MQTT
The MQTT service handles communication between the ACU6 Base System and the cloud-located Actia Device Manager. Most of the services it provides is internal to the base system and not exposed to the User VM.
Shoulder tap
Shoulder tap is a service provided by the MQTT service together with the Actia Device Manager that allows sending of short messages from the device manager to a user application running in the User VM on a device, somewhat similar to how the cellular network allows for SMS messages. Shoulder taps are only from device manager to the device, there is no response, and there is no guranteed delivery. It is a fire-and-forget kind of signaling.
Each shoulder tap consists of a UUID identifying the request and an optional data block of up to 64 bytes. The meaning of the UUID is up to the application developer to define, but it could be used to distinguish between different types of messages or to route the shoulder tap to different services inside of the User VM.
Shoulder tap is sent either manually from the Actia Device Manager web front-end or by using the API of the device manager. See the Actia Device Manager documentation for information on how to send a shoulder tap.
Example
Shoulder tap is normally received as part of the user application. For testing and debugging the demo application shoulder-tap-demo can be used.
$ # Wait for any shoulder tap
$ shoulder-tap-demo
....
<send a shoulder tap via the device manager>
Shoulder tap UUID: fb0233ed-f67f-46d2-8f13-81fc8a777387
Payload length: 5 bytes
Payload: 03 42 34 92 83