Serverless File Upload API on AWS Using Lambda, API Gateway, and S3 (Python)

Build and deploy a secure, serverless file upload API using Python, AWS Lambda, API Gateway, and S3 — with a simple frontend and no servers to manage.

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🧭 Introduction

For my third DevOps/cloud project, I decided to go fully serverless. In this project, I built a secure file upload API using AWS services — without managing any servers. The architecture uses:

• Lambda (Python) to process uploads

• API Gateway to expose a public REST endpoint

• S3 to store uploaded files

• A minimal HTML frontend for file selection and upload

🔗 GitHub Repo: Serverless_File-Upload_API

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🎯 Objective

To create a serverless solution where:

• Users upload files via browser

• Files are sent to a Lambda function via REST API

• Lambda stores the file in S3 bucket

Bonus goals:

• Use AWS SAM for infrastructure as code

• Keep frontend and backend loosely coupled

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🧱 Architecture Diagram

[HTML Upload Form]
      |
      v
[API Gateway Endpoint] ---> [Lambda Function] ---> [S3 Bucket]

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🖥️ Frontend – index.html

A simple HTML + JS form sends the file to API Gateway.

<input type="file" id="fileInput" />
<button onclick="uploadFile()">Upload</button>
<script>
  async function uploadFile() {
    const file = document.getElementById('fileInput').files[0];
    const response = await fetch("YOUR_API_GATEWAY_ENDPOINT", {
      method: "POST",
      headers: { "X-Filename": file.name },
      body: file
    });
    console.log(await response.json());
  }
</script>

📸 Screenshot: Upload form in browser

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🐍 Lambda – upload_handler.py

Lambda reads the incoming file, then uploads it to an S3 bucket using boto3.

import json, boto3, base64, os

s3 = boto3.client('s3')
BUCKET_NAME = os.environ.get('UPLOAD_BUCKET')

def lambda_handler(event, context):
    body = base64.b64decode(event['body'])
    filename = event['headers'].get('X-Filename', 'uploaded_file')

    s3.put_object(Bucket=BUCKET_NAME, Key=filename, Body=body)

    return {
        'statusCode': 200,
        'body': json.dumps({'message': 'Upload successful', 'file': filename})
    }

📸 Screenshot: CloudWatch logs showing successful upload

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📦 Infrastructure – template.yaml

Using AWS SAM to define the Lambda function, S3 bucket, and API Gateway trigger.

Resources:
  UploadFunction:
    Type: AWS::Serverless::Function
    Properties:
      Handler: upload_handler.lambda_handler
      Runtime: python3.9
      CodeUri: lambda/
      Environment:
        Variables:
          UPLOAD_BUCKET: !Ref UploadBucket
      Events:
        UploadAPI:
          Type: Api
          Properties:
            Path: /upload
            Method: post

  UploadBucket:
    Type: AWS::S3::Bucket

📸 Screenshot: API Gateway and Lambda deployed via SAM

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🚀 Deploying the App

Prerequisites:

• AWS CLI and AWS SAM CLI

• IAM user with Lambda + S3 permissions

Steps:

sam build
sam deploy --guided

Copy the API Gateway endpoint output and use it in your index.html frontend.

📸 Screenshot: Successful SAM deployment CLI output

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🔐 GitHub Repo

📂 github.com/PradeepMahadevaiah/Serverless_File-Upload_API

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🧠 Learnings & Next Steps

✅ Learned AWS SAM deployment, Lambda integration with API Gateway and S3
✅ Practiced environment variables and event decoding in Python Lambda
✅ Worked with API Gateway binary mode + file headers

🔄 Improvements coming:

• Add validation for file type and size

• Store metadata in DynamoDB

• Add Cognito auth or signed URLs

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🙌 Conclusion

This serverless file uploader is powerful, scalable, and easy to extend. It showcases how DevOps and cloud skills come together — from infrastructure as code to hands-on coding with Python.

Feel free to check out the GitHub repo or leave comments with your feedback or suggestions!

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