Virtual Networks Load Balancing

This article provides a comprehensive overview of load balancing within Azure Virtual Networks. Load balancing is a crucial technique for distributing network traffic across multiple resources, enhancing availability, scalability, and performance of applications.

Understanding Load Balancing Concepts

Load balancing aims to prevent any single resource from becoming a bottleneck. It achieves this by intelligently routing incoming traffic to available and healthy backend instances. Key benefits include:

High Availability: Ensures that if one instance fails, traffic is automatically redirected to healthy instances.
Scalability: Allows you to easily add or remove backend instances to handle varying traffic loads.
Performance: Distributes load evenly, reducing latency and improving response times.
Improved Resource Utilization: Prevents individual servers from being overloaded while others remain idle.

Types of Load Balancing in Azure

Azure offers several load balancing solutions tailored to different needs:

1. Azure Load Balancer

Azure Load Balancer is a Layer 4 (TCP/UDP) load balancer that distributes traffic based on IP address and port. It operates at the network level and is highly available and scalable.

Key Features:

Supports both internal and external load balancing scenarios.
Provides high availability for applications.
Offers health probes to detect unhealthy instances.
Supports inbound and outbound NAT rules.
Can be configured with a single public IP address or multiple.

Use Cases:

Load balancing virtual machines within an Azure Virtual Network.
Distributing traffic for critical business applications requiring high availability.
Implementing network address translation (NAT) for inbound or outbound connections.

Note: Azure Load Balancer does not inspect the content of the traffic. It makes decisions based on network layer information.

2. Azure Application Gateway

Azure Application Gateway is a Layer 7 (HTTP/HTTPS) load balancer that provides advanced routing capabilities based on application-level attributes like URL paths, host headers, and more.

Key Features:

Web Application Firewall (WAF): Protects against common web vulnerabilities.
SSL Termination: Offloads SSL decryption from backend servers.
Cookie-based Session Affinity: Ensures requests from the same client go to the same backend server.
URL-based Content Routing: Routes requests based on the URL path.
Host-based Routing: Routes requests to different backend pools based on the host header.
Redirection: Can redirect HTTP traffic to HTTPS.

Use Cases:

Load balancing web applications and APIs.
Implementing secure web application delivery with WAF.
Directing traffic to different microservices based on URL paths.

Tip: For HTTP/HTTPS traffic, Application Gateway offers more sophisticated routing and security features than Azure Load Balancer.

3. Azure Traffic Manager

Azure Traffic Manager is a DNS-based traffic load balancer. It directs users to the most appropriate endpoint based on a chosen traffic-routing method. Endpoints can be in Azure or even outside of Azure.

Key Features:

DNS-based Load Balancing: Distributes traffic at the DNS level.
Various Routing Methods: Performance, Weighted, Priority, Geographic, Multivalue, Subnet.
Endpoint Monitoring: Monitors the health of endpoints and fails over if an endpoint becomes unhealthy.
Global Traffic Distribution: Ideal for distributing traffic across different geographic regions.

Use Cases:

Distributing traffic globally for disaster recovery and high availability.
Directing users to the closest or best-performing endpoint.
Performing blue/green deployments or A/B testing across regions.

Important: Traffic Manager operates at the DNS level, meaning it resolves the IP address before the actual connection is established.

Choosing the Right Load Balancing Solution

The selection of the appropriate Azure load balancing service depends on your application's requirements:

Azure Load Balancer: For general TCP/UDP load balancing at the network level, especially for VMs within a VNet.
Azure Application Gateway: For advanced HTTP/HTTPS load balancing, WAF, SSL termination, and application-level routing.
Azure Traffic Manager: For global traffic management, DNS-based routing, and disaster recovery.

Common Configuration Scenarios

Scenario 1: Load Balancing Web Servers in a VNet with Azure Load Balancer

You can set up a standard Azure Load Balancer to distribute incoming internet traffic to a pool of virtual machines running a web application. This ensures that if one web server goes down, traffic is automatically routed to the remaining healthy servers.


# Example of a simple Azure Load Balancer configuration (conceptual)
resource "azurerm_load_balancer" "example" {
  name                = "myLoadBalancer"
  location            = azurerm_resource_group.example.location
  resource_group_name = azurerm_resource_group.example.name

  frontend_ip_configuration {
    name                 = "PublicIPAddress"
    public_ip_address_id = azurerm_public_ip.example.id
  }

  backend_address_pool {
    name = "pool"
  }

  load_balancing_rule {
    name                = "http"
    protocol            = "Tcp"
    frontend_port       = 80
    backend_port        = 80
    frontend_ip_configuration_name = "PublicIPAddress"
    backend_address_pool_name      = "pool"
    probe_name          = "http"
  }

  probe {
    name                = "http"
    protocol            = "Http"
    port                = 80
    request_path        = "/"
    interval_in_seconds = 5
    numberOfProbes      = 2
  }
}

Scenario 2: Routing Traffic based on URL Path with Application Gateway

An Application Gateway can be configured to direct requests to different backend pools based on the URL. For instance, requests to /images/* could go to an image serving pool, while /api/* goes to an API processing pool.

This provides flexibility and allows for microservices architectures.

Conclusion

Azure offers robust load balancing solutions that are essential for building scalable, highly available, and performant applications. Understanding the differences between Azure Load Balancer, Application Gateway, and Traffic Manager will help you choose the most suitable service for your specific needs.

For more detailed configuration and advanced features, please refer to the official Azure documentation.

Related Articles: