Invoice Reminder System

An Azure Function App built with C# .NET 8 that automatically processes invoices and sends payment reminders.

Features

• Automatic Invoice Processing: Monitors Azure Blob Storage for uploaded invoices
• AI-Powered Data Extraction: Uses Azure AI Document Intelligence to extract invoice details
• Smart Validation: Validates extracted data with configurable confidence thresholds
• Scheduled Reminders: Sends email reminders 2 days before due date
• Retry Mechanism: Handles transient failures with exponential backoff
• Comprehensive Logging: Full audit trail in CosmosDB and Application Insights

Architecture

The solution follows Clean Architecture principles with three main layers:

1. Core Layer (InvoiceReminderSystem.Core)

• Domain Models: Invoice, ExtractedData, ProcessingStatus, etc.
• Interfaces: Repository and service abstractions
• Enums: InvoiceStatus, EmailSendStatus, AttemptResult
• Exceptions: Custom exceptions for domain logic

2. Infrastructure Layer (InvoiceReminderSystem.Infrastructure)

• Repositories: CosmosDB data access
• Services:
  • DocumentIntelligenceService (invoice parsing)
  • AzureCommunicationEmailService (email sending via Azure Communication Services)
  • InvoiceValidationService (data validation)
  • InvoiceProcessingService (business logic orchestration)

3. Function App Layer (InvoiceReminderSystem.FunctionApp)

• InvoiceUploadFunction: Blob trigger for processing new invoices
• EmailReminderFunction: Timer triggers for sending reminders

Prerequisites

• .NET 8.0 SDK
• Azure Subscription with:
  • Azure Storage Account (Blob Storage)
  • Azure Cosmos DB Account
  • Azure AI Document Intelligence Service
  • Azure Communication Services (with Email enabled)
  • Application Insights (recommended)

Setup Instructions

1. Clone the Repository

git clone <repository-url>
cd InvoiceReminderSystem

2. Configure Azure Resources

CosmosDB Setup

# Create CosmosDB account
az cosmosdb create --name <account-name> --resource-group <rg-name>

# Create database
az cosmosdb sql database create --account-name <account-name> \
  --name InvoiceReminderDb --resource-group <rg-name>

# Create container with partition key
az cosmosdb sql container create --account-name <account-name> \
  --database-name InvoiceReminderDb --name Invoices \
  --partition-key-path /invoiceMonth --resource-group <rg-name>

Storage Account Setup

# Create storage account
az storage account create --name <storage-name> --resource-group <rg-name>

# Create blob container
az storage container create --name invoices \
  --account-name <storage-name>

Document Intelligence Setup

# Create Document Intelligence resource
az cognitiveservices account create --name <di-name> \
  --resource-group <rg-name> --kind FormRecognizer \
  --sku S0 --location <location>

3. Configure Application Settings

Update local.settings.json with your Azure resource credentials:

{
  "Values": {
    "CosmosDb:EndpointUri": "https://<your-account>.documents.azure.com:443/",
    "CosmosDb:PrimaryKey": "<your-key>",
    
    "DocumentIntelligence:Endpoint": "https://<your-di>.cognitiveservices.azure.com/",
    "DocumentIntelligence:ApiKey": "<your-key>",
    
    "AzureCommunicationServices:ConnectionString": "endpoint=https://<your-acs>.communication.azure.com/;accesskey=<key>",
    "AzureCommunicationServices:SenderAddress": "DoNotReply@<your-domain>.com",
    
    "BlobStorage:ConnectionString": "<your-connection-string>"
  }
}

4. Build and Run Locally

# Restore dependencies
dotnet restore

# Build solution
dotnet build

# Run Function App
cd src/InvoiceReminderSystem.FunctionApp
func start

5. Deploy to Azure

# Create Function App
az functionapp create --name <function-app-name> \
  --resource-group <rg-name> --storage-account <storage-name> \
  --consumption-plan-location <location> --runtime dotnet-isolated \
  --functions-version 4

# Deploy
func azure functionapp publish <function-app-name>

# Configure application settings in Azure Portal or via CLI
az functionapp config appsettings set --name <function-app-name> \
  --resource-group <rg-name> --settings \
  "CosmosDb:EndpointUri=<value>" \
  "CosmosDb:PrimaryKey=<value>" \
  # ... add all other settings

Usage

Upload an Invoice

Upload a PDF invoice to the invoices container in your Blob Storage:

az storage blob upload --account-name <storage-name> \
  --container-name invoices --name invoice-001.pdf \
  --file ./path/to/invoice.pdf

The system will automatically:

1. Detect the upload via Blob Trigger
2. Extract invoice data using Document Intelligence
3. Validate the extracted data
4. Store in CosmosDB with scheduled reminder date
5. Send email reminder 2 days before due date

Monitor Processing

View logs in Application Insights or check the CosmosDB container for invoice status:

• Uploaded: Invoice received
• Parsing: Extracting data
• Parsed: Data extracted
• Validated: Validation passed
• ValidationFailed: Data quality issues
• ReadyToSend: Waiting for scheduled date
• Sending: Email in progress
• Sent: Email delivered
• SendFailed: Email delivery failed

Configuration Options

Confidence Threshold

Adjust minimum confidence for extracted data (default: 85%):

"DocumentIntelligence:MinimumConfidenceThreshold": "0.85"

Email Retry Settings

Configure retry attempts (default: 3):

"SendGrid:MaxRetryAttempts": "3"

Timer Schedule

Modify CRON expressions in EmailReminderFunction.cs:

// Daily at 9 AM UTC
[TimerTrigger("0 0 9 * * *")]

// Every 5 minutes for retries
[TimerTrigger("0 */5 * * * *")]

Design Patterns Used

• Repository Pattern: Abstracts data access logic
• Dependency Injection: Loose coupling and testability
• SOLID Principles:
  • Single Responsibility: Each class has one job
  • Open/Closed: Extensible without modification
  • Liskov Substitution: Interfaces are substitutable
  • Interface Segregation: Small, focused interfaces
  • Dependency Inversion: Depend on abstractions

Error Handling

Validation Failures

Invoices with low confidence scores or missing data are marked as ValidationFailed and logged for manual review.

Transient Email Failures

Temporary issues (rate limits, network errors) trigger automatic retries with exponential backoff:

• Attempt 1: Retry after 1 minute
• Attempt 2: Retry after 5 minutes
• Attempt 3: Retry after 15 minutes

Permanent Email Failures

Issues like invalid email addresses are marked as SendFailed without retries.

Monitoring and Alerts

Set up Azure Monitor alerts for:

• High validation failure rates
• Email delivery failures
• Function execution failures

Example query for failed invoices:

InvoiceContainer
| where processingStatus.currentStatus == "ValidationFailed"
| project id, blobMetadata.blobName, processingStatus.validationErrors

Testing

Unit Tests

# Add test project
dotnet new xunit -n InvoiceReminderSystem.Tests

# Run tests
dotnet test

Integration Tests

Use Azurite for local storage emulation and CosmosDB Emulator for database testing.

Troubleshooting

Issue: Document Intelligence fails to extract email

Solution: The prebuilt invoice model may not extract email addresses reliably. Consider:

• Training a custom model with your invoice formats
• Adding email to a standard location in your invoices
• Using a different field (e.g., vendor contact info)

Issue: CosmosDB queries are slow

Solution: Ensure proper indexing:

{
  "indexingPolicy": {
    "includedPaths": [
      { "path": "/processingStatus/currentStatus/?" },
      { "path": "/emailNotification/scheduledSendDate/?" },
      { "path": "/emailNotification/sendStatus/?" }
    ]
  }
}

Issue: Emails not sending at expected time

Solution: Check timer function logs and ensure the scheduled send date is set correctly. Verify the CRON expression matches your timezone needs.

Contributing

1. Follow SOLID principles
2. Add unit tests for new features
3. Update documentation
4. Use meaningful commit messages

License

[Your License Here]

Support

For issues and questions, please create an issue in the repository.