Followed a guide on: https://docs.localstack.cloud/tutorials/elb-load-balancing/

Setting up Elastic Load Balancing (ELB) Application Load Balancers using LocalStack, deployed via the Serverless framework

This project demonstrates setting up Elastic Load Balancing (ELB) using LocalStack, focusing on creating a load balancer to distribute traffic among multiple instances of a Node.js Lambda function.

Prerequisites

Before you start, ensure you have the following installed:

• Docker: Required to run LocalStack, providing a local AWS cloud stack.
• AWS CLI: Used to interact with AWS services and LocalStack.
• Node.js: Necessary for running Node.js applications and Lambda functions.
• Serverless Framework: Utilized for deploying serverless applications to AWS or LocalStack.

Setup a Serverless project

Install Serverless framework:

npm install -g serverless

Verify the installation:

serverless --version

Create a new Serverless project using the serverless command:

serverless create --template aws-nodejs --path serverless-elb

This template includes a simple Node.js Lambda function that returns a message when invoked. It also generates a serverless.yml file that contains the project's configuration.

Install serverless-localstack plugin:

npm init -y
npm install -D serverless serverless-localstack serverless-deployment-bucket

To setup the plugins installed earlier, add the following properties to serverless.yml file:

service: serverless-elb

frameworkVersion: '3'

provider:
  name: aws
  runtime: nodejs12.x


functions:
  hello:
    handler: handler.hello

plugins:
  - serverless-deployment-bucket
  - serverless-localstack

custom:
  localstack:
    stages:
      - local

Configure a deploy script in your package.json file to simplify the deployment process. It lets you run the serverless deploy command directly over your local infrastructure. Update package.json file to the following:

{
  "name": "serverless-elb",
  "version": "1.0.0",
  "description": "",
  "main": "handler.js",
  "scripts": {
    "deploy": "sls deploy --stage local"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "serverless": "^3.25.0",
    "serverless-deployment-bucket": "^1.6.0",
    "serverless-localstack": "^1.0.1"
  }
}

With this configuration, you can now run the deployment script using:

npm run deploy

This will execute the serverless deploy --stage local command, deploying your Serverless project to LocalStack.

Create Lambda functions & ELB Application Load Balancers

Create two Lambda functions named hello1 and hello2 that will run on the Node.js 12.x runtime. Open the handler.js file and replace the existing code with the following:

'use strict';

module.exports.hello1 = async (event) => {
  console.log(event);
  return {
    "isBase64Encoded": false,
    "statusCode": 200,
    "statusDescription": "200 OK",
    "headers": {
        "Content-Type": "text/plain"
    },
    "body": "Hello 1"
  };
};

module.exports.hello2 = async (event) => {
  console.log(event);
    return {
    "isBase64Encoded": false,
    "statusCode": 200,
    "statusDescription": "200 OK",
    "headers": {
        "Content-Type": "text/plain"
    },
    "body": "Hello 2"
  };
};

Each function will log the incoming event and respond with a status code of 200 and a custom message.

Configure the serverless.yml file to create an Application Load Balancer (ALB) and attach the Lambda functions to it:

service: serverless-elb

provider:
  name: aws
  runtime: nodejs12.x
  deploymentBucket:
    name: testbucket

functions:
  hello1:
    handler: handler.hello1
    events:
    - alb:
        listenerArn: !Ref HTTPListener
        priority: 1
        conditions:
          path: /hello1
  hello2:
    handler: handler.hello2
    events:
    - alb:
        listenerArn: !Ref HTTPListener
        priority: 2
        conditions:
          path: /hello2

plugins:
  - serverless-deployment-bucket
  - serverless-localstack

custom:
  localstack:
    stages:
      - local

Create a VPC, a subnet, an Application Load Balancer, and an HTTP listener on the load balancer that redirects traffic to the target group. To do this, add the following resources to your serverless.yml file:

...
resources:
  Resources:
    LoadBalancer:
      Type: AWS::ElasticLoadBalancingV2::LoadBalancer
      Properties:
        Name: lb-test-1
        Subnets:
          - !Ref Subnet
    HTTPListener:
      Type: AWS::ElasticLoadBalancingV2::Listener
      Properties:
        DefaultActions:
          - Type: redirect
            RedirectConfig:
              Protocol: HTTPS
              Port: 443
              Host: "#{host}"
        LoadBalancerArn: !Ref LoadBalancer
        Protocol: HTTP
    Subnet:
      Type: AWS::EC2::Subnet
      Properties:
        VpcId: !Ref VPC
        CidrBlock: 12.2.1.0/24
        AvailabilityZone: !Select
          - 0
          - Fn::GetAZs: !Ref "AWS::Region"
    VPC:
      Type: AWS::EC2::VPC
      Properties:
        EnableDnsSupport: "true"
        EnableDnsHostnames: "true"
        CidrBlock: 12.2.1.0/24

Creating the infrastructure on LocalStack

Deploy the Serverless project and verify the resources created in LocalStack. Run the following command:

npm run deploy