Deploy and Update an AWS Lambda Function Using AWS SAM CLI – A Complete Guide

🔰 What is AWS Lambda?

AWS Lambda is a serverless compute service that lets you run code without provisioning or managing servers. You just write the function, and AWS handles the infrastructure, scaling, and execution.

💡 Why Lambda?

• No server management

• Auto-scaling

• High availability

• Pay-per-use pricing

• Tight integration with AWS services (API Gateway, S3, DynamoDB, SNS, etc.)

  
  

🛠️ Why Use AWS SAM CLI?

The AWS Serverless Application Model (SAM) CLI simplifies the entire lifecycle of serverless applications:

• Initialize projects

• Build and test locally

• Deploy and update in the cloud

It uses CloudFormation under the hood and allows local simulation using Docker.

📦 Prerequisites

Before you begin, ensure:

• ✅ AWS CLI is installed and configured (aws configure). ( Installation Guide )

• ✅ AWS SAM CLI is installed. ( Installation Guide )

• ✅ Docker is installed and running (⚠️ required for local testing)

⚠️ Important: Docker must be installed and running before using sam local invoke or sam local start-api.

✅ Step 1: Initialize a New SAM Project

sam init

Choose

1 - AWS Quick Start Templates

2 - Custom Template Location

Creates

✅ Step 2: Explore the Project

Edit hello_world/app.py:

import json
def lambda_handler(event, context):
   
    return {
        "statusCode": 200,
        "body": json.dumps({
            "message": "Hello World",
        }),
    }

Template snippet in template.yaml:

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: >
  hello-world-lambda

  Sample SAM Template for hello-world-lambda

# More info about Globals: https://github.com/awslabs/serverless-application-model/blob/master/docs/globals.rst
Globals:
  Function:
    Timeout: 3

Resources:
  HelloWorldFunction:
    Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
    Properties:
      CodeUri: hello_world/
      Handler: app.lambda_handler
      Runtime: python3.13
      Architectures:
        - x86_64
      Events:
        HelloWorld:
          Type: Api # More info about API Event Source: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#api
          Properties:
            Path: /hello
            Method: get

Outputs:
  # ServerlessRestApi is an implicit API created out of Events key under Serverless::Function
  # Find out more about other implicit resources you can reference within SAM
  # https://github.com/awslabs/serverless-application-model/blob/master/docs/internals/generated_resources.rst#api
  HelloWorldApi:
    Description: "API Gateway endpoint URL for Prod stage for Hello World function"
    Value: !Sub "https://${ServerlessRestApi}.execute-api.${AWS::Region}.amazonaws.com/Prod/hello/"
  HelloWorldFunction:
    Description: "Hello World Lambda Function ARN"
    Value: !GetAtt HelloWorldFunction.Arn
  HelloWorldFunctionIamRole:
    Description: "Implicit IAM Role created for Hello World function"
    Value: !GetAtt HelloWorldFunctionRole.Arn

✅ Step 3: Build the Project

       sam build

This compiles your Lambda and prepares deployment artifacts.

✅ Step 4: Test the Lambda Locally

You can verify Docker is running by:

dokcer info

🔹 Invoke the Lambda manually:

        sam local invoke HelloWorldFunction

🔹 Run the API locally:

      sam local start-api

Hit the api  http://127.0.0.1:3000/hello into postman.

✅ Step 5: Deploy to AWS

First-time deployment:

sam deploy --guided

✅ Step 6: Update the Lambda Code

Change app.py:

import json
def lambda_handler(event, context):
   
    return {
        "statusCode": 200,
        "body": json.dumps({
            "message": "Hello Juyel",
        }),
    }

Then rebuild and redeploy:

sam build

sam deploy

✅ Step 7: Clean Up Resources

sam delete

🧰 Quick Command Summary

Command	Purpose
sam init	Create a new SAM project
sam build	Compile and prepare for deploy
sam local invoke	Run Lambda locally (Docker required)
sam local start-api	Emulate API Gateway locally
sam deploy --guided	First-time deployment with setup
sam deploy	Fast deploy after configuration saved
sam delete	Remove all AWS resources

🎯 Conclusion

With AWS Lambda and SAM CLI, deploying serverless applications becomes fast and reliable. By leveraging Docker for local testing, and CloudFormation for repeatable deployments, you ensure your development process is both scalable and production-ready.

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