Broadcasting events to multiple solutions

I sat down with a customer who had an interesting integration problem the other day.

They are building a system that creates data useful for the entire enterprise. They want to push that data to different solutions. They also expose APIs, but they want to support the push-model where their system pushes the data to the different solutions.

Security came to mind. Some of that data is sensitive and shouldn’t be pushed to anyone requesting it.

Also, they do not want to become a bottleneck for the entire organization. They do not want to have to hand out access keys and revoke them.

In this article, we’ll dive into that problem and the solution we came up with, including a working proof of concept (POC).

As usual, code is in GitHub.

First draft

At first we thought about Azure Event Grid.

Event Grid is great for that scenario as it is a push solution: it delivers the events to subscribers.

Now there was security concerns. There are different delivery mechanisms (or handlers). The most obvious one, i.e. a web hook, would require the different solutions to expose a public endpoint for Event Grid to contact. At the time of this writing, i.e. February 2020, Event Grid doesn’t integrate with VNETs.

That was deemed unacceptable as it brought too much risks. The endpoint could be attacked. We can now secure web hooks with AAD authentication, but that remains a public endpoint.

Securing publishing

Before we talk about the second draft, let’s address the security concern: how do we secure publications?

At first, it seems a little baffling. An API seems so easy to secure: we just lock the door. But something that broadcast events… how do we make sure some events aren’t picked up by some actor.

A good approach when we have a solution that works in one context but not in another is to take a step back and understand the reasons why the solution works in the former context.

For APIs, we have that mental model:

Secure APIs

Basically, each API:

That is why it is easy to secure.

A much harder way to model the APIs from a security perspective is to have one endpoint for all type of data: a buffet. The only way to secure it is to apply security trimming, i.e. identifying the caller and hiding data they do not have the right to see in the response payload.

Cyber security people typically do not like security trimming because it pushes the access control mechanisms deeper inside an API’s implementation. In order to validate security compliance, we often need to validate the code of a solution.

Hence keeping the simpler approach in mind, we can now replicate it to event topics:

Secure Events

Again here, for this to work, each topic need to:

This is basically the dual of an API.

It requires us to think about all the data our broadcasting solution is producing in terms of data buckets. Each bucket has a security context. The sensitive data are apart from the non-sensitive ones.

This requires us to make some artificial separation. For instance, if we think about HR data, we would need to set appart non-sensitive attributes (e.g. the skills of an employee, their department, etc.) from the sensitive ones (e.g. salary, complaints, etc.). But even further, maybe there are levels of sensitive information. For instance, the salary of employees might be visible to any HR staff, but the salary of CxO might be accessible only to HR executives…

Before going too wild in partitioning the data, let’s remember that this isn’t Facebook. It isn’t about securing individuals but securing systems. We do not want to have HR complaints transiting in payroll systems. But having the salary of every employees, including executives, transiting in a payroll solution is probably fine.

With all of that in mind, we need to design data buckets that make sense.

Second draft

Now that we have a model to secure our events, we need to come back with the shortcomings of publishing events on public endpoints.

An easy solution we found is to use a different type of handler: event hub. Azure Event Grid can publish events to Azure Event Hub.

The customer we were working with is already using Kafka as a messaging infrastructure in a couple of solutions. So having Azure Event Hub with Kafka made a lot of sense and didn’t bring much friction.

So we came up with the following solution:

Broadcast many topics

Our Producing App emits events to different Azure Event Grid topics. Each topic represents a data bucket with a specific security context. A solution can then subscribe to some (or all) of those topics using Azure Event Grid Subscription. Some filtering can even be applied at the subscription for convenience (i.e. to offload the Consuming App from data it doesn’t need). The subscription delivers the events in an Azure Event Hub belonging to a specific solution. The Consuming App (or solution) can then pickup those events.

From a self-service perspective, this solution is quite attractive. The Event Grid Topics belong to the Producing App while both the subscriptions and the event hubs belong to the Consuming App.

The only thing we need to link the two is the act of creating a subscription. This is an administration task, perform when setting up the integration. The access control for that relies on Azure RBAC mechanisms.

We can look at the solution with a different view. Let’s take a single topic consumed by multiple solutions:

Broadcast to many solutions

We see that each solution owns the subscriptions and event hubs.

This solution ticked all the boxes. So we kept it. It is secure, it is self served and the team managing the producing app doesn’t become a bottle neck for the organisation. Security Governance needs to be applied when a subscription to a topic is created. This is similar to allowing API access.

Proof of concept

Now, let’s look at how this is implemented. Let’s deploy the proof of concept:

Deploy button

This ARM template doesn’t have any parameter. It deploys the following resources:


Here is what those resources are for:

Name Type Description
function-app-* App Service Azure Function sending events to the topic. It simulates the Producing App.
demo-app-plan App Service plan Consumption plan (i.e. serverless) of the Azure function
topic-* Event Grid Topic Represents a single data bucket
event-hub-* Event Hubs Namespace Represents the consuming app event hub
storage* Storage account Storage for the Azure function

Let’s look at the function. Looking at the code we can see the function simply fires several random events to the topic. The number of events is defined in environment variable EVENT_COUNT is set to 50 by default.

Let’s run the function once.

We’ll have to wait about 5 minutes to see the result of that in Azure Monitor.

After 5 minutes, let’s open the Event Grid resource. We should see a recent spike:

Event Grid Spike

The spike has a height of 50, which is the number of events the function fired.

We can move to the single subscription (at the bottom of the Overview page). We should see a similar spike.

If we now go to the Event Hub, we should see similar spikes.

We could play with filtering. The event type would be an easy property to filter on. The code generating it is:

EventType = random.Next(0, 10) < 4 ? "news" : "updates",

Basically, %40 of events are news while %60 are updates. Filtering on the event type on either “news” or “updates” should yield %40 x 50 = 20 or %60 x 50 = 30 events in the subscription / Event Hub (or close to, given the random nature of assignation).


We wanted to show the thought process in crafting an eventing solution from requirements to POC.

Different approaches could have been taken. For instance, instead of using Event Hubs, we could have used any of the supported handlers.

2 responses

  1. Jairo 2020-02-25 at 14:48

    Interesting Vincent! So in the end the same event will be published to different hubs with different versions of the same payload each one of them?

  2. Vincent-Philippe Lauzon 2020-02-27 at 08:49

    Yes, with the same identical payload.

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