Resizing image Lambda function

In this step, we will create a Lambda function that automatically resizes images after they are uploaded to S3. When a user uploads a book image via the book_create function, this ImageResize function is triggered by an S3 event, resizes the image to the specified dimensions, and stores the result in the BookImageResizeShop bucket for serving to the frontend.

How it works

1. User uploads an image → stored in BookImageShop bucket
2. S3 triggers the resize_image Lambda function (s3:ObjectCreated:*)
3. The function reads the original image, resizes it to 200×280px, and saves the resized version to BookImageResizeShop bucket
4. The frontend reads book images from the resize bucket

Edit SAM Template

1. Open the template.yaml file in the fcaj-book-shop folder.

2. First, add the width and height parameters under the Parameters section. These define the target dimensions for the resized images:

   width:
     Type: String
     Default: 200px
   
   height:
     Type: String
     Default: 280px
   

   

3. Next, add the ImageResize function resource under the Resources section:

   ImageResize:
     Type: AWS::Serverless::Function
     Properties:
       CodeUri: fcaj-book-shop/resize_image/function.zip
       PackageType: Zip
       Handler: index.handler
       Runtime: nodejs22.x
       FunctionName: resize_image
       Architectures:
         - x86_64
       Policies:
         - Statement:
             - Sid: ResizeUploadImage
               Effect: Allow
               Action:
                 - s3:GetObject
                 - s3:PutObject
                 - s3:DeleteObject
               Resource:
                 - !Sub "arn:aws:s3:::${bookImageShopBucketName}/*"
                 - !Sub "arn:aws:s3:::${bookImageResizeShopBucketName}/*"
       Events:
         ResizeImage:
           Type: S3
           Properties:
             Bucket: !Ref BookImageShop
             Events: s3:ObjectCreated:*
       Environment:
         Variables:
           WIDTH: !Ref width
           HEIGHT: !Ref height
           DES_BUCKET: !Ref BookImageResizeShop
   

   

4. Finally, add the ImageResizeInvokePermission resource to grant the BookImageShop S3 bucket permission to invoke this Lambda function:

   ImageResizeInvokePermission:
     Type: "AWS::Lambda::Permission"
     Properties:
       FunctionName: !Ref ImageResize
       Action: "lambda:InvokeFunction"
       Principal: "s3.amazonaws.com"
       SourceAccount: !Sub ${AWS::AccountId}
       SourceArn: !GetAtt BookImageShop.Arn
   

   

Prepare the Function Code

5. The directory structure should look like this:

   fcaj-book-shop
   ├── fcaj-book-shop
   │   ├── books_list
   │   │   └── books_list.py
   │   ├── book_create
   │   │   ├── book_create.py
   │   │   └── requirements.txt
   │   ├── book_delete
   │   │   └── book_delete.py
   │   └── resize_image
   │       └── function.zip
   └── template.yaml
   

   • Create the resize_image folder in the fcaj-book-shop/fcaj-book-shop/ directory.

   • Download the function.zip file below and place it in the resize_image folder:

   • Download the function.zip file here: function.zip

Build and Deploy

6. Run the following commands to validate and build the SAM template:

   sam validate
   sam build
   

   

7. Deploy the updated stack:

   sam deploy
   

   The changeset will show the following new resources:

   • ImageResizeRole (AWS::IAM::Role)
   • ImageResize (AWS::Lambda::Function)
   • ImageResizeInvokePermission (AWS::Lambda::Permission)
   • ImageResizeResizeImagePermission (AWS::Lambda::Permission)
   • BookImageShop (AWS::S3::Bucket) - Modify to add S3 event notification

   Enter y to confirm the deployment.

   

8. Wait for the deployment to complete. You should see Successfully created/updated stack.

   

Verify on AWS Console

9. Open the Lambda console. You should now see four functions: books_list, book_create, book_delete, and resize_image.

   

10. Click on the resize_image function. Review the Code source section — you should see the index.js file with the image resizing logic using the sharp library.

    

11. Review the Function overview section. Notice the S3 trigger on the left side — this confirms that the function is automatically triggered by S3 events.

    

12. Click the Configuration tab, then select Triggers from the left menu. Click Details to expand the trigger information. Verify:

    • Bucket arn: arn:aws:s3:::book-image-shop-by-tranvix
    • Event types: s3:ObjectCreated:*
    • Service principal: s3.amazonaws.com

    

13. Select Environment variables from the left menu. Verify the 3 environment variables:

    • DES_BUCKET = book-image-resize-shop-by-tranvix
    • HEIGHT = 280px
    • WIDTH = 200px

    

14. Select Permissions from the left menu. Click on the execution role name (e.g., fcaj-book-shop-ImageResizeRole-...).

    

15. At the IAM Role page, check the Permissions policies section. You should see:

    • AWSLambdaBasicExecutionRole (AWS managed) - for CloudWatch Logs
    • ImageResizeRolePolicy0 (Customer inline) - the custom policy we defined

    

16. Click on ImageResizeRolePolicy0 to expand it. Review the policy JSON that grants s3:GetObject, s3:PutObject, and s3:DeleteObject permissions on both the image shop and resize shop buckets.