Simulating AWS Lambda locally using LocalStack

Pre-requisties

1. Install Docker & Start the demon (if not already running).

2. Install LocalStack according to your system requirements.

   1. Start the localstack process

   localstack start -d

3. Install awslocal latest version.

Prepare your Lambda Function (Go Example)

Create a simple Go Lambda Handler

package main

import (
	"context"
	"encoding/json"
	"fmt"

	"github.com/aws/aws-lambda-go/events"
	"github.com/aws/aws-lambda-go/lambda"
)

type RequestBody struct {
	Num1 int `json:"num1"`
	Num2 int `json:"num2"`
}

func handler(ctx context.Context, request events.APIGatewayProxyRequest) (events.APIGatewayProxyResponse, error) {
	var body RequestBody
	err := json.Unmarshal([]byte(request.Body), &body)
	if err != nil {
		return events.APIGatewayProxyResponse{StatusCode: 400, Body: "Invalid request body"}, nil
	}

	product := body.Num1 * body.Num2
	responseBody := fmt.Sprintf("The product of %d and %d is %d", body.Num1, body.Num2, product)

	return events.APIGatewayProxyResponse{
		StatusCode: 200,
		Body:       responseBody,
	}, nil
}

func main() {
	lambda.Start(handler)
}

Initialize Go project and tidy dependencies.

go mod init localstack-go-lambda-example
go mod tidy

Build for Linux amd64.

GOOS=linux GOARCH=amd64 go build -o main main.go

Zip the binary (make sure to name the binary main).

zip main.zip main

Create AWS Lambda

Go 1.x runtime is now unsupported on AWS. Refer documentation when setting up in real environment.

awslocal lambda create-function \
    --function-name localstack-go-lambda-example \
    --runtime go1.x \
    --zip-file fileb://main.zip \
    --handler main \
    --role arn:aws:iam::000000000000:role/lambda-role

Output Example

{
    "FunctionName": "localstack-go-lambda-example",
    "FunctionArn": "arn:aws:lambda:us-west-1:000000000000:function:localstack-go-lambda-example",
    "Runtime": "go1.x",
    "Role": "arn:aws:iam::000000000000:role/lambda-role",
    "Handler": "main",
    "CodeSize": 5408451,
    "Description": "",
    "Timeout": 3,
    "MemorySize": 128,
    "LastModified": "2024-07-23T19:26:58.271251+0000",
    "CodeSha256": "dY9rW1dQKjY0sS6/ijsx8B1ZtT8iQlO9qhTfCSieX9I=",
    "Version": "$LATEST",
    "TracingConfig": {
        "Mode": "PassThrough"
    },
    "RevisionId": "c5969abb-b2bf-4a41-967a-6d043b2f41bd",
    "State": "Pending",
    "StateReason": "The function is being created.",
    "StateReasonCode": "Creating",
    "PackageType": "Zip",
    "Architectures": [
        "x86_64"
    ],
    "EphemeralStorage": {
        "Size": 512
    },
    "SnapStart": {
        "ApplyOn": "None",
        "OptimizationStatus": "Off"
    }
}

Invoke Lambda

Wait for a 5-10 seconds before invoking the lambda.

Lambda execution on localstack is synchronus so you will not see live output

awslocal lambda invoke --function-name localstack-go-lambda-example \
    --cli-binary-format raw-in-base64-out \
    --payload '{"body": "{\"num1\": 10, \"num2\": 10}"}' output.txt

Or use the following command, when sending payload from a file (make sure the file contents are bas64 encoded).

awslocal lambda invoke \
    --function-name localstack-go-lambda-example \
    --payload file://encoded_payload \
    response.json

You should see lambda output in output.txt

{
    "statusCode": 200,
    "headers": null,
    "multiValueHeaders": null,
    "body": "The product of 10 and 10 is 100"
}

Resources

1. https://docs.localstack.cloud/user-guide/aws/lambda/#introduction

2. https://stackoverflow.com/questions/58133166/error-fork-exec-var-task-main-no-such-file-or-directory-while-executing-aws