1 files changed, 75 insertions, 0 deletions
diff --git a/crates/common/tedge_config/src/config_setting.rs b/crates/common/tedge_config/src/config_setting.rs
new file mode 100644
index 00000000..aa68d66a
--- /dev/null
+++ b/crates/common/tedge_config/src/config_setting.rs
@@ -0,0 +1,75 @@
+pub trait ConfigSetting {
+    /// This is something like `device.id`.
+    const KEY: &'static str;
+
+    const DESCRIPTION: &'static str;
+
+    /// The underlying value type of the configuration setting.
+    type Value;
+}
+
+pub trait ConfigSettingAccessor<T: ConfigSetting> {
+    /// Read a configuration setting
+    fn query(&self, setting: T) -> ConfigSettingResult<T::Value>;
+
+    fn query_optional(&self, setting: T) -> ConfigSettingResult<Option<T::Value>> {
+        match self.query(setting) {
+            Ok(value) => Ok(Some(value)),
+            Err(ConfigSettingError::ConfigNotSet { .. }) => Ok(None),
+            Err(err) => Err(err),
+        }
+    }
+
+    /// Update a configuration setting
+    fn update(&mut self, _setting: T, _value: T::Value) -> ConfigSettingResult<()>;
+
+    /// Unset a configuration setting / reset to default
+    fn unset(&mut self, _setting: T) -> ConfigSettingResult<()>;
+}
+
+/// Extension trait that provides methods to query a setting as a String or
+/// update a setting provided a String value.
+pub trait ConfigSettingAccessorStringExt<T: ConfigSetting>: ConfigSettingAccessor<T> {
+    /// Read a configuration setting and convert it into a String.
+    fn query_string(&self, setting: T) -> ConfigSettingResult<String>;
+
+    fn query_string_optional(&self, setting: T) -> ConfigSettingResult<Option<String>> {
+        match self.query_string(setting) {
+            Ok(value) => Ok(Some(value)),
+            Err(ConfigSettingError::ConfigNotSet { .. }) => Ok(None),
+            Err(err) => Err(err),
+        }
+    }
+
+    /// Update a configuration setting from a String value
+    fn update_string(&mut self, setting: T, value: String) -> ConfigSettingResult<()>;
+}
+
+pub type ConfigSettingResult<T> = Result<T, ConfigSettingError>;
+
+#[derive(thiserror::Error, Debug)]
+pub enum ConfigSettingError {
+    #[error(
+        r#"A value for `{key}` is missing.
+    A value can be set with `tedge config set {key} <value>`"#
+    )]
+    ConfigNotSet { key: &'static str },
+
+    #[error("Readonly setting: {message}")]
+    ReadonlySetting { message: &'static str },
+
+    #[error("Conversion from String failed")]
+    ConversionFromStringFailed,
+
+    #[error("Conversion into String failed")]
+    ConversionIntoStringFailed,
+
+    #[error("Derivation for `{key}` failed: {cause}")]
+    DerivationFailed { key: &'static str, cause: String },
+
+    #[error("Config value {key}, cannot be configured: {message} ")]
+    SettingIsNotConfigurable {
+        key: &'static str,
+        message: &'static str,
+    },
+}