Azure Event Grid's push delivery - concepts

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This article describes the main Event Grid concepts related to push delivery.

Events

An event is the smallest amount of information that fully describes something that happened in a system. Every event has common information like source of the event, time the event took place, and a unique identifier. Every event also has specific information that is only relevant to the specific type of event. For example, an event about a new file being created in Azure Storage has details about the file, such as the lastTimeModified value. An Event Hubs event has the URL of the Capture file. An event about a new order in your Orders microservice might have an orderId attribute and a URL attribute to the order’s state representation.

CloudEvents

Event Grid uses CNCF’s open standard CloudEvents 1.0 specification using the HTTP protocol binding with the JSON format. The CloudEvents is an extensible event specification with documented extensions for specific requirements. When using Event Grid, CloudEvents is the preferred event format because of its well-documented use cases (modes for transferring events, event formats, etc.), extensibility, and improved interoperability. CloudEvents improves interoperability by providing a common event format for publishing and consuming events. It allows for uniform tooling and standard ways of routing & handling events.

The following table shows the current support for CloudEvents specification:

CloudEvents content mode Supported?
Structured JSON Yes
Binary No

The maximum allowed size for an event is 1 MB. Events over 64 KB are charged in 64-KB increments. For the properties that are sent in an event, see CloudEvents schema.

Publishers

A publisher is the application that sends events to Event Grid. It can be the same application where the events originated, the event source. Azure services publish events to Event Grid to announce an occurrence in their service. You can publish events from your own application. Organizations that host services outside of Azure can publish events through Event Grid too.

Event sources

An event source is where the event happens. Each event source is related to one or more event types. For example, Azure Storage is the event source for blob created events. IoT Hub is the event source for device created events. Your application is the event source for custom events that you define. Event sources are responsible for sending events to Event Grid.

Topics

A topic holds events that have been published to Event Grid. You typically use a topic resource for a collection of related events. To respond to certain types of events, subscribers (an Azure service or other applications) decide which topics to subscribe to. There are several kinds of topics: custom topics, system topics, and partner topics.

Custom topics

Custom topics are also topics that are used with your applications. They were the first kind of topic designed to build event-driven integrations for custom applications. As a self-standing resource, they expose their own endpoint to which events are published.

Custom topics support push delivery. You may also want to refer to article Custom topics.

System topics

System topics are built-in topics provided by Azure services such as Azure Storage, Azure Event Hubs, and Azure Service Bus. You can create system topics in your Azure subscription and subscribe to them. For more information, see Overview of system topics.

Event subscriptions

A subscription tells Event Grid which events on a topic you're interested in receiving. When creating a subscription, you provide an endpoint for handling the event. Endpoints can be a webhook or an Azure service resource. You can filter the events that are sent to an endpoint. You can filter by event type or event subject, for example. For more information, see Event subscriptions and CloudEvents schema. Event subscriptions for custom, system, and partner topics as well as Domains feature the same resource properties.

For examples of creating subscriptions for custom, system, and partner topics as well as Domains, see:

For information about getting your current Event Grid subscriptions, see Query Event Grid subscriptions.

Event subscription expiration

You can set an expiration time for event subscriptions associated to custom, system, partner, and domain topics as well as to Domain subscriptions. The event subscription is automatically expired after that date. Set an expiration for event subscriptions that are only needed for a limited time and you don't want to worry about cleaning up those subscriptions. For example, when creating an event subscription to test a scenario, you might want to set an expiration.

For an example of setting an expiration, see Subscribe with advanced filters.

Event handlers

From an Event Grid perspective, an event handler is the place where the event is sent. The handler takes some further action to process the event. When using push delivery, Event Grid supports several handler types. You can use a supported Azure service, or your own webhook as the handler. Depending on the type of handler, Event Grid follows different mechanisms to guarantee the delivery of the event. For HTTP webhook event handlers, the event is retried until the handler returns a status code of 200 - OK. For Azure Storage Queue, the events are retried until the Queue service successfully processes the message push into the queue.

For information about delivering events to any of the supported Event Grid handlers, see Event handlers in Azure Event Grid.

Security

Event Grid provides security for subscribing to topics and when publishing events to topics. When subscribing, you must have adequate permissions on the Event Grid topic. If using push delivery, the event handler is an Azure service, and a managed identity is used to authenticate Event Grid, the managed identity should have an appropriate RBAC role. For example, if sending events to Event Hubs, the managed identity used in the event subscription should be a member of the Event Hubs Data Sender role. When publishing, you must have a SAS token or key authentication for the topic. For more information, see Event Grid security and authentication.

Event delivery

If Event Grid can't confirm that an event has been received by the subscriber's endpoint when using push delivery, it redelivers the event. For more information, see Event Grid message delivery and retry.

Batching

When you use a custom topic, events must always be published in an array. This can be a batch of one for low-throughput scenarios.

Availability zones

Azure availability zones are physically separate locations within each Azure region that are tolerant to local failures. They're connected by a high-performance network with a round-trip latency of less than 2 milliseconds. Each availability zone is composed of one or more data centers equipped with independent power, cooling, and networking infrastructure. If one zone is affected, regional services, capacity, and high availability are supported by the remaining two zones. For more information about availability zones, see Regions and availability zones.

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