File Backup System

An event-driven AWS serverless solution that automatically backs up files uploaded to an S3 bucket and sends real-time notifications via SNS.

Overview

When a new object is uploaded to the source S3 bucket, a Lambda function is triggered that:

1. Copies the object into a dedicated backup S3 bucket, preserving the original key.
2. Publishes a JSON notification to an SNS topic (if configured) containing the source bucket, object key, backup bucket, and a UTC timestamp.
3. Skips objects that already exist in the backup bucket (idempotent).

Architecture

Source S3 Bucket
      │
      │ s3:ObjectCreated:*
      ▼
Lambda Function (dotnet8)
      ├──► Copy object ──► Backup S3 Bucket
      └──► Publish notification ──► SNS Topic

AWS Resources

Resource	Type	Description
Bucket	AWS::S3::Bucket	Source bucket — triggers the Lambda on new uploads
BackupBucket	AWS::S3::Bucket	Destination bucket — receives backup copies
NotificationTopic	AWS::SNS::Topic	SNS topic for backup notifications
S3Function	AWS::Serverless::Function	Lambda function (.NET 8) that performs the backup

Both S3 buckets use DeletionPolicy: Retain to prevent accidental data loss on stack deletion.

Project Structure

FileBackupSystem/
└── src/
    └── Modules/
        └── FileBackupSystemModule/
            ├── Function.cs                   # Lambda handler
            ├── FileBackupSystemModule.csproj  # .NET 8 project file
            ├── serverless.template            # AWS SAM / CloudFormation template
            └── aws-lambda-tools-defaults.json # Lambda Tools CLI defaults

Environment Variables

The Lambda function reads the following environment variables at startup:

Variable	Required	Description
BACKUP_BUCKET	Yes	Name of the S3 bucket to copy objects into
SNS_TOPIC_ARN	No	ARN of the SNS topic to publish notifications to

The SAM template wires these up automatically from the provisioned resources.

Prerequisites

• .NET 8 SDK
• AWS CLI configured with appropriate credentials
• AWS SAM CLI
• Amazon.Lambda.Tools global tool (see below)

# Install (if not already installed)
dotnet tool install -g Amazon.Lambda.Tools

# Or update to the latest version
dotnet tool update -g Amazon.Lambda.Tools

Deployment

Using Amazon.Lambda.Tools (Recommended)

dotnet lambda deploy-serverless is the recommended approach for .NET Lambda functions. It runs dotnet publish internally, ensuring all required runtime artifacts (including .deps.json) are correctly packaged before deployment.

cd src/Modules/FileBackupSystemModule
dotnet lambda deploy-serverless --stack-name file-backup-system --resolve-s3 true 

Using Visual Studio

Right-click the FileBackupSystemModule project in Solution Explorer and select Publish to AWS Lambda.

Using SAM CLI

Warning

sam build does not run dotnet publish for .NET Lambda functions. Deploying with sam build alone will produce an incomplete package that is missing the required .deps.json file, causing a Runtime.ExitError at cold start. Run dotnet publish first to work around this.

cd src/Modules/FileBackupSystemModule

# Publish the .NET project first so sam build picks up the correct artifacts
dotnet publish -c Release

sam build --template-file serverless.template
sam deploy --guided --template-file serverless.template

SAM Template Parameters

Parameter	Default	Description
BackupBucketName	(auto-generated)	Name of the backup S3 bucket. Leave blank to let CloudFormation generate a name.

Deployment Configuration

Default values for Lambda Tools CLI and Visual Studio are stored in:

• aws-lambda-tools-defaults.json — region, build configuration, and template path.

Stack Outputs

After deployment, the CloudFormation stack exposes the following outputs:

Output	Description
Bucket	Name of the source S3 bucket
BackupBucket	Name of the backup S3 bucket
S3FunctionArn	ARN of the Lambda function
NotificationTopicArn	ARN of the SNS notification topic

Teardown

Warning

Both S3 buckets (Bucket and BackupBucket) have DeletionPolicy: Retain in the template. They will not be deleted when the stack is removed — you must empty and delete them manually afterwards if no longer needed.

Using Amazon.Lambda.Tools

cd src/Modules/FileBackupSystemModule
dotnet lambda delete-serverless --stack-name file-backup-system --region eu-west-1

Using SAM CLI

cd src/Modules/FileBackupSystemModule
sam delete --stack-name file-backup-system --region eu-west-1

Manually deleting the retained S3 buckets

After the stack is deleted, remove the buckets via the AWS CLI:

# Repeat for both the source bucket and the backup bucket
aws s3 rm s3://<bucket-name> --recursive
aws s3api delete-bucket --bucket <bucket-name> --region eu-west-1

Tech Stack

• Runtime: .NET 8 (AWS Lambda)
• IaC: AWS SAM (CloudFormation)
• Trigger: Amazon S3 event notifications
• Storage: Amazon S3
• Notifications: Amazon SNS
• Region: eu-west-1 (configurable)