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title S3 Image Resizer with Lambda (Hot Reload)
description Learn how to build and test a serverless image resizing pipeline locally using LocalStack Pro with Lambda hot reload support.

S3 Image Resizer with Lambda (Hot Reload)

Introduction

In this tutorial, you’ll learn how to build and run a serverless image resizing pipeline locally using LocalStack Pro.
The workflow is simple:

  1. Upload an image to an S3 source bucket.
  2. An AWS Lambda function automatically resizes the image.
  3. The resized image is written back to an S3 target bucket.
  4. Using hot reload, you can modify Lambda code locally and instantly test changes without redeployment.

This pattern is ideal for developing and testing serverless media-processing workflows like thumbnails, avatars, and dynamic image scaling.

Prerequisites

Make sure you have the following installed and configured:

  • LocalStack Pro (hot reload support requires Pro)
  • Docker (to run LocalStack services)
  • Node.js or Python (depending on the Lambda runtime used)
  • AWS CLI or awslocal
  • Make (optional, for running build scripts)
  • Basic understanding of AWS Lambda and S3 event triggers

Architecture Diagram

          ┌────────────────────┐
          │    Source S3       │
          │ (e.g. input-bucket)│
          └───────┬────────────┘
                  │
           (S3 Event Trigger)
                  │
          ┌───────▼────────────┐
          │     Lambda         │
          │ (Image Resizer)    │
          └───────┬────────────┘
                  │
          ┌───────▼────────────┐
          │  Target S3 Bucket  │
          │ (e.g. output-bucket)│
          └────────────────────┘

Steps

1. Clone the Repository

git clone https://github.com/localstack-samples/sample-lambda-s3-image-resizer-hot-reload.git
cd sample-lambda-s3-image-resizer-hot-reload

2. Install Dependencies

If using Python:

python -m venv .venv
source .venv/bin/activate
pip install -r requirements-dev.txt

If using Node.js Lambdas:

npm install

3. Start LocalStack

Make sure your LocalStack Pro token is set:

localstack auth set-token <your-auth-token>
localstack start

4. Build and Deploy the Lambda

Run the provided scripts to build and deploy:

deployment/build-lambdas.sh
deployment/awslocal/deploy.sh

This sets up S3 buckets, event triggers, and deploys the Lambda function locally.

5. Upload a Sample Image

Once deployment completes, upload a sample image to the input bucket:

awslocal s3 cp ./samples/test-image.jpg s3://input-bucket/

The Lambda will trigger automatically and write the resized image to s3://output-bucket/.

Testing the Application

Step 1: Verify the Resized Image

  • List files in the output bucket:
awslocal s3 ls s3://output-bucket/
  • Download and inspect the resized image:
awslocal s3 cp s3://output-bucket/test-image.jpg ./output/
  • Confirm the dimensions (e.g., via any image viewer or identify command from ImageMagick):
identify ./output/test-image.jpg

Step 2: Test Hot Reload

  • Modify your Lambda code locally (for example, change the resize dimensions or add a watermark).

  • Hot reload automatically detects changes—no redeploy needed.

  • Upload another image (or the same one under a new name):

awslocal s3 cp ./samples/test-image2.jpg s3://input-bucket/

Verify that the new logic is applied (e.g., updated size or watermark visible).

Conclusion

You’ve built and tested a complete serverless image pipeline on LocalStack, featuring:

S3 → Lambda → S3 event-driven workflow

Hot reload for rapid local iteration

End-to-end testing of serverless image processing

This pattern can easily extend to real-world use cases—such as photo galleries, CMS platforms, or social apps—where dynamic image transformation is essential.