What’s the Actual Problem?

Let’s say 50 users hit the “ /increment ” endpoint at the exact same time.

Each request:

1. Reads the current value from the database.
2. Adds 1.
3. Saves the new value back.

If these operations are not atomic — meaning they don’t happen as one indivisible unit — you’ll get inconsistent results.

Example:
If the value is 100, and 50 users hit the route at the same time…
You might expect 150, but you’ll often get something like 104.

Why? Because multiple requests read the same value before it’s updated, then overwrite each other.

🔥 The Fix: Think Atomic, Not Async

To solve this, we need atomic operations that ensure data is updated correctly even under simultaneous access.