What Is an Application Load Balancer and Why You Probably Need One

Written by Demola Malomo (opens in a new tab)

At some point, every backend application runs into the same limitation. One server is no longer enough.

Traffic increases, response times start to slip, and users begin to notice. The natural solution is to add more servers. But the moment you do that, a new problem appears.

How do users reach the right server?

You cannot ask users or client applications to choose between IP addresses. You also cannot afford to update endpoints every time a server is added, removed, or fails. This is exactly the problem an Application Load Balancer, usually called an ALB, is designed to solve.

An ALB sits in front of your application servers and provides a single, stable entry point. Users send requests to the load balancer, and it forwards those requests to healthy backend servers behind the scenes. From the outside, your system still looks like one application.

Without a load balancer, scaling past a single server quickly becomes fragile and hard to manage.

The Core Problem Load Balancers Solve

To understand why ALBs matter, it helps to start with a simple scenario.

Imagine you run an API on a single EC2 instance. Users connect directly to that server’s IP address, and everything works fine at first.

As traffic grows, you add a second server. Then a third.

The question is, how do users reach the right server?

You cannot expect users to randomly pick an IP address. Client applications should not need changes every time infrastructure changes. If one server goes down, some users will inevitably hit a broken endpoint.

A load balancer solves this by:

• Providing one stable endpoint for all users
• Distributing traffic across multiple backend servers
• Automatically removing unhealthy servers from rotation

From the user’s perspective, nothing changes. There is still just one service. Internally, traffic is being managed intelligently.

With that problem in mind, let’s look at what an Application Load Balancer actually does.

What an Application Load Balancer Actually Is

An Application Load Balancer (opens in a new tab) is a managed AWS service that operates at the application layer. It understands HTTP and HTTPS traffic, not just raw network packets.

Instead of blindly forwarding requests, an ALB can inspect paths, headers, and response codes. This context allows it to make routing decisions that are far more flexible than lower-level load balancers.

In practical terms, an ALB:

• Listens for incoming HTTP or HTTPS requests
• Keeps track of which backend servers are healthy
• Forwards each request to an appropriate healthy server

All of this happens transparently. Your application code does not need to know that a load balancer exists.

Where the ALB Fits in Your Architecture

In a typical AWS setup, an ALB sits in front of your EC2 instances (opens in a new tab) inside a VPC (opens in a new tab).

A common layout looks like this:

• The ALB is placed in public subnets
• EC2 instances live in private subnets
• The ALB is the only component exposed to the internet

Users communicate with the ALB. The ALB communicates with your servers. Your servers never communicate directly with the internet.

This structure improves security and keeps responsibilities clear. The load balancer handles exposure and routing, while your application servers focus solely on running the application.

Health Checks: How the ALB Knows Where to Send Traffic

Routing traffic only works if the load balancer knows which servers are healthy.

This is where health checks come in.

You define a simple endpoint on your application, commonly something like /health or /status. The ALB periodically sends requests to this endpoint on every backend server.

If a server fails its health checks, the ALB automatically stops sending traffic to it. If Auto Scaling is configured, the unhealthy instance can be replaced without user impact.

This ensures users are never routed to instances that are technically running but functionally broken. For most production systems, health checks alone make a load balancer essential.

ALB and Auto Scaling Work Together

Application Load Balancers and Auto Scaling are designed to work as a pair.

When Auto Scaling launches a new EC2 instance, it starts up, registers with the load balancer’s target group, and begins receiving traffic once health checks pass.

When Auto Scaling needs to remove an instance, the ALB first stops sending new requests to it. Existing requests are allowed to complete before the instance is terminated.

To users, this entire process is invisible. Traffic continues to flow without interruption. Without a load balancer, Auto Scaling would add servers that no one can reliably reach.

Path-Based Routing: One Load Balancer, Multiple Services

Beyond simple traffic distribution, ALBs support path-based routing.

This allows you to route requests to different backend services based on the URL path, all behind a single load balancer and domain.

For example:

• Requests to /api/* can be routed to backend API servers
• Requests to /admin/* can be routed to an internal admin service
• Requests to /static/* can be routed to a separate service

This pattern is common in microservice architectures and keeps DNS, networking, and certificates simple.

HTTPS and Certificate Management

ALBs also handle HTTPS termination.

Instead of managing TLS certificates on every EC2 instance, you attach a certificate to the load balancer using AWS Certificate Manager (opens in a new tab). The ALB handles encryption and forwards traffic to your servers over HTTP inside the VPC.

This reduces operational overhead and avoids many common certificate-related issues.

While ALBs are powerful and offer many features, it's important to note that it's not a one-size-fits-all solution. There are issues that ALBs do not handle.

What an ALB Does Not Do

It's important to be clear about the limits of an ALB.

An ALB does not:

• Fix slow or inefficient application code
• Replace caching strategies
• Eliminate the need for good database design

The load balancer handles traffic routing and availability. Performance issues inside your application still need to be solved at the application level.

Final Thoughts

An Application Load Balancer often feels optional early on. As soon as reliability and scaling matter, it becomes essential.

It provides:

• A single, stable entry point for users
• Safe and intelligent traffic distribution
• Automatic removal of unhealthy servers
• Seamless integration with Auto Scaling
• Simpler HTTPS management

If you are running more than one EC2 instance, or planning to scale beyond a single server, an ALB is not overengineering. It's how you build something that can grow without constant rework.