Load balancing concepts (for example, Application Load Balancer)

Load balancing is the process of distributing incoming network traffic across multiple servers or computing resources. This distribution helps ensure no single server bears too much demand. By spreading the load, load balancing improves responsiveness and increases the availability of applications.

Types of Load Balancers in AWS

AWS provides three types of load balancers:

• Classic Load Balancer (CLB): Offers basic load balancing across multiple EC2 instances and operates at both the request level and connection level.
• Application Load Balancer (ALB): Operates at the request level, routing traffic to targets within Amazon Virtual Private Cloud (Amazon VPC) based on the content of the application request.
• Network Load Balancer (NLB): Operates at the connection level, capable of handling millions of requests per second, and is suited for TCP traffic where extreme performance is required.

Application Load Balancer (ALB)

ALB is best suited for load balancing of HTTP and HTTPS traffic. ALB offers advanced routing features, which makes it highly efficient for web applications with complex routing needs.

Key Features of ALB

• Content-Based Routing: ALB allows you to route traffic based on the content of the request. This includes the URL path, hostname, HTTP headers, HTTP method, query parameters, and source IP addresses.
• Host-Based Routing: You can route traffic to different backend services based on the host field in the HTTP header. This enables running multiple domains from the same load balancer.
• Path-Based Routing: Route requests to different services based on the request path. It’s useful for microservices architecture where each service is responsible for a specific path.
• HTTPS & HTTP/2 Support: ALB provides security by offering HTTPS termination and supports the latest protocol HTTP/2 for improved performance.
• WebSocket Support: It maintains a persistent connection between the client and the server, providing full-duplex communication channels over a single TCP connection.
• Health Checks: ALB periodically checks the health of resources behind it to ensure traffic is sent to healthy instances.

Configuring an ALB

To create an ALB in AWS, you’ll go through the process of defining your load balancer, configuring listener settings, setting up target groups, registering targets, and finally, monitoring your load-balanced services.

Here’s a simplified example of how you might set up an ALB using the AWS Management Console:

1. Create Load Balancer: Select the ‘Application Load Balancer’ option and configure the basic settings like name, scheme (internet-facing or internal), and the VPC.
2. Configure Listeners and Routing: Set up listener rules to define how the ALB routes requests. A common configuration is listening on port 80 and 443 for HTTP and HTTPS traffic, respectively.
3. Target Groups: Create target groups, which are sets of registered targets (like EC2 instances) that ALB will forward requests to. You set criteria on the target group for health checks.
4. Register Targets: Add EC2 instances to the target groups. ALB routes the incoming traffic to these registered targets based on the rules you configured.
5. Adjust Scaling and Security: Set up Auto Scaling for your EC2 instances to handle the load effectively and ensure that necessary security features like AWS WAF or Shield are in place for protection against web exploits.
6. Monitoring: Once the ALB is in place, use Amazon CloudWatch to monitor the load balancer’s performance. You can set up alerts for high latency, unhealthy targets, or any other metric that would indicate an issue with your application’s performance.

Example CloudWatch Alarm for Unhealthy Hosts:

{
“AlarmName”: “Unhealthy Hosts”,
“MetricName”: “UnHealthyHostCount”,
“Namespace”: “AWS/ApplicationELB”,
“Statistic”: “Average”,
“Period”: 300,
“EvaluationPeriods”: 1,
“Threshold”: 1,
“ComparisonOperator”: “GreaterThanOrEqualToThreshold”,
“AlarmActions”: [“arn:aws:sns:us-west-2:111122223333:my-sns-topic”],
“AlarmDescription”: “Alarm when the number of Unhealthy hosts exceeds 1”
}

When preparing for the AWS Certified Solutions Architect – Associate exam, understanding how to implement and configure load balancing using an ALB in AWS is vital. You should grasp the different types of load balancers available, the specific features and use cases of ALBs, and how to monitor and scale your application effectively with load balancing.