Azure Event Grid

Azure Event Grid is a fully managed event routing service built on a publish-subscribe model. It enables event-driven architectures by connecting event sources (publishers) to event handlers (subscribers) with reliable delivery, filtering, and fan-out. Event Grid is a key building block in Azure serverless architectures, providing the glue between services.

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Events vs Messages

Before diving into Event Grid, it is important to understand the difference between events and messages:

Characteristic	Event	Message
Purpose	Notification that something happened	Command or data transfer
Content	Lightweight — describes what happened	Contains the full payload
Expectation	Publisher does not expect a specific action	Sender expects the message to be processed
Coupling	Loose — publisher does not know the subscribers	Tighter — sender knows the receiver
Example	"Blob uploaded to container"	"Process this order"

Event Grid handles events. Service Bus and Queue Storage handle messages. Choose the right service based on whether you are notifying or commanding.

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How Event Grid Works

The architecture has four components:

1. Event Sources (Publishers)

Services that emit events:

Source	Example Events
Azure Blob Storage	BlobCreated, BlobDeleted
Azure Resource Manager	ResourceWriteSuccess, ResourceDeleteSuccess
Azure Container Registry	ImagePushed, ImageDeleted
Azure IoT Hub	DeviceCreated, DeviceTelemetry
Azure Event Hubs	CaptureFileCreated
Azure Key Vault	SecretNewVersionCreated, CertificateNearExpiry
Azure SignalR	ClientConnected
Custom topics	Any event you define

2. Topics

A topic is an endpoint where events are sent:

• System topics — automatically created for Azure services (Blob Storage, Resource Group, etc.)
• Custom topics — endpoints you create for your own applications
• Partner topics — events from third-party SaaS providers (Auth0, Datadog, etc.)
• Domains — manage thousands of related topics under a single resource (multi-tenant scenarios)

3. Event Subscriptions

A subscription connects a topic to a handler with optional filtering:

# Subscribe an Azure Function to blob events
az eventgrid event-subscription create \
    --name sub-process-images \
    --source-resource-id /subscriptions/{sub}/resourceGroups/{rg}/providers/Microsoft.Storage/storageAccounts/{account} \
    --endpoint /subscriptions/{sub}/resourceGroups/{rg}/providers/Microsoft.Web/sites/{funcApp}/functions/{funcName} \
    --endpoint-type azurefunction \
    --included-event-types Microsoft.Storage.BlobCreated \
    --subject-begins-with /blobServices/default/containers/images/ \
    --subject-ends-with .jpg

4. Event Handlers (Subscribers)

Services that receive and process events:

Handler	Description
Azure Functions	Execute serverless code in response to events
Logic Apps	Trigger a workflow
Event Hubs	Stream events for analytics
Service Bus	Queue events for reliable processing
Storage Queue	Simple queue-based processing
Webhooks	Any HTTP endpoint (custom applications)
Power Automate	Low-code automation flows

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Event Schema

Event Grid supports two schemas:

Event Grid Schema

{
    "id": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
    "eventType": "Microsoft.Storage.BlobCreated",
    "subject": "/blobServices/default/containers/images/blobs/photo.jpg",
    "eventTime": "2024-03-15T10:30:00.1234567Z",
    "data": {
        "api": "PutBlob",
        "contentType": "image/jpeg",
        "contentLength": 524288,
        "blobType": "BlockBlob",
        "url": "https://mystorage.blob.core.windows.net/images/photo.jpg"
    },
    "dataVersion": "1.0",
    "metadataVersion": "1",
    "topic": "/subscriptions/{sub}/resourceGroups/{rg}/providers/Microsoft.Storage/storageAccounts/mystorage"
}

CloudEvents Schema

The CNCF standard schema, increasingly adopted across the industry: