Routing with a custom Event Grid Topic

Posted: August 24, 2017  |  Categories: Functions Logic Apps Microsoft Azure

Event Grid Topic is a part of Event Grid, a new Platform Service, which provides intelligent event routing through filters and event types. Moreover, it offers a uniform publish-subscribe model similar to the model of the BizTalk runtime. However, we are talking events here and not messaging. Event Grid is a managed service in Azure with service fabric underneath. Some of the characteristics of Event Grid are discussed in one of Tom Kerhove’s latest’s posts : Exploring Azure Event Grid.

Event Grid offers custom event routing capabilities with an Event Grid Topic. Consequently, a Topic can be provisioned through the Azure Portal. And once the Topic becomes available you can hook it up with one of more subscribers.

Routing with Event Grid Topic

Custom events can be pushed to an Event Grid Topic, which can have multiple subscribers. Subsequently, the subscription is set on either Event Type and/or filters (Prefix, Suffix). Hence, a broadcast of a single event to multiple handlers can be accomplished. Therefore, each handler can operate on the event.

The consumption of the events will be through the custom Event Grid Topic as shown above. Futhermore, consumers can be a Function, Logic App, WebHook or Azure Automation currently. And the mechanism of subscriptions in Logic App and Functions is through WebHooks, which I will eloborate more about in event subscribers.

Custom Events to Event Grid Topic

Custom events need to adhere to a schema, which includes five mandatory string properties and a required data object. Subsequently, a custom event needs these properties. Therefore, a .NET client for instance can leverage the System.Net.Http namespace using a HttpClient. Hence to be able to sent a custom event with a .NET client to the Event Grid Topic, you’ll need to know the endpoint (URL) and SAS-Key.

Let me explain here. First of all, a Event Grid Topic requires either a SAS-key or key authentication, however the last one is easier to implement in a .NET client. Hence, you add a default request header with the key “aeg-sas-key” with the value in key1 found in the Azure Event Grid Topic Overview.

To actually sent the event, you can use the PostAsync method. This method of the HttpClient requires the content (event data) and Endpoint URL, which also can be found in the Azure Event Grid Topic Overview.

Event content

The content has to event schema. Therefore, the payload could look like:


In bold you can see the event details (data) and Id, Subject, EventType and EventTime, which are the mandatory string properties. You might ask yourself now, there are only four string properties and one data, where’s the topic property? Probably once the event above is published to the Event Grid Topic, the topic property is added to the event.

“Id”: ” a72f1473-d763-43c0-ad49-13dacf9158d3″,
“Subject”: “WindDetails”,
“EventType”: “WindSpeedEvent”,
“EventTime”: “2017-08-24T14:01:57.4354747Z”,
  “topic”: “/subscriptions/0bf166ac-9aa8-4597-bb2a-a845afe01415/resourceGroups/RG_EventGridSample/providers/Microsoft.EventGrid/topics/SampleTopic”

The Event Grid uses HTTP response codes to acknowledge receipt of events. Hence the event above sent to a custom event topic will provide the following response :

{StatusCode: 200, ReasonPhrase: ‘OK’, Version: 1.1, Content: System.Net.Http.StreamContent, Headers:
x-ms-request-id: e6c1fbf3-f295-49b3-ad13-b26c22b60313
Date: Thu, 24 Aug 2017 14:41:19 GMT
Server: Microsoft-HTTPAPI/2.0
Content-Length: 0

The Http Status code is 200, which is OK (event delivered). In addition I suggest you read Event Grid message delivery and retry.

Event Subscribers

A custom Event Grid Topic can have one of more subscriptions (event handlers). A subscription is an instruction for the Topic to tell it “I want this event”. In addition, the instruction can contain filters (pre- post) and/or an EventType. The Event Grid itself supports multiple subscriber types like WebHooks. And depending on subscriber type, the Event Grid has a mechanism to guarantee delivering the event to the subscriber. Do for WebHooks it’s a 200-OK, similar to when a .NET client is delivering an event to the Event Grid Topic.

Azure Function or Logic App can use the WebHook mechanism to subscribe to events on a custom Event Grid Topic. As a result, the subscription is created in the Azure Event Grid Topic containing an URL for which the Event Grid Topic can deliver the events to (POST). In addition, the Event Type can be specified, i.e. by default this is all. And finally filters can be applied, which are optional. To conclude a custom event with a certain type can be published to an Event Grid Topic, which has one of more subscribers interested in the events of a certain type.

Note the subscription of a Logic to events in the Event Grid Topic is done through a Logic App Trigger. However, to subscriber to a specific Event Type you will need to edit the subscription in the Event Grid Topic to set it from all to the required one.

A function can subscribe to events using a WebHook trigger. You create a subscription in the Event Grid Topic by providing the URL of the WebHook trigger function and Event Type (and optional filters if necessary).

Sample Scenario

To have a better understanding of routing custom events with a Event Grid topic let us look how custom events are sent and how they are consumed by multiple subscribers. Therfore, I will discuss the following scenario with you using serveral Azure services like :

  • An Event Grid Topic
  • An Azure Functions
  • A Logic Apps
  • A .NET Client
  • A WebHook (RequestBin)
  • An Azure Service Bus Queue

The .Net client sends a custom event to a custom Event Grid Topic provisioned in Azure. Subsequently, a Azure Function, Logic App and WebHook (RequestBin) will subscribe to an event of the Type WindSpeedEvent. First of all the Function will process the event by enriching it with a calculated value of Beaufort, and sent the enriched event to a Service Bus queue. Furthermore, the Logic App will evaluate the windspeed and send an email if windspeed is higher than a specified value. And finally RequestBin will just consume the event.

The following diagram shows the event flow of the scenario.

Sent a custom event

A custom event can be sent to a Event Grid Topic using a .NET client. In our scenario the custom event is of the type WindSpeedEvent, containing a few fields, including WindSpeed in meters per second and no known Beaufort (0) yet.

The Event Topic has three subscribers:

  • Logic App
  • Function
  • WebHook (Request Bin)

Each will receive the event, as each has subscribed to the Topic with events of Type WindSpeedEvent. Hence, in the Azure Function Monitor Pane I observed the consumption of the event.

Subsequently, in the Logic Run History I observed the consumption of the event.

Finally, when refreshing the RequestBin page I see the event in its raw format. And this is smilar to the Event Grid Quickstart Create and route custom events with Azure Event Grid.

To conclude, each subscriber recieves the event of the Type WindSpeedEvent.

The Event Grid Topic in our scenario has three subscribers, see the screenshot of the Event Grid Topic Overview below.


Custom Event handling with an Event Grid Topic is easy to comprehend. Also it opens doors to many scenario’s ranging from IOT to Website Traffic monitoring. In this post I focused only on a custom event handled by several subscribers. However, Event Grid has more to offer in handling events from other sources like Azure Subscriptions, resource groups, and other. Finally, more publishers and handlers will be available in the future. To conclude Event Grid in my opinion is a great addition to other serverless capabilities in Azure. However I like to emphasise it is event capability in Azure compatible with other serverless components like Logic Apps and Functions.

Author: Steef-Jan Wiggers

Steef-Jan Wiggers is all in on Microsoft Azure, Integration, and Data Science. He has over 15 years’ experience in a wide variety of scenarios such as custom .NET solution development, overseeing large enterprise integrations, building web services, managing projects, designing web services, experimenting with data, SQL Server database administration, and consulting. Steef-Jan loves challenges in the Microsoft playing field combining it with his domain knowledge in energy, utility, banking, insurance, healthcare, agriculture, (local) government, bio-sciences, retail, travel, and logistics. He is very active in the community as a blogger, TechNet Wiki author, book author, and global public speaker. For these efforts, Microsoft has recognized him a Microsoft MVP for the past 8 years.

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