Arduino -Sending and Receiving JSON Data over MQTT

To send an receive JSON data with Arduino you can use the ArduinoJson library. The online documentation contains several usage examples, and there is even a book that supports the project. In this example we are going to send a JSON object using MQTT, receive that object and decode it.

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MQTT-SN Topic Names and Identifiers

MQTT Topics are UTF-8 strings structured in a hierarchy similar to folders and files in a file system using the forward slash ( / )as a delimiter. To be valid a topic must be at least 1 character long and generally in real world applications they 10s of characters in length. The topic is sent with every message published, and so in order to reduce the length of published messages MQTT-SN uses shorter topic names. it does this by using a variety of topic identifiers in place of standard topic name used in MQTT.

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Publishing MQTT Data to to a Web Page

Although it may not be obvious the JavasScript MQTT client makes it easy to publish MQTT data to a web page on the Internet and also to create a control panel that you can host on the Internet. Most Web applications currently use PHP as the scripting engine. Although there are MQTT modules available in PHP ( I have never used them), and are probably not supported on most web hosts.

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Python MQTT Manual Reconnect

As mentioned previously in understanding the loop there are some conditions when it is better to call the client loop yourself rather than using the loop_start() or loop_forever() calls. Using these built in function calls means that you do not need to handle reconnects as they are part of the calls. However when using the  manual loop() method this you will need to handle reconnects yourself. The code below will handle connects and reconnects.

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MQTTv5 Clean Start (Clean Sessions) and Session Expiry

In MQTT v3.1.1 we have the concept of clean sessions or non persistent/persistent connections. On connect the client indicates to the server using the clean session flag if the session state should be kept by the server when the client disconnects. Session state includes any client subscriptions and also messages depending on the QOS used and is covered in detail in the Understanding clean sessions tutorial , I recommend you read before proceeding if you are unfamiliar with clean sessions.

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Paho Python MQTT Client Changes for MQTTv5 Support

The Paho Python client version 1.5.1 included support for MQTTv5. Because of the new capabilities of MQTTv5 there have been changes to many of the common functions like connect,subscribe and publish etc. The aim of this tutorial is to point out the main changes, and what you need to do in your code to use the new features.

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MQTTv5 Properties by Message Type

Properties (MQTTv5) are probably one of the most important additions to the MQTT protocol, and are available in most MQTT message types including acknowledgement messages. The property fields are dependent upon message type, so a CONNECT message will have different property fields than a PUBLISH message. I have listed the property fields for each message type with links to the online documentation describing the filed and its use.

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Receiving Messages with the Paho MQTT Python Client

Messages are received by the on_message callback, and so this callback must be defined and bound in the main script. All callbacks rely on the client loop and this must also have been started using loop_start() or loop_forever(), or run manually within the main script. See Understanding the loop for more details Provided this is the case it is easy to receive the messages and display them on the console using the example code below:

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Introduction to MQTT +Sparkplug For IIOT

Eclipse recently announced a working group to drive the adoption of the Eclipse Sparkplug specification to standardise interoperability in the Industrial Internet of things arena. The MQTT protocol has fast become the de facto standard for messaging in IOT applications. However MQTT was designed to be as open as possible and didn’t provide any restrictions on topic names and message structures.

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Using the Arduino PubSub MQTT Client

There are several  MQTT clients available for Arduino and we are going to use the PubSub MQTT client. Before you can use this client you need to install it into the Arduino Library. Go to the Library manager and so a search for MQTT. You will find quite a few listings scroll down the select the PubSub client.

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