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A system topic in Event Grid represents one or moreevents published by Azure services such as Azure Storage and Azure Event Hubs. For example, a system topic can representall blob events or onlyblob created andblob deleted events published for aspecific storage account. In this example, when a blob is uploaded to the storage account, the Azure Storage service publishes ablob created event to the system topic in Event Grid, which then forwards the event to topic'ssubscribers that receive and process the event.
Note
Only Azure services can publish events to system topics. Therefore, you don't get an endpoint or access keys that you can use to publish events like you do forcustom topics orevent domains.
Here's the current list of Azure services that support creation of system topics on them.
System topics are visible as Azure resources and provide the following capabilities:
Note
ContosoStorage in an Azure subscriptionContosoSubscription, Event Grid creates the system topic in theContosoSubscription. It's not possible to create a system topic in an Azure subscription that's different from the event source's Azure subscription.You can create a system topic in two ways:
Create anevent subscription on an Azure resource as an extension resource, which automatically creates a system topic with the name in the format:<Azure resource name>-<GUID>. The system topic created in this way is automatically deleted when the last event subscription for the topic is deleted.
Create a system topic for an Azure resource, and then create an event subscription for that system topic. When you use this method, you can specify a name for the system topic. The system topic isn't deleted automatically when the last event subscription is deleted. You need to manually delete it.
When you use the Azure portal, you're always using this method. When you create an event subscription using theEvents page of an Azure resource, the system topic is created first and then the subscription for the topic is created. You can explicitly create a system topic first by using theEvent Grid System Topics page and then create a subscription for that topic.
When you useCLI,REST, orAzure Resource Manager template, you can choose either of the above methods.
Important
We recommend that you create a system topic first and then create a subscription on the topic, as it's the latest way of creating system topics.
The system topic creation fails if you set up Azure policies in such a way that the Event Grid service can't create it. For example, you can have a policy that allows creation of only certain types of resources (for example: Azure Storage, Azure Event Hubs, and so on) in the subscription.
In such cases, event flow functionality is preserved. However, metrics and diagnostic functionalities of system topics are unavailable.
If you require this functionality, allow creation of resources of the system topic type, and create the missing system topic as described in theLifecycle of system topics section.
For Azure event sources that are in a specific region/location, system topic is created in the same location as the Azure event source. For example, if you create an event subscription for an Azure blob storage in East US, the system topic is created in East US. For global Azure event sources such as Azure subscriptions, resource groups, or Azure Maps, Event Grid creates the system topic inglobal location.
In general, system topic is created in the same resource group that the Azure event source is in. For event subscriptions created at Azure subscription scope, system topic is created in theDefault-EventGrid resource group in theWest US 2 region. If the resource group doesn't exist, Azure Event Grid creates it before creating the system topic.
See the following articles:
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