tedge-configuration-plugin specification (#2097)

Signed-off-by: Rina Fujino <18257209+rina23q@users.noreply.github.com>

docs/src/references/tedge-configuration-management.md

@@ -0,0 +1,311 @@
+---
+title: New Configuration Management
+tags: [Reference, Configuration]
+sidebar_position: 7
+draft: true
+---
+
+# Configuration Management Plugin
+
+Thin-edge provides an operation plugin to manage device configuration files.
+
+* This management is bi-directional:
+  * A device can act as a reference,
+    with all the managed files being uploaded to the [tedge file transfer repository](tedge-file-transfer-service.md)
+    and stored there as a configuration snapshot.
+  * A configuration update can be pushed from the tedge file transfer repository to any devices of the same type,
+    i.e. supporting the same kind of configuration files.
+  * This plugin, combined with a cloud mapper, enables configuration management from the cloud.
+* With this operation plugin, the device owner defines the list of files
+  (usually configuration files, but not necessarily).
+* The plugin configuration itself can be managed both locally and from the cloud,
+  meaning, the device owner can update the list of files to be managed from the cloud using the cloud mapper.
+* The configuration files are managed according to their type,
+  a name chosen by the device owner to categorize each configuration.
+  By default, the full path of a configuration file on the device is used as its type.
+* When files are downloaded from the tedge file transfer repository to the device where the plugin is installed,
+  __these files are stored in the target path with a temporary name first__.
+  They are atomically renamed, only after a fully successful download to avoid breaking the system with half-downloaded files.
+* When a downloaded file is copied to its target, the Unix user, group and mode are preserved.
+* Once an update has been downloaded from the tedge file transfer repository to the target device,
+  __the plugin publishes an operation status update message on the local thin-edge MQTT bus__.
+  The device software must subscribe to these messages if any action is required,
+  such as checking the content of the file, to pre-processing it, or restarting a daemon.
+* The configuration plugin can be installed on both the main thin-edge device and the child-device.
+* The plugin has a dependency on the `tedge.toml` configuration file to get the MQTT hostname, port, and device identifier.
+
+In summary, the responsibilities of the plugin are:
+  * to define the list of files under configuration management,
+  * to notify the local MQTT bus when this list is updated,
+  * to upload these files to the tedge file transfer repository on demand,
+  * to download the files pushed from the tedge file transfer repository,
+  * to ensure that the target files are atomically updated after a successful download,
+  * to notify the device software when the configuration is updated.
+
+By contrast, the plugin is not responsible for:
+  * checking that the uploaded files are well-formed,
+  * restarting the configured processes,
+  * establishing any direct connection to clouds.
+
+A user-specific component installed on the device
+  can implement more sophisticated configuration use-cases by:
+  * listening for configuration updates on the local thin-edge MQTT bus,
+  * restarting the appropriate processes when needed,
+  * declaring intermediate files as the managed files,
+    to have the opportunity to check or update their content
+    before moving them to the actual targets.
+
+## Installation
+
+As part of this plugin installation:
+* On systemd-enabled devices, the service definition file for this plugin is also installed.
+
+Once installed, the `tedge-configuration-plugin` runs as a daemon on the device,
+listening to configuration snapshot commands on the `<root>/<identifier>/cmd/config_snapshot/+` [MQTT topic](mqtt-api.md#command-examples) 
+and to configuration update commands on the `<root>/<identifier>/cmd/config_update/+` [MQTT topic](mqtt-api.md#command-examples).
+
+## Configuration
+
+The `c8y-configuration-plugin` configuration is stored by default under `/etc/tedge/plugins/tedge-configuration-plugin.toml`
+
+This [TOML](https://toml.io/en/) file defines the list of files to be managed by the plugin.
+Each configuration file is defined by a record with:
+* The full `path` to the file.
+* An optional configuration `type`. If not provided, the `path` is used as `type`.
+  This `type` is used to declare the supported configuration file and then to trigger operations on that file.
+  All the configuration `type`s are declared as the supported config list to the local MQTT bus on startup
+  and on changes to the `plugins/tedge-configuration-plugin.toml` file.
+* Optional unix file ownership: `user`, `group` and octal `mode`.
+  These are only used when a configuration file pushed via a `config_update` command doesn't exist on the device,
+  and a new one is created with these ownership parameters.
+  When a configuration file is already present on the device,
+  this plugin preserves its existing ownership, ignoring these parameters.
+
+```toml title="file: /etc/tedge/plugins/tedge-configuration-plugin.toml"
+files = [
+  { path = '/etc/tedge/tedge.toml', type = 'tedge.toml' },
+  { path = '/etc/tedge/mosquitto-conf/c8y-bridge.conf', type = 'c8y-bridge' },
+  { path = '/etc/tedge/mosquitto-conf/tedge-mosquitto.conf', type = 'tedge-mosquotto' },
+  { path = '/etc/mosquitto/mosquitto.conf', type = 'mosquitto', user = 'mosquitto', group = 'mosquitto', mode = 0o644 }
+]
+```
+
+On start and whenever this file is updated, the configuration plugin sends
+the supported config types declaration message with a retained flag
+to the `config_snapshot` and `config_update` command topics
+with the set of `type`s listed in that configuration file
+(implicitly adding the `tedge-configuration-plugin` type also to that set).
+The message can be observed over the MQTT bus of the thin-edge device.
+
+Given that `root.topic` and `device.topic` are set to `te` and `device/main//` for the main device,
+the message to declare the supported configuration types is as follows.
+
+```sh te2mqtt
+tedge mqtt pub -r 'te/device/main///cmd/config_snapshot' '{
+  "types": ["tedge-configuration-plugin", "tedge.toml", "/etc/tedge/mosquitto-conf/c8y-bridge.conf", "/etc/tedge/mosquitto-conf/tedge-mosquitto.conf", "mosquitto"]
+}'
+```
+
+```sh te2mqtt
+tedge mqtt pub -r 'te/device/main///cmd/config_update' '{
+  "types": ["tedge-configuration-plugin", "tedge.toml", "/etc/tedge/mosquitto-conf/c8y-bridge.conf", "/etc/tedge/mosquitto-conf/tedge-mosquitto.conf", "mosquitto"]
+}'
+```
+
+:::note
+* The file `/etc/tedge/plugins/tedge-configuration-plugin.toml` itself doesn't need to be listed.
+  This is implied, so the list can *always* be configured from the configuration plugin.
+  The `type` for this self configuration file is `tedge-configuration-plugin`.
+* If the file `/etc/tedge/plugins/tedge-configuration-plugin.toml`
+  is not found, empty, ill-formed or not-readable
+  then only `tedge-configuration-plugin.toml` is declared as a supported configuration type.
+:::
+
+The behavior of the `tedge-configuration-plugin` is also controlled
+by the configuration of thin-edge:
+
+* `tedge config get mqtt.bind.address`: the address of the local MQTT bus.
+* `tedge config get mqtt.bind.port`: the TCP port of the local MQTT bus.
+* `tedge config get root.topic`: the root of the [MQTT topic scheme](mqtt-api.md) to publish and subscribe.
+* `tedge config get device.topic`: the identifier of the [MQTT topic scheme](mqtt-api.md) to publish and subscribe.
+
+## Handling config snapshot commands
+
+During a config snapshot operation, the plugin uploads a requested configuration file to the tedge file transfer repository.
+
+The plugin subscribes to config snapshot commands on the `<root>/<identifier>/cmd/config_snapshot/+` MQTT topics.
+For example, it subscribes to the following topic for the main device.
+
+```sh te2mqtt
+tedge mqtt sub 'te/device/main///cmd/config_snapshot/+'
+```
+
+When a new config snapshot command with the ID "1234" is published by another component, it appears as follows.
+
+```sh te2mqtt
+tedge mqtt pub -r 'te/device/main///cmd/config_snapshot/1234' '{
+  "status": "init",
+  "tedgeUrl": "http://127.0.0.1:8000/tedge/file-transfer/main/config_snapshot/mosquitto-1234",
+  "type": "mosquitto"
+}'
+```
+
+Upon receiving a configuration snapshot command, the plugin performs the following actions:
+   1. The plugin uses the `type` information (`mosquitto`) to look up the target path from the `tedge-configuration-plugin.toml` file
+   and retrieves the requested configuration content from the corresponding `path`(`/etc/mosquitto/mosquitto.conf`).
+   2. It then performs a `PUT` request to the `tedgeUrl` specified in the command's payload to upload the content.
+
+Throughout the process, the plugin updates the command status via MQTT by publishing a retained message
+to the same `<root>/<identifier>/cmd/config_snapshot/<id>` topic where the command is received.
+The payload contains all the received data along with the `path` information.
+
+When the plugin receives a new config snapshot command, it updates the status to `executing`.
+After successfully uploading the file to the file transfer repository,
+the plugin updates the status to `successful`.
+If any unexpected error occurs, the plugin updates the status to `failed`,
+providing a comprehensive `reason` for the failure.
+
+As a result, the operation status update message for the example above looks like this:
+
+```sh te2mqtt
+tedge mqtt pub -r 'te/device/main///cmd/config_snapshot/1234' '{
+  "status": "successful",
+  "tedgeUrl": "http://127.0.0.1:8000/tedge/file-transfer/main/config_snapshot/mosquitto-1234",
+  "type": "mosquitto",
+  "path": "/etc/mosquitto/mosquitto.conf"
+}'
+```
+
+### Flow
+
+```mermaid
+sequenceDiagram
+  participant Mapper/others
+  participant Plugin
+  participant Tedge Agent
+
+  Mapper/others->>Plugin: tedge config_snapshot command (Status: init)
+  Plugin->>Mapper/others: Status: executing
+  alt No error
+    Plugin->>Tedge Agent: File upload [HTTP]
+    Tedge Agent-->> Plugin: Status OK [HTTP]
+    Plugin->>Mapper/others: Status: successful
+  else Any error occurs
+    Plugin->>Mapper/others: Status: failed
+  end
+```
+
+## Handling config update commands
+
+During a config update operation, the plugin downloads a requested configuration file
+from the tedge file transfer repository and moves it to the target path.
+
+The plugin subscribes to config update commands on the `<root>/<identifier>/cmd/config_update/+` MQTT topics.
+For example, it subscribes to the following topic for the main device.
+
+```sh te2mqtt
+tedge mqtt sub 'te/device/main///cmd/config_update/+'
+```
+
+When a new config update command with the ID "1234" is published by another component,
+it appears as below.
+
+```sh te2mqtt
+tedge mqtt pub -r 'te/device/main///cmd/config_update/1234' '{
+  "status": "init",
+  "tedgeUrl": "http://127.0.0.1:8000/tedge/file-transfer/main/config_update/mosquitto-1234",
+  "remoteUrl": "http://www.my.url",
+  "type": "mosquitto"
+}'
+```
+
+Upon receiving a configuration update command, the plugin performs the following actions:
+   1. It performs a `GET` request to the `tedgeUrl` specified in the command to retrieve the content.
+   2. The plugin then uses the `type` information (`mosquitto`) to to look up the target path from the `tedge-configuration-plugin.toml` file
+   and applies the new configuration content to the corresponding `path`(`/etc/mosquitto/mosquitto.conf`).
+
+Throughout the process, the plugin updates the command status via MQTT
+by publishing a retained message to the same `<root>/<identifier>/cmd/config_update/<id>` topic
+where the command is received.
+The payload contains all the received data along with the `path` information.
+
+Upon receiving a new config update command, the plugin updates the status to `executing`.
+After successfully completing all operation steps, the plugin updates the status to `successful`.
+If any unexpected error occurs,
+the plugin updates the status to `failed` along with a comprehensive `reason` for the failure.
+
+As a result, the operation status update message for the example above looks like this.
+
+```sh te2mqtt
+tedge mqtt pub -r 'te/device/main///cmd/config_update/1234' '{
+  "status": "successful",
+  "tedgeUrl": "http://127.0.0.1:8000/tedge/file-transfer/main/config_update/mosquitto-1234",
+  "remoteUrl": "http://www.my.url",
+  "type": "mosquitto",
+  "path": "/etc/mosquitto/mosquitto.conf"
+}'
+```
+
+### Flow
+
+```mermaid
+sequenceDiagram
+  participant Mapper/others
+  participant Plugin
+  participant Tedge Agent
+
+  Mapper/others->>Tedge Agent: Make a target config file ready
+  Mapper/others->>Plugin: tedge config_update command (Status: init)
+  Plugin->>Mapper/others: Status: executing
+  alt No error
+    Plugin->>Tedge Agent: GET the config file [HTTP]
+    Tedge Agent-->>Plugin: Return the content [HTTP]
+    Plugin->>Plugin: Apply the config
+    Plugin->>Mapper/others: Status: successful
+  else Any error occurs
+    Plugin->>Mapper/others: Status: failed
+  end
+```
+
+## Usage
+
+```sh
+tedge-configuration-plugin --help
+```
+
+```run command="tedge-configuration-plugin --help" lang="text" title="Output"
+Thin-edge device configuration manager
+
+USAGE:
+    tedge-configuration-plugin [OPTIONS]
+
+OPTIONS:
+        --config-dir <CONFIG_DIR>
+            [default: /etc/tedge]
+
+        --debug
+            Turn-on the debug log level.
+
+            If off only reports ERROR, WARN, and INFO If on also reports DEBUG and TRACE
+
+    -h, --help
+            Print help information
+
+    -V, --version
+            Print version information
+
+On start, `tedge-configuration-plugin` notifies of its managed configuration files and
+sends this list via MQTT.
+`tedge-configuration-plugin` subscribes to the topics for the commands `config_snapshot`
+and `config_update`.
+
+The thin-edge `CONFIG_DIR` is used:
+  * to find the `tedge.toml` where the following configs are defined:
+    ** `mqtt.bind.address` and `mqtt.bind.port` to connect to the tedge MQTT broker
+    ** `root.topic` and `device.topic`: for the MQTT topics to publish to and subscribe from
+  * to find/store the `tedge-configuration-plugin.toml`: the plugin configuration file
+```
+
+## Logging
+
+The `tedge-configuration-plugin` reports progress and errors on its `stderr`.