Create Your First AWS Lambda Function Step by Step

By Ravindra Bagale, trainer · LinkedIn · Last updated: 23 October 2026

1. A story: one discount calculator, five teams

Five teams in one company — Python, Node, Java, .NET, Go — all need the same rule: price in, discount out. Instead of five always-on servers, they share one Lambda pattern. Each team ships its own runtime. The trigger and the IAM role stay the same shape.

2. The execution role — why Lambda needs IAM

Why do we need this?

Your code often calls AWS: write logs, read S3, put DynamoDB items. Lambda must prove it is allowed. Access keys inside code leak. A role does not.

What is it?

An execution role is an IAM role that Lambda assumes when your function runs. Trust policy: lambda.amazonaws.com may call sts:AssumeRole. Permissions policy: what the code may do. The AWS managed policy AWSLambdaBasicExecutionRole allows creating log groups and writing log streams — without it, even a Hello function looks "broken" because logs never appear.

3. Handler shape in five languages

What is it?

Every runtime has a handler: a function AWS calls with the event and a context object. Below, each snippet returns {"total": price * (1 - discount)}.

Python 3.12

import json
def lambda_handler(event, context):
    price = float(event["price"])
    discount = float(event.get("discount", 0.1))
    return {"total": round(price * (1 - discount), 2)}

Node.js 20.x

exports.handler = async (event) => {
  const price = Number(event.price);
  const discount = Number(event.discount ?? 0.1);
  return { total: Math.round(price * (1 - discount) * 100) / 100 };
};

Java 17 (handler class)

package example;
import java.util.Map;
import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestHandler;
public class Discount implements RequestHandler<Map<String, Object>, Map<String, Object>> {
  public Map<String, Object> handleRequest(Map<String, Object> e, Context c) {
    double price = ((Number) e.get("price")).doubleValue();
    double discount = e.containsKey("discount") ? ((Number) e.get("discount")).doubleValue() : 0.1;
    return Map.of("total", Math.round(price * (1 - discount) * 100.0) / 100.0);
  }
}

.NET 8 (C#)

using Amazon.Lambda.Core;
[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]
namespace Discount;
public class Function {
  public Dictionary<string, double> FunctionHandler(Dictionary<string, double> e, ILambdaContext ctx) {
    var price = e["price"]; var discount = e.GetValueOrDefault("discount", 0.1);
    return new Dictionary<string, double> { ["total"] = Math.Round(price * (1 - discount), 2) };
  }
}

Go 1.x

package main
import ("context"; "github.com/aws/aws-lambda-go/lambda")
type In struct { Price float64 `json:"price"`; Discount float64 `json:"discount"` }
type Out struct { Total float64 `json:"total"` }
func Handle(ctx context.Context, e In) (Out, error) {
  d := e.Discount; if d == 0 { d = 0.1 }
  return Out{Total: e.Price * (1 - d)}, nil
}
func main() { lambda.Start(Handle) }

4. Lab: discount function in the console (Python)

How to do it (step by step)

1. Lambda → Create function → Author from scratch. Name: discount-lab. Runtime: Python 3.12. Create a new role with basic Lambda permissions.
2. Paste the Python handler above → Deploy.
3. Configuration → General configuration → Edit → Memory 256 MB, Timeout 10 s → Save. Why: imports and JSON need a little headroom.
4. Configuration → Environment variables → Edit → Add DEFAULT_DISCOUNT = 0.1 → Save. Optional: read it with os.environ["DEFAULT_DISCOUNT"].
5. Test → Create event name ten-percent with JSON:

{"price": 1000, "discount": 0.1}

6. Test. Expected return: {"total": 900.0}. Note billed duration in the result pane.
7. Monitor → View CloudWatch logs → open the latest stream → confirm REPORT line with Duration and Memory Size.
8. Configuration → Permissions → click the role name → confirm it has AWSLambdaBasicExecutionRole. Why: that is how logs are allowed.

Same test via AWS CLI

aws lambda invoke --function-name discount-lab   --payload '{"price":500,"discount":0.2}' --cli-binary-format raw-in-base64-out out.json   --region ap-south-1 && cat out.json
# Expected: {"total": 400.0}

Cleanup

1. Delete function discount-lab.
2. IAM → Roles → delete its execution role if unused.

5. Interview questions

Revision, comparison and interview cards

Revise this lesson in a few minutes. Save the cards on your phone or download them as PNG files. The QR code on each card opens this lesson.

More cards: Ravi Sir's Cards.