System Design: Load Balancers

You have built a great app. Suddenly, 10,000 users sign up. Your server crashes. Why?

The Traffic Cop

A Load Balancer (LB) is like a traffic cop at a busy intersection.

Instead of all traffic hitting one server, the LB sits in front and distributes requests.

• Request 1 -> Server A
• Request 2 -> Server B
• Request 3 -> Server C

Algorithms

How does the LB decide?

1. Round Robin: A -> B -> C -> A. Simple, fair.
2. Least Connections: Send to the server with fewest active users.
3. IP Hash: User X always goes to Server A (Useful for caching/sessions).

Nginx as LB

You don't need expensive hardware. Nginx can do this.

upstream myapp {
    server 10.0.0.1;
    server 10.0.0.2;
    server 10.0.0.3;
}

server {
    location / {
        proxy_pass http://myapp;
    }
}

Conclusion

Scaling horizontally (adding more cheap servers) is almost always better than scaling vertically (buying one super-computer).