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To publish an MQTT message, a client sends a PUBLISH packet to a broker with a topic name and a payload. Choose QoS according to the delivery guarantee you need, and set the retain flag when new subscribers should receive the topic’s latest state. The payload’s format is up to your application.
What happens when you publish an MQTT message?
MQTT is a client/server publish-subscribe messaging transport. A publisher is an MQTT client; it sends a PUBLISH control packet to an MQTT server, commonly called a broker. The broker forwards the application message to clients with subscriptions matching the published Topic Name. The OASIS MQTT Version 5.0 specification defines the packet flow and required responses.
A PUBLISH packet has a fixed header, a variable header, and a payload. The Topic Name identifies the information channel; the Payload carries application data. MQTT does not prescribe that data’s format, and an empty payload is valid. The fixed header includes the DUP, QoS, and RETAIN controls.
How do you publish a message?
At a practical level, connect an MQTT client to a broker, select a topic and payload, choose a QoS level and retain setting, then send the publish. For example, the Eclipse Paho sample demonstrates publishing a message to a topic at QoS 0; its command syntax depends on the installed Paho version and broker endpoint. See the Eclipse Paho Python sample client and adapt it to your environment.
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Before sending, decide what the data represents. Use a topic such as sensors/room1/temperature to identify the channel, and encode the payload in the format expected by subscribers, such as JSON or plain text. MQTT transports the payload; your application defines how to interpret it.
Which MQTT QoS should you choose?
| QoS | Guarantee | What to expect | Typical consideration |
|---|---|---|---|
| 0 | At most once | No acknowledgement packet is expected. | Choose when occasional message loss is acceptable. |
| 1 | At least once | The receiver responds with PUBACK; duplicate delivery is possible. |
Choose when delivery matters and the application can handle duplicates. |
| 2 | Exactly once delivery in the MQTT exchange | The exchange begins with PUBREC and continues through the QoS 2 handshake. |
Use when the protocol-level delivery guarantee is needed; business effects still need suitable application-side design. |
These guarantees describe MQTT delivery behavior, not whether a business operation happens only once. For example, a subscriber that processes a payment or changes device state should still use suitable transaction handling or idempotency. Selecting QoS 2 alone does not make an application operation idempotent.
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When should you set RETAIN?
Set RETAIN=1 when a new matching non-shared subscriber should receive the latest state published for a topic. The broker stores or replaces that topic’s retained application message; this is useful for state that changes irregularly, such as a device’s current status. It is not a history or general-purpose queue of every event.
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What do DUP, topic, and payload mean?
- Topic Name: Identifies the channel used for subscription matching. Subscribers receive messages when their subscriptions match it.
- Payload: Holds the application message. The content and encoding are application-specific; an empty payload is permitted.
- DUP: Indicates that the sender is attempting to re-deliver a PUBLISH packet. It must be 0 for QoS 0. It is not a unique-message identifier and does not mean the application payload has never been seen before.
Which transport and connection details matter?
MQTT requires an underlying transport that provides an ordered, lossless byte stream between client and server. The specification names TCP/IP, TLS, and WebSocket as suitable examples; UDP alone is unsuitable because it can lose or reorder data. Its non-normative transport discussion identifies port 1883 for non-TLS MQTT and 8883 for TLS MQTT. Actual endpoints and security requirements depend on the broker deployment.
For a production connection, verify the broker’s address and port, TLS configuration, authentication and authorization, and any broker limits that apply to message size or retained messages. Protecting the connection with TLS does not replace configuring appropriate access controls.
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Publishing through a managed service
If you do not want to operate a broker, a managed service may expose publishing through an API. For example, the AWS IoT Publish API provides parameters including qos, retain, contentType, messageExpiry, and responseTopic. This is an AWS IoT service option rather than a change to MQTT’s core publish model.
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A quick decision checklist
- Choose QoS based on whether loss is acceptable and whether your application can tolerate duplicates.
- Enable retain only when future subscribers need the latest topic state; publish a zero-byte retained message to clear that state.
- Define a topic structure and payload encoding that publishers and subscribers both understand.
- Use an ordered, lossless transport and configure TLS and access controls as required by your deployment.
- Check broker limits and confirm the publish syntax for your client library or managed-service API.
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