How to broadcast to device groups with Event Grid MQTT when clients don’t know their own attributes?

Question

How to broadcast to device groups with Event Grid MQTT when clients don’t know their own attributes?

NicholasDyhre-2538 25

Hi,

I’m evaluating Azure Event Grid MQTT (preview) for a large IoT deployment, and I’ve run into a design challenge that I can’t resolve with the current documentation.

My setup

I have many physical devices and device groups (> 10).
Physical devices connect to MQTT clients in azure.
Azure Event Grid MQTT Clients has information not available on the physical devices, such as attributes.

My requirements

Group messaging:
1. Goal: I want to broadcast messages to groups of devices based on their attributes in Event Grid (e.g., all devices with some_attribute=1 or customerId=42).
Restrictions:
1. The devices should only receive message from topics that match their attributes, e.g. clients with attributes. customerId=42 should only be able to listen on someTopic/customerId/42. someTopic/customerId/32 should be forbidden.
2. The physical hardware devices should not be required to know the attributes of azure-MQTT-Client. So when i setup subscription, the number “42” should not be required for the physical device to know.

The problem

Topic Spaces let me define patterns like:
cloud2device/${client.attributes.customerId}/#
But devices cannot use placeholders like ${client.attributes.customerId} in their subscription filters.
They must specify the resolved value, which they don’t know.
A client’s attributes can change, but physical devices should remain stateless.

My question

Given these constraints Is there a built-in way for the broker to automatically route messages to clients based on their attributes (without the device knowing the attribute) which will allow me to accomplish group broadcast with Event Grid MQTT?

Any guidance, or confirmation of current limitations, would be much appreciated.

Thanks!

Harish Badijana 320 Reputation points Microsoft External Staff Moderator

2025-08-28T22:19:09.0566667+00:00

@ND

Thank you for using Q&A Platform.

It sounds like you're navigating some complexities with Azure Event Grid MQTT for your IoT deployment specifically, how to group broadcast messages to devices without them being aware of their attributes.

You can achieve partial automation using Client Groups and Permission Bindings

Client Groups i.e., Define groups

{

"query": "attributes.customerId = 42"

}

This groups all clients with customerId=42

Permission Bindings, Bind the group to a topic space

{

"topicSpace": "someTopic/customerId/42",

"permissions": ["subscribe"]

This allows only clients in the group to subscribe to that topic—even if they don’t explicitly know the value.

Cloud-to-Device Messaging, Use topic templates

cloud2device/${client.attributes.customerId}/#

This lets cloud apps publish messages to the correct topic based on client attributes.

I hope this helps in resolving the issue, do let me know if you have any further questions on this
NicholasDyhre-2538 25 Reputation points

2025-08-29T08:33:24.5933333+00:00
Harish Badijana

thank you for the quick reply. However, I don’t think this approach scales for my scenario:

As documented here, there’s a hard limit of 10 client groups per namespace: https://free.blessedness.top/en-us/azure/event-grid/mqtt-client-groups#:~:text=Currently%2C%20a%20maximum%20of%2010%20client%20groups%20per%20namespace%20as%20supported

In my scenario, the number of distinct attribute-based-groups values (e.g., customerId) can easily exceed 10 (100+). Defining one client group per customer is therefore not feasible.

Adding permission bindings, doesn’t remove the need for clients to subscribe to the exact topic (e.g. to know customerId) – only restrict them for what they are allowed to subscribe to.

I need broker-side attribute-based group resolution:

Devices should be able to subscribe to topics without needing attribute values. E.g. My physical device should not need to know the customerId.

Client attributes should determine which events a device receives. For example, a client with customerId=42 should only receive events published to topics matching customerId=42, and must not receive events published to other values such as customerId=33.

Are there any ways to use Event Grid MQTT broker to broadcast to attribute-based groups at scale?

Thanks again for the guidance.
Harish Badijana 320 Reputation points Microsoft External Staff Moderator

2025-09-03T20:17:02.9566667+00:00

@ND Just checking to see if you have a chance to check my previous response and helped, do let me know if you have any further questions on this.
Harish Badijana 320 Reputation points Microsoft External Staff Moderator

2025-09-04T22:39:55.4833333+00:00

@ND Following up to see if you have a chance to check my previous response and helped, do let me know if you have any further questions on this.

Answer accepted by question author

1 additional answer

Your answer

Harish Badijana 320 Reputation points Microsoft External Staff Moderator

2025-08-28T22:19:09.0566667+00:00

@ND

Thank you for using Q&A Platform.

It sounds like you're navigating some complexities with Azure Event Grid MQTT for your IoT deployment specifically, how to group broadcast messages to devices without them being aware of their attributes.

You can achieve partial automation using Client Groups and Permission Bindings

Client Groups i.e., Define groups

{

"query": "attributes.customerId = 42"

}

This groups all clients with customerId=42

Permission Bindings, Bind the group to a topic space

{

"topicSpace": "someTopic/customerId/42",

"permissions": ["subscribe"]

This allows only clients in the group to subscribe to that topic—even if they don’t explicitly know the value.

Cloud-to-Device Messaging, Use topic templates

cloud2device/${client.attributes.customerId}/#

This lets cloud apps publish messages to the correct topic based on client attributes.

I hope this helps in resolving the issue, do let me know if you have any further questions on this
NicholasDyhre-2538 25 Reputation points

2025-08-29T08:33:24.5933333+00:00

Harish Badijana

thank you for the quick reply. However, I don’t think this approach scales for my scenario:

As documented here, there’s a hard limit of 10 client groups per namespace: https://free.blessedness.top/en-us/azure/event-grid/mqtt-client-groups#:~:text=Currently%2C%20a%20maximum%20of%2010%20client%20groups%20per%20namespace%20as%20supported

In my scenario, the number of distinct attribute-based-groups values (e.g., customerId) can easily exceed 10 (100+). Defining one client group per customer is therefore not feasible.

Adding permission bindings, doesn’t remove the need for clients to subscribe to the exact topic (e.g. to know customerId) – only restrict them for what they are allowed to subscribe to.

I need broker-side attribute-based group resolution:

Devices should be able to subscribe to topics without needing attribute values. E.g. My physical device should not need to know the customerId.

Client attributes should determine which events a device receives. For example, a client with customerId=42 should only receive events published to topics matching customerId=42, and must not receive events published to other values such as customerId=33.

Are there any ways to use Event Grid MQTT broker to broadcast to attribute-based groups at scale?

Thanks again for the guidance.
Harish Badijana 320 Reputation points Microsoft External Staff Moderator

2025-09-03T20:17:02.9566667+00:00

@ND Just checking to see if you have a chance to check my previous response and helped, do let me know if you have any further questions on this.
Harish Badijana 320 Reputation points Microsoft External Staff Moderator

2025-09-04T22:39:55.4833333+00:00

@ND Following up to see if you have a chance to check my previous response and helped, do let me know if you have any further questions on this.

Answer 1

@ND

Azure Event Grid MQTT supports routing messages from clients to Namespace topics or Custom topics

Once routed, you can configure event subscriptions to push or pull messages to Azure services or custom endpoints. However, this routing is not attribute-aware at the broker level. It does not automatically resolve client attributes to determine delivery eligibility.

FYI, there is no built-in mechanism in Event Grid MQTT today that allows the broker to automatically resolve client attributes and match them against published topic value. This means your requirement—where a device subscribes generically and the broker filters based on attributes—is not natively supported.

While not ideal, here are some potential approaches

Use Event Grid Routing and Azure Functions

Route all MQTT messages to a central Event Grid topic.

Use an Azure Function to inspect client attributes and forward messages to appropriate subtopics or endpoints.

This adds latency and complexity but allows dynamic filtering.

Custom Broker Layer

Deploy a custom MQTT broker in Azure.

Use Azure services to manage attribute-based routing.

This gives full control but sacrifices native Event Grid integration.

Push Model via Cloud-to-Device Messaging

Use cloud-side logic to publish messages to topics like cloud2device/${client.attributes.customerId}/#.

Clients subscribe to generic topics, and permissions restrict access.

Still requires clients to know or be configured with resolved topic values.

I hope this helps in resolving the issue, do let me know if you have any further questions on this

Answer 2

Hello @ND,

welcome to this moderated Azure community forum.

It seems this 10 client groups limitation is a 'soft limit'.

It's probably not there because some technology cannot cope with it, it's just a decision from the team because there should be some limit.

You could try to ask for a new limit by discussing your use case with the product team so they better understand why a different number is needed.

This can be done by submitting a support ticket.

If the response helped, do "Accept Answer". If it doesn't work, please let us know the progress. All community members with similar issues will benefit by doing so. Your contribution is highly appreciated.

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How to broadcast to device groups with Event Grid MQTT when clients don’t know their own attributes?

1 additional answer

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