Load Balancer

Definition: A load balancer is a component that receives incoming traffic and distributes it across multiple servers/instances instead of sending everything to one server.
It helps applications handle more traffic, improve performance, and remain available when one server becomes unhealthy. (Microsoft Learn)

Simple example

Without a load balancer:

Users
  │
  ▼
Server 1

If 10,000 users arrive, Server 1 has to handle everyone.

With a load balancer:

                 ┌──→ Server 1
Users → Load ────┼──→ Server 2
        Balancer └──→ Server 3

The load balancer decides where each incoming request should go. It can also check server health and stop sending traffic to an unhealthy server. (AWS Documentation)

3 examples

1. E-commerce website

1000 requests
      ↓
Load Balancer
 ┌────┼────┐
 ↓    ↓    ↓
S1   S2   S3

Instead of one server becoming overloaded during a sale, requests are distributed across several servers.

2. Server failure

Load Balancer
 ├── Server 1 ✓
 ├── Server 2 ❌
 └── Server 3 ✓

If Server 2 becomes unhealthy, the load balancer can stop sending requests to it and continue using the healthy servers. (AWS Documentation)

3. Scaling

You have:

Load Balancer
   ├── Server 1
   └── Server 2

Traffic increases, so you add Server 3:

Load Balancer
   ├── Server 1
   ├── Server 2
   └── Server 3

You can add/remove backend resources as demand changes without changing how clients reach the application. (AWS Documentation)

How does it decide where to send requests?

Common strategies include:

• Round Robin: Server 1 → Server 2 → Server 3 → Server 1…

• Least Connections: Send the request to the server currently handling the fewest connections.

• Health-based: Don’t send traffic to unhealthy servers.

• Weighted: Give more traffic to more powerful servers. (Amazon Web Services, Inc.)

Easy way to remember:

Load balancer = traffic manager for your servers.
Instead of 1000 users → 1 server, it makes 1000 users → multiple servers.

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