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Vipul
Vipul

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Inside Load Balancers - The Hidden Traffic Controllers of the Internet

#architecture #beginners #networking #systemdesign

Whenever thousands or even millions of users open an application at the same time, one big question arises:

"How does a single website handle so much traffic without crashing?"

The answer is usually a Load Balancer.

A load balancer sits between users and backend servers and distributes incoming traffic intelligently.

Instead of this:

Users --> One Server
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Modern systems work like this:

Users --> Load Balancers --> Multiple Servers
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This prevents any single server from becoming overloaded.

What Actually Happens Behind the Scenes?

Suppose you open:

https://myapp.com
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Your request first reaches the load balancer, not the application server directly.

The load balancer then decides:

"Which backend server should handle this request?"

It checks things like:

  • Current server load
  • Active connections
  • Server health
  • Response times

and forwards your request to the best available server.

How Does It Choose Servers?

Load balancers use different algorithms.

Round Robin

Request 1 --> Server A
Request 2 --> Server B
Request 3 --> Server C
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Least Connections

Traffic goes to the server handling the fewest users currently.

Weighted Distribution

More powerful servers receive more traffic.

One of the Most Important Features:

Health Checks

Load balancers continuously monitor backend servers.
If a server crashes or becomes unhealthy:

Server B --> Down
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the load balancer automatically stops sending traffic to it.

This is one of the main reasons modern applications achieve high availability without users noticing failures.

Layer 4 vs Layer 7

Some load balancers only understand:

  • IP addresses
  • TCP/UDP ports

These are called:
Layer 4 Load Balancers

Others understand full HTTP requests including:

  • URLs
  • Headers
  • Cookies

These are:
Layer 7 Load Balancers
This allows advanced routing like:

/api --> API Servers
/image --> Image Servers
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HTTPS and SSL Termination

Handling HTTPS encryption on every backend server is expensive.

Client HTTPS --> Load Balancer decrypts traffic --> Backend servers receive HTTP
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This process is called:
SSL Termination
It improves performance and simplifies certificate management.

Why Load Balancers Matter

Without load balancers:

  • Servers crash during traffic spikes
  • Applications become unreliable
  • Scaling becomes difficult

With load balancers:

  • Traffic is distributed efficiently
  • Failed servers are isolated automatically
  • Applications scale horizontally

That's why almost every modern platform -- from streaming services to cloud applications -- depends heavily on load balancing behind the scenes.

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