Beginner’s Guide to Load Balancing with AWS Application Load Balancer (ALB)

A load balancer is a tool that distributes incoming network traffic across multiple servers. This ensures that no single server gets too much traffic, improving performance, reliability, and availability. In AWS, this service is called Elastic Load Balancing (ELB). It automatically handles traffic and can scale as needed.

In the world of cloud computing especially with AWS - load balancers are a key part of building reliable, scalable apps. And among the different types available, the Application Load Balancer (ALB) is one of the most popular choices.

In this blog, I’ll break down the basics of load balancers, then zoom in on AWS ALB, how it works, and why it matters even if you're just starting out.

Types of Load Balancers (Brief Overview)

Before we get into the details of ALB, let’s take a quick look at the different types of load balancers that AWS offers. There are three main types:

1. Application Load Balancer (ALB)

This is the one we’ll focus on. ALB is best when your application uses HTTP or HTTPS. It works at the application layer (Layer 7), so it can make smart decisions based on the content of the request—like which URL path or hostname is being accessed.

2. Network Load Balancer (NLB)

NLB is designed for high performance and ultra-low latency, and it works at the transport layer (Layer 4). It's a better fit for TCP/UDP traffic or situations where speed matters more than application-level routing.

3. Classic Load Balancer (CLB)

This is the old-school one. It supports both Layer 4 and Layer 7, but it’s slowly being phased out in favor of ALB and NLB. You probably won’t use it in new projects.

What is AWS Application Load Balancer (ALB)?

The AWS Application Load Balancer, or ALB, is a service that helps you manage traffic to your web applications.

Think of it like this: imagine a restaurant with multiple chefs in the kitchen. When too many orders come in at once, a manager steps in and assigns orders to the chefs evenly so no one gets overwhelmed. That’s basically what an ALB does for your servers — it takes incoming requests (like people visiting your website) and distributes them across multiple backend servers.

But ALB does more than just “balance” traffic. Since it operates at the Application Layer (Layer 7) of the OSI model, it actually understands the content of the request. That means it can:

• Route users based on the URL path (like /login or /products)

• Route based on the hostname (like app1.example.com vs app2.example.com)

• Forward traffic to different backend services depending on what the user is trying to do

ALB is designed for modern web applications, especially those that follow a microservices or container-based architecture.

If your app runs on HTTP or HTTPS, and you want flexibility in how requests are routed, ALB is usually the best fit.

Key Features of ALB (Explained Simply)

The AWS Application Load Balancer isn’t just a traffic splitter — it comes with smart features that help you build better, more flexible applications. Let’s look at a few of the most important ones:

1. Path-Based Routing

This means ALB can look at the URL path in a request and forward it to the right backend.
For example:

• Requests to /login can go to one server

• Requests to /products can go to another

This is super helpful if your application is broken into different parts or microservices.

2. Host-Based Routing

ALB can also route traffic based on the hostname.
For example:

• admin.yourapp.com can go to the admin service

• shop.yourapp.com can go to the shopping service

Perfect for apps that serve multiple domains or subdomains.

3. Target Groups

ALB sends traffic to something called target groups, which are basically a set of backend resources like EC2 instances, containers (ECS/EKS), or even Lambda functions.
Each target group can handle a different part of your app.

4. Health Checks

ALB keeps an eye on your targets by sending regular health checks. If a server goes down, ALB will stop sending traffic to it and only route requests to healthy targets.
This makes your app more resilient and available.

How ALB Works

Let’s say you run a food delivery app.

You have different kitchens for pizza, burgers, and desserts — and a smart receptionist who listens to the customer’s order, figures out what kind of food they want, and sends it to the right kitchen.

That receptionist? That’s your Application Load Balancer.

Here’s How ALB Works in a Real Web App:

1. A user makes a request — like visiting yourapp.com/login

2. The ALB receives that request

3. It checks:

   • What’s the URL path? (/login)

   • What’s the hostname? (yourapp.com)

4. Based on rules you’ve set, ALB chooses a target group

5. ALB picks a healthy target (like an EC2 instance) within that group

6. It forwards the request to that target, and the user gets a response

7. If one of your targets is unhealthy, ALB won’t use it — keeping your app available

Benefits of Using ALB

So far, we’ve seen what ALB does and how it works. But why should you actually use it? What makes ALB worth adding to your architecture?

Let’s break down some of the real advantages:

1. Smart Routing = Better Control

With path-based and host-based routing, ALB gives you fine-grained control over where traffic goes. This is especially useful when your app is broken into different microservices or needs to support multiple domains.

2. Improved Security

You can easily set up HTTPS by attaching an SSL certificate to your ALB. That means encrypted traffic between your users and the load balancer — which is a must for login forms, personal data, etc.

4. Automatic Health Checks

If one of your backend servers crashes or goes offline, ALB will automatically stop sending traffic to it. It’ll only use healthy targets — helping keep your app available and responsive.

5. Scales Automatically

You don’t need to worry about traffic spikes. ALB can scale up and down based on demand, distributing requests efficiently even when traffic suddenly increases.

6. Pay-As-You-Go

Just like most AWS services, you only pay for what you use. There’s no upfront cost — you’re charged based on the amount of traffic handled and the time the load balancer is running.

All of this makes ALB a smart, flexible choice for developers and teams who want to build apps that are scalable, secure, and reliable.

Conclusion

If you’re building anything more than a simple static site, chances are you’ll need some kind of load balancing — and AWS Application Load Balancer (ALB) is a great place to start.

It’s flexible, easy to integrate with modern apps, and smart enough to route traffic exactly where it needs to go. Whether you're running EC2 instances, containers, or even Lambda functions, ALB helps you build applications that are resilient, scalable, and secure. You don’t need to be an expert to start using it. With a bit of hands-on practice (yes, the AWS Free Tier supports it!), you can set up your own ALB and see it in action.

So if you're just getting started with AWS or cloud architecture in general — learning how ALB works is a solid step forward.