Understand event domains for managing Event Grid topics
An event domain provides an endpoint for thousands of individual topics related to the same application. You can think of it as a meta-topic that can have thousands of individual topics. It provides one publishing endpoint for all the topics in the domain. When publishing an event, the publisher must specify the target topic in the domain to which it wants to publish. The publisher can send an array or a batch of events where events are sent to different topics in the domain. See the Publishing events to an event domain section for details.
Domains also give you authentication and authorization control over each topic so you can partition your tenants. This article describes how to use event domains to manage the flow of custom events to your various business organizations, customers, or applications. Use event domains to:
- Manage multitenant event-driven architectures at scale.
- Manage your authentication and authorization.
- Partition your topics without managing each individually.
- Avoid individually publishing to each of your topic endpoints.
Note
Event domain is not intended to support broadcast scenario where an event is sent to a domain and each topic in the domain receives a copy of the event. When publishing events, the publisher must specify the target topic in the domain to which it wants to publish. If the publisher wants to publish the same event payload to multiple topics in the domain, the publisher needs to duplicate the event payload, and change the topic name, and publish them to Event Grid using the domain endpoint, either individually or as a batch.
Example use case
Event domains are most easily explained using an example. Let's say you run Contoso Construction Machinery, where you manufacture tractors, digging equipment, and other heavy machinery. As a part of running the business, you push real-time information to customers about equipment maintenance, systems health, and contract updates. All of this information goes to various endpoints including your app, customer endpoints, and other infrastructure that your customers have set up.
Event domains allow you to model Contoso Construction Machinery as a single Event Grid entity. Each of your customers is represented as a topic within the domain. Authentication and authorization are handled using Microsoft Entra ID. Each of your customers can subscribe to their topic and get their events delivered to them. Managed access through the event domain ensures they can only access their topic.
It also gives you a single endpoint, which you can publish all of your customer events to. Event Grid will take care of making sure each topic is only aware of events scoped to its tenant.
Access management
With a domain, you get fine grain authorization and authentication control over each topic via Azure role-based access control (Azure RBAC). You can use these roles to restrict each tenant in your application to only the topics you wish to grant them access to. Azure RBAC in event domains works the same way managed access control works in the rest of Event Grid and Azure. Use Azure RBAC to create and enforce custom role definitions in event domains.
Built in roles
Event Grid has two built-in role definitions to make Azure RBAC easier for working with event domains. These roles are EventGrid EventSubscription Contributor and EventGrid EventSubscription Reader. You assign these roles to users who need to subscribe to topics in your event domain. You scope the role assignment to only the topic that users need to subscribe to. For information about these roles, see Built-in roles for Event Grid.
Subscribing to topics
Subscribing to events for a topic within an event domain is the same as creating an event subscription on a custom topic or subscribing to an event from an Azure service.
Important
Domain topic is considered an auto-managed resource in Event Grid. You can create an event subscription at the domain scope without creating the domain topic. In this case, Event Grid automatically creates the domain topic on your behalf. Of course, you can still choose to create the domain topic manually. This behavior allows you to worry about one less resource when dealing with a huge number of domain topics. When the last subscription to a domain topic is deleted, the domain topic is also deleted irrespective of whether the domain topic was manually created or auto-created.
Domain scope subscriptions
Event domains also allow for domain-scope subscriptions. An event subscription on an event domain receives all events sent to the domain regardless of the topic the events are sent to. Domain scope subscriptions can be useful for management and auditing purposes.
Publishing to an event domain
When you create an event domain, you're given a publishing endpoint similar to if you had created a topic in Event Grid. To publish events to any topic in an event domain, push the events to the domain's endpoint the same way you would for a custom topic. The only difference is that you must specify the topic you'd like the event to be delivered to. For example, publishing the following array of events would send event with "id": "1111" to topic foo while the event with "id": "2222" would be sent to topic bar.
When using the cloud event schema, specify the name of the Event Grid topic in the domain as a value for the source property. In the following example, source property is set to foo for the first event and to bar for the second event.
If you want to use a different field to specify the intended topic in the domain, specify input schema mapping when creating the domain. For example, if you're using the REST API, use the properties.inputSchemaMapping property when to map that field to properties.topic. If you're using the .NET SDK, use EventGridJsonInputSchemaMapping. Other SDKs also support the schema mapping.
[{
"source": "foo",
"id": "1111",
"type": "maintenanceRequested",
"subject": "myapp/vehicles/diggers",
"time": "2018-10-30T21:03:07+00:00",
"data": {
"make": "Contoso",
"model": "Small Digger"
},
"specversion": "1.0"
},
{
"source": "bar",
"id": "2222",
"type": "maintenanceCompleted",
"subject": "myapp/vehicles/tractors",
"time": "2018-10-30T21:04:12+00:00",
"data": {
"make": "Contoso",
"model": "Big Tractor"
},
"specversion": "1.0"
}]
Event domains handle publishing to topics for you. Instead of publishing events to each topic you manage individually, you can publish all of your events to the domain's endpoint. Event Grid makes sure each event is sent to the correct topic.
Pricing
Event domains use the same operations pricing that all other features in Event Grid use. Operations work the same in event domains as they do in custom topics. Each ingress of an event to an event domain is an operation, and each delivery attempt for an event is an operation.
Related content
To learn about setting up event domains, creating topics, creating event subscriptions, and publishing events, see Manage event domains.