Once a channel is provisioned and thing is connected to it, it can start to publish messages on the channel. The following sections will provide an example of message publishing for each of the supported protocols.

HTTP#

To publish message over channel, thing should send following request:

curl -s -S -i --cacert docker/ssl/certs/ca.crt -X POST -H "Authorization: <thing_token>" https://localhost/http/channels/<channel_id>/messages -d '[{"bn":"some-base-name:","bt":1.276020076001e+09, "bu":"A","bver":5, "n":"voltage","u":"V","v":120.1}, {"n":"current","t":-5,"v":1.2}, {"n":"current","t":-4,"v":1.3}]'

Note that if you're going to use senml message format, you should always send messages as an array.

MQTT#

To send and receive messages over MQTT you could use Mosquitto tools, or Paho if you want to use MQTT over WebSocket.

To publish message over channel, thing should call following command:

mosquitto_pub -u <thing_id> -P <thing_key> -t channels/<channel_id>/messages -h localhost -m '[{"bn":"some-base-name:","bt":1.276020076001e+09, "bu":"A","bver":5, "n":"voltage","u":"V","v":120.1}, {"n":"current","t":-5,"v":1.2}, {"n":"current","t":-4,"v":1.3}]'

To subscribe to channel, thing should call following command:

mosquitto_sub -u <thing_id> -P <thing_key> -t channels/<channel_id>/messages -h localhost

If you want to use standard topic such as channels/<channel_id>/messages with SenML content type (JSON or CBOR), you should use following topic channels/<channel_id>/messages.

If you are using TLS to secure MQTT connection, add --cafile docker/ssl/certs/ca.crt to every command.

CoAP#

CoAP adapter implements CoAP protocol using underlying UDP and according to RFC 7252. To send and receive messages over CoAP, you can use Copper CoAP user-agent. To set the add-on, please follow the installation instructions provided here. Once the Mozilla Firefox and Copper are ready and CoAP adapter is running locally on the default port (5683), you can navigate to the appropriate URL and start using CoAP. The URL should look like this:

coap://localhost/channels/<channel_id>/messages?authorization=<thing_auth_key>

To send a message, use POST request. To subscribe, send GET request with Observe option set to 0. There are two ways to unsubscribe: 1) Send GET request with Observe option set to 1. 2) Forget the token and send RST message as a response to CONF message received by the server.

The most of the notifications received from the Adapter are non-confirmable. By RFC 7641:

Server must send a notification in a confirmable message instead of a non-confirmable message at least every 24 hours. This prevents a client that went away or is no longer interested from remaining in the list of observers indefinitely.

CoAP Adapter sends these notifications every 12 hours. To configure this period, please check adapter documentation If the client is no longer interested in receiving notifications, the second scenario described above can be used to unsubscribe.

Subtopics#

In order to use subtopics and give more meaning to your pub/sub channel, you can simply add any suffix to base /channels/<channel_id>/messages topic.

Example subtopic publish/subscribe for bedroom temperature would be channels/<channel_id>/messages/bedroom/temperature.

Subtopics are generic and multilevel. You can use almost any suffix with any depth.

Topics with subtopics are propagated to NATS broker in the following format channels.<channel_id>.<optional_subtopic>.

Our example topic channels/<channel_id>/messages/bedroom/temperature will be translated to appropriate NATS topic channels.<channel_id>.bedroom.temperature.

You can use multilevel subtopics, that have multiple parts. These parts are separated by . or / separators. When you use combination of these two, have in mind that behind the scene, / separator will be replaced with .. Every empty part of subtopic will be removed. What this means is that subtopic a///b is equivalent to a/b. When you want to subscribe, you can use NATS wildcards * and >. Every subtopic part can have * or > as it's value, but if there is any other character beside these wildcards, subtopic will be invalid. What this means is that subtopics such as a.b*c.d will be invalid, while a.b.*.c.d will be valid.

Authorization is done on channel level, so you only have to have access to channel in order to have access to it's subtopics.

Note: When using MQTT, it's recommended that you use standard MQTT wildcards + and #.

For more information and examples checkout official nats.io documentation