Azure Load Balancer

Azure Load Balancer is one of the most essential components in cloud infrastructure. It distributes network traffic across multiple backend resources to ensure high availability and consistent performance. Whether you’re handling web traffic, internal services, or hybrid networks, Azure Load Balancer is the go-to solution for fast and reliable traffic routing at layer 4.

Types of Azure Load Balancers

Azure offers two main types of load balancers, each serving a different purpose:

Both types use health probes to monitor the availability of backend resources and only forward traffic to healthy instances.

Architecture and Use Cases

Azure Load Balancer sits in front of your services and spreads incoming requests across multiple instances. Some common patterns include:

It integrates well with availability zones and availability sets, ensuring your services remain online even during outages in a single zone or host.

Core Components of Azure Load Balancer

Azure Load Balancer is built on four core components. Each plays a role in how traffic is distributed and managed.

1. Frontend IP Configuration

This sets the public or private IP address that clients use to reach your service. A public IP is internet-facing, while a private IP handles internal traffic inside a virtual network.

2. Backend Pool

The backend pool is the group of resources that receive traffic. It can include:

There are two SKUs:

Standard SKU allows connection to any virtual network (VNet) in the same region using peering, while Basic is restricted to a single VNet.

3. Health Probes

Health probes check the health of each backend instance and determine if it should receive traffic. There are three types:

If a backend fails a probe, it's temporarily removed from the pool until it's healthy again.

4. Load Balancing Rules

Rules define how traffic moves from frontend to backend. They map a front-end IP address and port combination to back-end IP address and port combinations

Azure Load Balancer uses a five-tuple hash to distribute traffic to available servers. This includes:

When a client connects, the five-tuple is used to consistently route them to the same backend (unless session persistence is disabled).

Session Persistence

Session persistence determines how Azure Load Balancer handles returning clients. By default, the load balancer can distribute each request independently, but this isn’t ideal for all applications.

You can modify session persistence with three options:

This is especially useful for scenarios where stateful sessions are stored in the VM, or when the app does not support external session management.

Security Implications and Risks

Azure Load Balancer itself does not inspect or modify packets. It plays an important role in secure architectures when paired with other tools:

On the risk side, a misconfigured load balancer can route traffic to unintended or unpatched services. It's also important to secure your backend pool VMs, since Load Balancer does not encrypt traffic or block malicious connections by itself.

Performance and Operational Impact

Azure Load Balancer is built to scale. It can handle millions of connections with ultra-low latency and offers up to 1000 rules per configuration. You don’t need to worry about the underlying infrastructure as it’s fully managed and fault tolerant.

That said, it’s not immune to misconfigurations. Improper health probe settings or missing NSG rules can silently break traffic flow. It’s important to test thoroughly in dev or staging before going live.

When to Use Load Balancer vs. Other Azure Services

Azure Load Balancer is great for simple, fast traffic distribution. But it’s not the only load balancing option in Azure. Here’s when you might choose another service: