Why load balancing matters in AWS - Why It Works

Introduction

When many people use a website or app at the same time, the servers can get overwhelmed and slow down or stop working. Load balancing helps by spreading the work evenly across multiple servers so everything runs smoothly and users have a good experience.

When your website gets many visitors at once and you want to avoid slow loading or crashes

When you run an online store and need to keep it available during sales or busy times

When you want to make sure your app keeps working even if one server stops responding

When you want to add more servers easily to handle more users without downtime

When you want to improve the speed of your service by sending users to the closest or fastest server

Commands

This command creates a new Application Load Balancer named 'my-load-balancer' in two subnets with a security group to control access.

Terminal

aws elbv2 create-load-balancer --name my-load-balancer --subnets subnet-12345678 subnet-87654321 --security-groups sg-12345678

Expected Output

{

    "LoadBalancers": [

        {

            "LoadBalancerArn": "arn:aws:elasticloadbalancing:us-east-1:123456789012:loadbalancer/app/my-load-balancer/50dc6c495c0c9188",

            "DNSName": "my-load-balancer-1234567890.us-east-1.elb.amazonaws.com",

            "CanonicalHostedZoneId": "Z35SXDOTRQ7X7K",

            "CreatedTime": "2024-06-01T12:00:00.000Z",

            "LoadBalancerName": "my-load-balancer",

            "Scheme": "internet-facing",

            "VpcId": "vpc-1a2b3c4d",

            "State": {

                "Code": "active"

            },

            "Type": "application",

            "IpAddressType": "ipv4"

        }

    ]

}

→

--name - Sets the name of the load balancer

→

--subnets - Specifies the subnets where the load balancer will run

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--security-groups - Defines the security groups controlling access

This command creates a target group named 'my-targets' that listens on HTTP port 80 inside the specified VPC. Targets like servers will be registered here.

Terminal

aws elbv2 create-target-group --name my-targets --protocol HTTP --port 80 --vpc-id vpc-1a2b3c4d

Expected Output

{

    "TargetGroups": [

        {

            "TargetGroupArn": "arn:aws:elasticloadbalancing:us-east-1:123456789012:targetgroup/my-targets/6d0ecf831eec9f09",

            "TargetGroupName": "my-targets",

            "Protocol": "HTTP",

            "Port": 80,

            "VpcId": "vpc-1a2b3c4d",

            "HealthCheckProtocol": "HTTP",

            "HealthCheckPort": "traffic-port",

            "HealthCheckEnabled": true,

            "HealthCheckPath": "/",

            "Matcher": {

                "HttpCode": "200"

            },

            "TargetType": "instance"

        }

    ]

}

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--name - Sets the name of the target group

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--protocol - Defines the protocol used by targets

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--port - Defines the port used by targets

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--vpc-id - Specifies the VPC where targets are located

This command adds two EC2 instances as targets to the target group so the load balancer can send traffic to them.

Terminal

aws elbv2 register-targets --target-group-arn arn:aws:elasticloadbalancing:us-east-1:123456789012:targetgroup/my-targets/6d0ecf831eec9f09 --targets Id=i-0abcdef1234567890 Id=i-0abcdef1234567891

Expected Output

{}

→

--target-group-arn - Specifies the target group to register targets with

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--targets - Lists the instance IDs to add as targets

This command creates a listener on the load balancer that listens on HTTP port 80 and forwards incoming traffic to the target group.

Terminal

aws elbv2 create-listener --load-balancer-arn arn:aws:elasticloadbalancing:us-east-1:123456789012:loadbalancer/app/my-load-balancer/50dc6c495c0c9188 --protocol HTTP --port 80 --default-actions Type=forward,TargetGroupArn=arn:aws:elasticloadbalancing:us-east-1:123456789012:targetgroup/my-targets/6d0ecf831eec9f09

Expected Output

{

    "Listeners": [

        {

            "ListenerArn": "arn:aws:elasticloadbalancing:us-east-1:123456789012:listener/app/my-load-balancer/50dc6c495c0c9188/9f1a2b3c4d5e6f7g",

            "LoadBalancerArn": "arn:aws:elasticloadbalancing:us-east-1:123456789012:loadbalancer/app/my-load-balancer/50dc6c495c0c9188",

            "Port": 80,

            "Protocol": "HTTP",

            "DefaultActions": [

                {

                    "Type": "forward",

                    "TargetGroupArn": "arn:aws:elasticloadbalancing:us-east-1:123456789012:targetgroup/my-targets/6d0ecf831eec9f09"

                }

            ]

        }

    ]

}

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--load-balancer-arn - Specifies which load balancer to attach the listener to

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--protocol - Sets the protocol the listener uses

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--port - Sets the port the listener listens on

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--default-actions - Defines what the listener does with incoming traffic

This command checks the status and details of the load balancer to confirm it is active and ready to use.

Terminal

aws elbv2 describe-load-balancers --names my-load-balancer

Expected Output

{

    "LoadBalancers": [

        {

            "LoadBalancerName": "my-load-balancer",

            "DNSName": "my-load-balancer-1234567890.us-east-1.elb.amazonaws.com",

            "State": {

                "Code": "active"

            },

            "Type": "application"

        }

    ]

}

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--names - Specifies the load balancer to describe

Key Concept

If you remember nothing else from this pattern, remember: load balancing spreads user traffic evenly across servers to keep your app fast and reliable.

Common Mistakes

Not registering any targets to the target group

The load balancer has nowhere to send traffic, so users get errors or no response

Always register at least one healthy target (server) to the target group before using the load balancer

Creating a load balancer without specifying subnets

The load balancer cannot run without knowing where to place itself in the network

Specify at least two subnets in different availability zones for high availability

Not creating a listener on the load balancer

Without a listener, the load balancer does not know which port or protocol to accept traffic on

Create a listener that listens on the desired port and forwards traffic to the target group

Summary

Create a load balancer to distribute incoming traffic across servers.

Create a target group and register your servers as targets.

Create a listener on the load balancer to forward traffic to the target group.

Check the load balancer status to ensure it is active and ready.