MQTT Messages are routed to an Event Grid topic as CloudEvents according to the following logic:

For MQTT v3 messages or MQTT v5 messages of a payload format indicator=0, the payload will be forwarded in the data_base64 object and encoded as a base 64 string according to the following schema sample.

{"specversion": "1.0","id": "9aeb0fdf-c01e-0131-0922-9eb54906e20", // unique id stamped by the service."time": "2019-11-18T15:13:39.4589254Z", // timestamp when the message was received by the service."type": "MQTT.EventPublished", // set type for all MQTT messages enveloped by the service."source": "testnamespace", // namespace name"subject": "campus/buildings/building17", // topic of the MQTT publish request."data_base64":     {IlRlbXAiOiAiNzAiLAoiaHVtaWRpdHkiOiAiNDAiCg==}}

For MQTT v5 messages of content type= “application/json; charset=utf-8” or of a payload format indicator=1, the payload will be forwarded in the data object, and the message will be serialized as a JSON (or a JSON string if the payload isn't a JSON). Setting the content type and/or the payload format indicator enables you to filter on your payload properties as the payload is forwarded within the data field as is. Learn more aboutfiltering on the message's payload.

{"specversion": "1.0","id": "9aeb0fdf-c01e-0131-0922-9eb54906e20", // unique id stamped by the service."time": "2019-11-18T15:13:39.4589254Z", // timestamp when the message was received by the service."type": "MQTT.EventPublished", // set type for all MQTT messages enveloped by the service."source": "testnamespace", // namespace name"subject": "campus/buildings/building17", // topic of the MQTT publish request. "data":     {"Temp": 70,"humidity": 40}}

For MQTT v5 messages that are already enveloped in a CloudEvent according to theMQTT Protocol Binding for CloudEvents whether using the binary content mode or the structured content mode in JSON encoding (utf-8), the event will be forwarded with the original default CloudEvents attributes after enrichments according to the following schema sample.

{"specversion": "1.0","id": "9aeb0fdf-c01e-0131-0922-9eb54906e20", // original id stamped by the client. "time": "2019-11-18T15:13:39.4589254Z", // timestamp when the message was received by the client"type": "Custom.Type", // original type value stamped by the client."source": "Custom.Source", // original source value stamped by the client."subject": " Custom.Subject", // original subjectvalue stamped by the client."data":     {"Temp": "70","humidity": "40"}}

Next steps:

Use the following articles to learn more about routing:

QuickStart:

• Tutorial: Route MQTT messages to Azure Event Hubs using namespace topics
• Tutorial: Route MQTT messages to Azure Functions using custom topics

Concepts:

• Routing
• Routing Filtering
• Routing Enrichments