Event-Driven Architecture

Asynchronous event processing patterns with data residency compliance and regional event routing.

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Overview

Event-driven architectures enable loose coupling and scalability but introduce complexity for data sovereignty. Events may contain regulated data and must be processed within approved boundaries. This module covers sovereign-aware event patterns.

Learning Objectives

After completing this section, you will be able to:

• ✅ Design event-driven architectures with sovereignty controls
• ✅ Implement regional event routing
• ✅ Configure event stores with data residency
• ✅ Handle cross-region event choreography

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Event-Driven Architecture

Component	Purpose	Sovereignty Role
Event Grid	Event routing	Regional deployment
Service Bus	Ordered messaging	Topic-based routing
Event Hubs	Streaming ingestion	Regional capture
Cosmos DB	Event store	Multi-region writes disabled
Azure Functions	Serverless processing	Regional execution

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Regional Event Routing

Event Grid Configuration

# Create regional Event Grid topic
New-AzEventGridTopic `
    -Name "eu-events" `
    -ResourceGroupName "events-rg" `
    -Location "westeurope" `
    -InputSchema "CloudEventSchemaV1_0"

# Create subscription with regional filter
New-AzEventGridSubscription `
    -TopicName "eu-events" `
    -ResourceGroupName "events-rg" `
    -EventSubscriptionName "eu-processor" `
    -Endpoint "/subscriptions/{sub}/resourceGroups/functions-rg/providers/Microsoft.Web/sites/eu-processor/functions/ProcessEvent" `
    -EndpointType "AzureFunction" `
    -AdvancedFilter @(
        @{
            Key = "data.region"
            OperatorType = "StringIn"
            Values = @("EU", "westeurope", "northeurope")
        }
    )

Event Routing Pattern

graph TB
    subgraph Sources ["Event Sources"]
        A[Application A]
        B[Application B]
        C[IoT Devices]
    end

    subgraph Router ["Regional Router"]
        D{Event Grid<br/>Regional Filter}
    end

    subgraph EU ["EU Region"]
        E[EU Event Grid]
        F[EU Processor]
        G[EU Event Store]
    end

    subgraph US ["US Region"]
        H[US Event Grid]
        I[US Processor]
        J[US Event Store]
    end

    A --> D
    B --> D
    C --> D
    D -->|"region=EU"| E
    D -->|"region=US"| H
    E --> F --> G
    H --> I --> J

    style EU fill:#E8F4FD,stroke:#0078D4
    style US fill:#FFF4E6,stroke:#FF8C00

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Service Bus Patterns

Ordered Message Processing

// Session-based ordering for sovereignty compliance
await using var processor = serviceBusClient.CreateSessionProcessor(
    queueName: "sovereign-events",
    new ServiceBusSessionProcessorOptions
    {
        MaxConcurrentSessions = 10,
        MaxConcurrentCallsPerSession = 1, // Strict ordering
        SessionIdleTimeout = TimeSpan.FromMinutes(5)
    });

processor.ProcessMessageAsync += async args =>
{
    var message = args.Message;
    var region = message.ApplicationProperties["DataRegion"].ToString();

    // Validate regional processing
    if (!IsApprovedRegion(region, Environment.GetEnvironmentVariable("DEPLOYMENT_REGION")))
    {
        throw new InvalidOperationException($"Cannot process {region} data in this region");
    }

    await ProcessEventAsync(message);
    await args.CompleteMessageAsync(message);
};

Dead Letter Handling

# Dead letter configuration for compliance
deadLetterConfiguration:
  maxDeliveryCount: 3
  deadLetterQueue: "sovereignty-violations-dlq"

  alerting:
    enabled: true
    threshold: 1  # Alert on first violation
    notification: "security-team@contoso.com"

  retention:
    days: 90  # Regulatory retention
    archiveToStorage: true

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Event Store Design

Cosmos DB Configuration

# Create sovereign event store
New-AzCosmosDBAccount `
    -ResourceGroupName "events-rg" `
    -Name "eu-event-store" `
    -Location "West Europe" `
    -DefaultConsistencyLevel "Strong" `
    -EnableMultipleWriteLocations $false `  # Prevent cross-region writes
    -PublicNetworkAccess "Disabled"

# Create event container with TTL
New-AzCosmosDBSqlContainer `
    -AccountName "eu-event-store" `
    -ResourceGroupName "events-rg" `
    -DatabaseName "events" `
    -Name "event-store" `
    -PartitionKeyPath "/aggregateId" `
    -DefaultTtl 2592000  # 30 days

Event Schema

{
  "id": "evt-12345",
  "aggregateId": "order-67890",
  "aggregateType": "Order",
  "eventType": "OrderPlaced",
  "eventVersion": 1,
  "timestamp": "2024-01-15T10:30:00Z",
  "dataRegion": "EU",
  "dataClassification": "Confidential",
  "causationId": "cmd-11111",
  "correlationId": "session-22222",
  "metadata": {
    "userId": "user-33333",
    "clientIp": "10.0.0.1"
  },
  "payload": {
    "orderId": "order-67890",
    "customerId": "cust-44444",
    "total": 150.00
  }
}

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Cross-Region Choreography

Saga Pattern with Sovereignty

sequenceDiagram
    participant EU as EU Order Service
    participant Saga as Saga Coordinator
    participant US as US Inventory Service

    EU->>Saga: Order Created (EU data)
    Saga->>Saga: Extract non-PII metadata
    Saga->>US: Reserve Inventory (metadata only)
    US->>Saga: Inventory Reserved
    Saga->>EU: Complete Order (EU)

    Note over EU,US: PII remains in EU region<br/>Only metadata crosses regions

Compensating Transactions

// Saga with regional compensation
public class SovereignOrderSaga
{
    public async Task ExecuteAsync(OrderCommand command)
    {
        try
        {
            // Step 1: Create order in origin region
            var order = await _euOrderService.CreateOrderAsync(command);

            // Step 2: Reserve inventory (metadata only)
            var reservation = await _inventoryService.ReserveAsync(
                new ReservationRequest
                {
                    OrderReference = order.Id, // Reference, not data
                    Items = command.Items.Select(i => i.Sku).ToList()
                });

            // Step 3: Complete in origin region
            await _euOrderService.ConfirmOrderAsync(order.Id, reservation.Id);
        }
        catch (Exception ex)
        {
            // Compensate in reverse order
            await _euOrderService.CancelOrderAsync(order.Id);
            throw;
        }
    }
}

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Implementation Checklist

• Deploy regional Event Grid topics
• Configure Service Bus with sessions
• Create Cosmos DB event store
• Implement regional filters
• Set up dead letter handling
• Configure event archival
• Implement saga coordinators
• Set up cross-region monitoring
• Define retry policies

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Next Steps

• Data Mesh → — Domain data ownership
• Observability Stack → — Event monitoring

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Reference: Azure Event Grid — Microsoft Learn