MQTT

MQTT (Message Queuing Telemetry Transport) is a standard for IoT messaging based on the publish-subscribe model. It's one of the most used protocols in IoT.

It consists of clients that connect to a broker instead of directly connecting with each other. This broker stores information about all clients connected to it and the topics they are subscribed to.

When clients connect over MQTT, it opens a realtime connection with the broker. The communication then happens in terms of Subscribe and Publish messages. When a device subscribes to a topic, it tells the broker it wants to receive all the messages published on that topic. When any device publishes a message on a topic, the broker looks up for devices that are subscribed to that topic and forwards the message to them. Pretty straight forward!

However when you are connecting your devices to Grandeur over MQTT, there are a few things to keep in mind.

Modes

There are two modes to connect: device mode and admin mode.

Device mode

When a device uses its device token to connect, it connects in the device mode. In device mode, it can only subscribe and publish data on its own variables.

Device mode is the most common mode for devices to connect with Grandeur.

Admin mode

When a device uses an auth token to connect, it connects in the admin mode. In admin mode, it can subscribe and publish data to not just its own variables, but also other devices. This means a device in admin mode can freely monitor and control all other devices in the project.

Topic

A topic structure looks like this: DEVICE_ID/VARIABLE . First part is the ID of the device you want to subscribe or publish to. Second part is the variable of that device.

Apart from this, keep the following things in mind as well to avoid unexpected errors:

1. You cannot leave the topic string empty.
2. A topic does not start or end with a forward slash (/).
3. Topic cannot have an empty level either like "device//data".