spec: c8y mqtt service connection spec

Author: albinsuresh

design/decisions/0007-c8y-mqtt-service-connect.md

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+# Connect to C8Y MQTT service endpoint
+
+* Date: __2024-06-23__
+* Status: __New__
+
+## Requirement
+
+Cumulocity has introduced a new [MQTT service endpoint](https://cumulocity.com/docs/device-integration/mqtt-service)
+that allows users to send free-form messages (any arbitrary topic and payload) to Cumulocity.
+It does not replace the existing core MQTT endpoint and hence a separate connection is required.
+To support the same, `tedge connect` must be enhanced to connect to this new endpoint as well,
+in addition to the existing connection to the core endpoint.
+
+## MQTT service specifications/limitations
+
+- The mqtt service endpoint is exposed at the same tenant URL via port 9883 (with TLS).
+- Both username/password based and cert based authentication is supported.
+- Only connections with clean session enabled are supported. Persistent connection attempts are rejected.
+- Subscribing to wildcard topics (`#` and `+`) and system topics (starting with `$SYS`) are not supported.
+- Publishing/subscribing to legacy C8Y topics (SmartREST and JSON over MQTT) are not supported either.
+- QoS 2 is not supported.
+- Retained flag is accepted, but ignored.
+- There are limits to the number of topics that can be created
+  and the number of messages that can be queued per topic.
+  Publishes are rejected when the queue is full.
+- Each published message has a time-to-live, after which it is cleared from the queue.
+  The subscribed clients are not notified of expired messages.
+
+## Solutions
+
+To establish a connection to the mqtt service endpoint, the following info must be provided in the connect request:
+
+* host: Same as legacy mqtt endpoint
+* port: 9883 by default
+* auth:
+  * for username/password based authentication
+    * username: `<tenant_id>/<user_name>`
+    * password
+  * for cert based authentication
+    * device cert path
+    * device key path
+    * server root ca path
+    * username: `<tenant_id>`
+* bridge topics: `<bridge_topic_prefix>/#`
+
+:::note
+Even for cert based auth, the `username` must be provided with the tenant id.
+:::
+
+The following are the solution approaches that are considered:
+
+### Approach 1: Connect to mqtt service along with core mqtt connection
+
+The mqtt service bridge connection is established along with `tedge connect c8y` command.
+
+:::note
+For the builtin bridge functionality, whether to spawn a different mapper,
+or establish a secondary connection from the existing mapper itself is to be decided.
+The same applies for the trivial mapping rule that adds the timestamp to the payload.
+:::
+
+#### Configuration
+
+The mqtt service connection related configs are defined under the `[c8y]` table in `tedge.toml` :
+
+| Config | Description | Default |
+| ------ | ----------- | ------- |
+| `c8y.mqtt_service.enabled` | Whether the mqtt server connection must be established or not | `false` |
+| `c8y.mqtt_service.url` | The config URL for the MQTT service | `<c8y.url>:9883` |
+| `c8y.mqtt_service.topic_prefix` | topic prefix used to bridge mqtt topics | `c8y-mqtt` |
+
+To connect to the mqtt service, it must be enabled before `tedge connect c8y` is executed:
+
+```sh
+tedge config set c8y.mqtt_service.enabled "true"
+```
+
+:::note
+The tenant id which must be passed in the username is retrieved using the `https://<c8y.url>/tenant/currentTenant` REST API.
+An alternative is to take this as an additional `c8y.tenant_id` configuration.
+:::
+
+#### Connection
+
+The command to connect to C8Y remains the same:
+
+```sh
+tedge connect c8y
+```
+
+The connection to the mqtt service is established only if it is enabled.
+
+**Pros**
+
+- Makes the mqtt service connection seamless for existing users, with minimal configuration.
+- Reuse most of the existing configurations like `c8y.url`, `c8y.device.key_path`, `c8y.device.cert_path` etc.
+- Users can choose not to connect to the new endpoint, by keeping it disabled.
+
+**Cons***
+
+- The mqtt service connection is tied to the direct `c8y` connection, to retrieve the tenant id
+  (although this dependency can be removed if tenant id is accepted via config).
+
+### Approach 2: Connect to mqtt service as an independent cloud endpoint
+
+Do not establish both the core mqtt connection and the mqtt service connection together with `tedge connect c8y` command,
+but treat the mqtt service as an independent/generic MQTT broker endpoint like connecting to AWS or AZ endpoints.
+
+#### Configuration
+
+As an independent cloud endpoint, the following config values must be provided:
+
+```sh
+tedge config set remote.url example.cumulocity.com:9883
+tedge config set remote.username t123456
+tedge config set remote.bridge.topic_prefix c8y-mqtt
+```
+
+The following setting can also be configured, though they all have default values:
+
+| Config | Description | Default |
+| ------ | ----------- | ------- |
+| `remote.device.cert_path` | The device cert path used for the authentication | Same as `device.cert_path` |
+| `remote.device.key_path` | The device cert path used for the authentication | Same as `device.key_path` |
+
+In addition to the above, pretty much all the configs defined in the `tedge.toml` for `az` and `aws` can be defined
+for this generic `remote` endpoint as well, as none of them are cloud specific but applies to any mqtt broker.
+
+#### Connection
+
+```sh
+tedge connect remote
+```
+
+**Pros**
+
+- Clear separation from `tedge connect c8y` allows the second connection to be established and disconnected on demand.
+- Existing users are not affected, as the `tedge connect c8y` behavior stays the same.
+- Allows customers to connect exclusively to the new mqtt service, if they don't need the core mqtt connection.
+- There is no real dependency on Cumulocity as multiple `remote` profiles can be used to connect to any generic broker.
+
+**Cons**
+
+- Many of the config parameters defined in the `c8y` profile like the `url`, `cert_path` etc
+  will have to be duplicated in `remote` profile as well.
+- When the device is connected to multiple c8y cloud profiles,
+  corresponding `remote` profiles must be configured explicitly per c8y instance.
+
+### Approach 3: Connect to all configured cloud instances with tedge connect
+
+This is more of an extension of `Approach 2`, where instead of explicitly connecting to each cloud instance/profile,
+a `tedge connect` command (without the cloud arg), detects all the cloud profiles that are configured/enabled
+and connects to each endpoint one by one.
+Only the cloud instances with a `url` explicitly configured are considered as enabled.
+An additional `enabled` flag can be added to each cloud profile, to skip connecting to it, even when it is configured.
+
+#### Configuration 
+
+TBD
+
+#### Connection 
+
+TBD
+