From 955009f992f8e6da1c3b7211ab597ca38535e1d8 Mon Sep 17 00:00:00 2001 From: Nicolas Belouin Date: Tue, 1 Aug 2023 15:48:36 +0200 Subject: [PATCH] New proposal for MQTT Discovery Handler This is a new proposal for an MQTT Discovery Handler, this proposal tries to keep things as simple as possible while still solving most use cases This is part of project-akri/akri#106 Signed-off-by: Nicolas Belouin --- proposals/mqtt.md | 83 +++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 83 insertions(+) create mode 100644 proposals/mqtt.md diff --git a/proposals/mqtt.md b/proposals/mqtt.md new file mode 100644 index 0000000..f15fca0 --- /dev/null +++ b/proposals/mqtt.md @@ -0,0 +1,83 @@ +# [MQTT](https://mqtt.org/) Discovery Handler + +## MQTT and its use + +MQTT is a simple message bus with a broad use in IoT devices, such device will usually connect to an existing MQTT broker on the network. +This allows reuse of the same broker for multiple devices. + +Let's quickly introduce some key concepts of MQTT: + +- An MQTT message consists of a slash (`/`) separated UTF-8 topic and a completely free-form payload with no built-in notion of sender +- Topics are only used for message routing and are not "created" or "destroyed" +- By default, messages are not stored, and a message with no client subscribing to its topic is simply discarded +- There are no specified way of querying connected clients or current subscriptions +- An MQTT client can use wildcards when subscribing to topics + +Because of these properties, it is impossible to discover devices that do not publish any message. + +Usually a device will publish into topics that are specific to that device instance so that it becomes possible to know the origin of the message. +For example a weather station might publish into topics like `weather//temperature` and `weather//pressure` with `` the location of station. + +Many power-constrained devices are not maintaining a long-lived connection to the MQTT broker. + +In the rest of the document we assume that devices to discover follow this kind of pattern. + +Using the above description of MQTT and device behavior, if we consider the Discovery Handler to be an MQTT client, we can define a discoverable device as an MQTT client that will "frequently" publish to an MQTT topic. + +## Discovery Details and Properties + +In order to discover MQTT based devices, we need the following pieces of information: + +- The MQTT broker to connect to (and its credentials) +- The topic patterns to subscribe to +- How long shall we wait to consider a silent device as disconnected + +Here is the expected schema for the discovery details: + +```yaml +discoveryDetails: + mqttBrokerUri: + type: string + topics: + type: array + items: + type: string + timeoutSeconds: + type: int64 + minimum: 0 + exclusiveMinimum: true +``` + +If the following keys are present in the discovery properties they will be used for connection and authentication to the MQTT broker: + +- `mqtt_broker_username`: username for authentication against the broker +- `mqtt_broker_password`: password for authentication against the broker +- `mqtt_broker_certificate`: client TLS certificate for authentication against the broker +- `mqtt_broker_key`: client TLS key for authentication against the broker +- `mqtt_broker_ca`: TLS CA bundle for validating the broker TLS certificate (if unspecified, system-wide bundle is used) + +## Instance properties + +The ID of the device is the full topic used by the device. + +The following properties shall be forwarded to the broker or any instance requesting the device: + +- `MQTT_BROKER_URI`: the MQTT broker URI +- `MQTT_TOPIC`: The topic the message was received on + +## Discovery process + +The discovery handler must subscribe to all topics pattern specified in the `topics` array on the `mqttBrokerUri` MQTT broker. + +The discovery handler must keep track of all discovered devices. + +When receiving a message on a watched topic, the discovery handler must: + +- Add the device to its discovered device list if not already here +- Reset the timer of the device to `timeoutSeconds` + +When the timer of a device expires, the discovery handler must: + +- Delete the device from its discovered device list + +Every time the discovered device list changes, the discovery handler must send that list back to the Agent.