Merge #347

347: MQTT/Miniconf refactor r=ryan-summers a=ryan-summers

This PR refactors the MQTT/Miniconf API to make use of the refactored miniconf API. Routing has been moved to the `net` module.

**TODO**:
- [x] Test on hardware
- [x] Document modules
- [x] Merge dependency changes, update dependencies to use `rev`
- [x] Update `miniconf.py` to be `stabilizer.py`, update API

Co-authored-by: Ryan Summers <ryan.summers@vertigo-designs.com>

Cargo.lock

@@ -203,7 +203,7 @@ dependencies = [
 [[package]]
 name = "derive_miniconf"
 version = "0.1.0"
-source = "git+https://github.com/quartiq/miniconf.git?rev=314fa5587d#314fa5587d1aa28e1ad70106f19e30db646e9f28"
+source = "git+https://github.com/quartiq/miniconf.git?rev=c6f2b28#c6f2b28f735e27b337eaa986846536e904c6f2bd"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -416,11 +416,9 @@ dependencies = [
 [[package]]
 name = "miniconf"
 version = "0.1.0"
-source = "git+https://github.com/quartiq/miniconf.git?rev=314fa5587d#314fa5587d1aa28e1ad70106f19e30db646e9f28"
+source = "git+https://github.com/quartiq/miniconf.git?rev=c6f2b28#c6f2b28f735e27b337eaa986846536e904c6f2bd"
 dependencies = [
  "derive_miniconf",
- "heapless 0.6.1",
- "minimq",
  "serde",
  "serde-json-core",
 ]
@@ -428,7 +426,7 @@ dependencies = [
 [[package]]
 name = "minimq"
 version = "0.2.0"
-source = "git+https://github.com/quartiq/minimq.git?rev=933687c2e4b#933687c2e4bc8a4d972de9a4d1508b0b554a8b38"
+source = "git+https://github.com/quartiq/minimq.git?rev=b3f364d#b3f364d55dea35da6572f78ddb91c87bfbb453bf"
 dependencies = [
  "bit_field",
  "embedded-nal",
@@ -658,6 +656,12 @@ dependencies = [
  "semver",
 ]
 
+[[package]]
+name = "ryu"
+version = "1.0.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "71d301d4193d031abdd79ff7e3dd721168a9572ef3fe51a1517aba235bd8f86e"
+
 [[package]]
 name = "semver"
 version = "0.9.0"
@@ -684,10 +688,12 @@ dependencies = [
 
 [[package]]
 name = "serde-json-core"
-version = "0.2.0"
+version = "0.3.0"
-source = "git+https://github.com/rust-embedded-community/serde-json-core.git?rev=ee06ac91bc#ee06ac91bc43b72450a92198a00d9e5c5b9946d2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "39af17f40c2a28d2c9a7918663ddc8a10f54cc6f109ead5c3f010869761df186"
 dependencies = [
- "heapless 0.5.6",
+ "heapless 0.6.1",
+ "ryu",
  "serde",
 ]
 
@@ -742,6 +748,7 @@ dependencies = [
  "log",
  "mcp23017",
  "miniconf",
+ "minimq",
  "nb 1.0.0",
  "panic-semihosting",
  "paste",

Cargo.toml

@@ -56,19 +56,15 @@ version = "0.9.0"
 
 [patch.crates-io.miniconf]
 git = "https://github.com/quartiq/miniconf.git"
-rev = "314fa5587d"
+rev = "c6f2b28"
 
 [dependencies.smoltcp-nal]
 git = "https://github.com/quartiq/smoltcp-nal.git"
 rev = "8468f11"
 
-[patch.crates-io.minimq]
+[dependencies.minimq]
 git = "https://github.com/quartiq/minimq.git"
-rev = "933687c2e4b"
+rev = "b3f364d"
-
-[patch.crates-io.serde-json-core]
-git = "https://github.com/rust-embedded-community/serde-json-core.git"
-rev = "ee06ac91bc"
 
 [features]
 semihosting = ["panic-semihosting", "cortex-m-log/semihosting"]

miniconf.py

@@ -9,11 +9,12 @@ import argparse
 import asyncio
 import json
 import logging
+import sys
+import uuid
 
 from gmqtt import Client as MqttClient
 
-logger = logging.getLogger(__name__)
+LOGGER = logging.getLogger(__name__)
-
 
 class Miniconf:
     """An asynchronous API for controlling Miniconf devices using MQTT."""
@@ -32,27 +33,33 @@ class Miniconf:
             client: A connected MQTT5 client.
             prefix: The MQTT toptic prefix of the device to control.
         """
+        self.uuid = uuid.uuid1()
+        self.request_id = 0
         self.client = client
         self.prefix = prefix
         self.inflight = {}
         self.client.on_message = self._handle_response
-        self.client.subscribe(f'{prefix}/response/#')
+        self.client.subscribe(f'{prefix}/response/{self.uuid.hex}')
 
-    def _handle_response(self, _client, topic, payload, *_args, **_kwargs):
+    def _handle_response(self, _client, _topic, payload, _qos, properties):
         """Callback function for when messages are received over MQTT.
 
         Args:
             _client: The MQTT client.
-            topic: The topic that the message was received on.
+            _topic: The topic that the message was received on.
             payload: The payload of the message.
+            _qos: The quality-of-service level of the received packet
+            properties: A dictionary of properties associated with the message.
         """
-        if topic not in self.inflight:
+        # Extract corrleation data from the properties
-            # TODO use correlation_data to distinguish clients and requests
+        correlation_data = json.loads(properties['correlation_data'][0].decode('ascii'))
-            logger.warning('Unexpected response on topic: %s', topic)
+
-            return
+        # Get the request ID from the correlation data
+        request_id = correlation_data['request_id']
+
+        self.inflight[request_id].set_result(json.loads(payload))
+        del self.inflight[request_id]
 
-        self.inflight[topic].set_result(payload.decode('ascii'))
-        del self.inflight[topic]
 
     async def command(self, path, value):
         """Write the provided data to the specified path.
@@ -62,29 +69,37 @@ class Miniconf:
             value: The value to write to the path.
 
         Returns:
-            The received response to the command.
+            The response to the command as a dictionary.
         """
         setting_topic = f'{self.prefix}/settings/{path}'
-        response_topic = f'{self.prefix}/response/{path}'
+        response_topic = f'{self.prefix}/response/{self.uuid.hex}'
-        if response_topic in self.inflight:
+
-            # TODO use correlation_data to distinguish clients and requests
+        # Assign a unique identifier to this update request.
-            raise NotImplementedError(
+        request_id = self.request_id
-                    'Only one in-flight message per topic is supported')
+        self.request_id += 1
+        assert request_id not in self.inflight, 'Invalid ID encountered'
+
+        correlation_data = json.dumps({
+            'request_id': request_id,
+        }).encode('ascii')
 
         value = json.dumps(value)
-        logger.info('Sending %s to "%s"', value, setting_topic)
+        LOGGER.info('Sending %s to "%s"', value, setting_topic)
         fut = asyncio.get_running_loop().create_future()
-        self.inflight[response_topic] = fut
+
+        self.inflight[request_id] = fut
         self.client.publish(setting_topic, payload=value, qos=0, retain=True,
-                            response_topic=response_topic)
+                            response_topic=response_topic,
+                            correlation_data=correlation_data)
         return await fut
 
 
 def main():
+    """ Main program entry point. """
     parser = argparse.ArgumentParser(
-            description='Miniconf command line interface.',
+        description='Miniconf command line interface.',
-            formatter_class=argparse.RawDescriptionHelpFormatter,
+        formatter_class=argparse.RawDescriptionHelpFormatter,
-            epilog='''Examples:
+        epilog='''Examples:
 %(prog)s dt/sinara/stabilizer afe/0='"G2"' iir_ch/0/0=\
 '{"y_min": -32767, "y_max": 32767, "y_offset": 0, "ba": [1.0, 0, 0, 0, 0]}'
 ''')
@@ -100,19 +115,22 @@ def main():
     args = parser.parse_args()
 
     logging.basicConfig(
-            format='%(asctime)s [%(levelname)s] %(name)s: %(message)s',
+        format='%(asctime)s [%(levelname)s] %(name)s: %(message)s',
-            level=logging.WARN - 10*args.verbose)
+        level=logging.WARN - 10*args.verbose)
 
     loop = asyncio.get_event_loop()
 
     async def configure_settings():
         interface = await Miniconf.create(args.prefix, args.broker)
-        for kv in args.settings:
+        for key_value in args.settings:
-            path, value = kv.split("=", 1)
+            path, value = key_value.split("=", 1)
             response = await interface.command(path, json.loads(value))
-            print(response)
+            print(f'{path}: {response}')
+            if response['code'] != 0:
+                return response['code']
+        return 0
 
-    loop.run_until_complete(configure_settings())
+    sys.exit(loop.run_until_complete(configure_settings()))
 
 
 if __name__ == '__main__':

src/bin/dual-iir.rs

@@ -13,7 +13,7 @@ use hardware::{
     InputPin, AFE0, AFE1,
 };
 
-use net::{Action, MiniconfInterface};
+use net::{Action, MqttInterface};
 
 const SCALE: f32 = i16::MAX as _;
 
@@ -54,7 +54,7 @@ const APP: () = {
         digital_input1: DigitalInput1,
         adcs: (Adc0Input, Adc1Input),
         dacs: (Dac0Output, Dac1Output),
-        mqtt_config: MiniconfInterface<Settings>,
+        mqtt: MqttInterface<Settings>,
 
         #[init([[[0.; 5]; IIR_CASCADE_LENGTH]; 2])]
         iir_state: [[iir::Vec5; IIR_CASCADE_LENGTH]; 2],
@@ -66,7 +66,7 @@ const APP: () = {
         // Configure the microcontroller
         let (mut stabilizer, _pounder) = hardware::setup(c.core, c.device);
 
-        let mqtt_config = MiniconfInterface::new(
+        let mqtt = MqttInterface::new(
             stabilizer.net.stack,
             "",
             &net::get_device_prefix(
@@ -93,7 +93,7 @@ const APP: () = {
             afes: stabilizer.afes,
             adcs: stabilizer.adcs,
             dacs: stabilizer.dacs,
-            mqtt_config,
+            mqtt,
             digital_input1: stabilizer.digital_inputs.1,
             settings: Settings::default(),
         }
@@ -150,14 +150,10 @@ const APP: () = {
         }
     }
 
-    #[idle(resources=[mqtt_config], spawn=[settings_update])]
+    #[idle(resources=[mqtt], spawn=[settings_update])]
     fn idle(mut c: idle::Context) -> ! {
         loop {
-            match c
+            match c.resources.mqtt.lock(|mqtt| mqtt.update()) {
-                .resources
-                .mqtt_config
-                .lock(|config_interface| config_interface.update())
-            {
                 Some(Action::Sleep) => cortex_m::asm::wfi(),
                 Some(Action::UpdateSettings) => {
                     c.spawn.settings_update().unwrap()
@@ -167,9 +163,9 @@ const APP: () = {
         }
     }
 
-    #[task(priority = 1, resources=[mqtt_config, afes, settings])]
+    #[task(priority = 1, resources=[mqtt, afes, settings])]
     fn settings_update(mut c: settings_update::Context) {
-        let settings = &c.resources.mqtt_config.mqtt.settings;
+        let settings = c.resources.mqtt.settings();
 
         // Update the IIR channels.
         c.resources.settings.lock(|current| *current = *settings);

src/bin/lockin-external.rs

@@ -16,7 +16,7 @@ use stabilizer::hardware::{
 };
 
 use miniconf::Miniconf;
-use stabilizer::net::{Action, MiniconfInterface};
+use stabilizer::net::{Action, MqttInterface};
 
 #[derive(Copy, Clone, Debug, Deserialize, Miniconf)]
 enum Conf {
@@ -60,7 +60,7 @@ const APP: () = {
         afes: (AFE0, AFE1),
         adcs: (Adc0Input, Adc1Input),
         dacs: (Dac0Output, Dac1Output),
-        mqtt_config: MiniconfInterface<Settings>,
+        mqtt: MqttInterface<Settings>,
         settings: Settings,
 
         timestamper: InputStamper,
@@ -73,7 +73,7 @@ const APP: () = {
         // Configure the microcontroller
         let (mut stabilizer, _pounder) = setup(c.core, c.device);
 
-        let mqtt_config = MiniconfInterface::new(
+        let mqtt = MqttInterface::new(
             stabilizer.net.stack,
             "",
             &net::get_device_prefix(
@@ -113,7 +113,7 @@ const APP: () = {
             afes: stabilizer.afes,
             adcs: stabilizer.adcs,
             dacs: stabilizer.dacs,
-            mqtt_config,
+            mqtt,
             timestamper: stabilizer.timestamper,
 
             settings,
@@ -195,14 +195,10 @@ const APP: () = {
         }
     }
 
-    #[idle(resources=[mqtt_config], spawn=[settings_update])]
+    #[idle(resources=[mqtt], spawn=[settings_update])]
     fn idle(mut c: idle::Context) -> ! {
         loop {
-            match c
+            match c.resources.mqtt.lock(|mqtt| mqtt.update()) {
-                .resources
-                .mqtt_config
-                .lock(|config_interface| config_interface.update())
-            {
                 Some(Action::Sleep) => cortex_m::asm::wfi(),
                 Some(Action::UpdateSettings) => {
                     c.spawn.settings_update().unwrap()
@@ -212,9 +208,9 @@ const APP: () = {
         }
     }
 
-    #[task(priority = 1, resources=[mqtt_config, settings, afes])]
+    #[task(priority = 1, resources=[mqtt, settings, afes])]
     fn settings_update(mut c: settings_update::Context) {
-        let settings = &c.resources.mqtt_config.mqtt.settings;
+        let settings = c.resources.mqtt.settings();
 
         c.resources.afes.0.set_gain(settings.afe[0]);
         c.resources.afes.1.set_gain(settings.afe[1]);

src/net/messages.rs

@@ -0,0 +1,91 @@
+use heapless::{consts, String, Vec};
+use serde::Serialize;
+
+use core::fmt::Write;
+
+#[derive(Debug, Copy, Clone)]
+pub enum SettingsResponseCode {
+    NoError = 0,
+    MiniconfError = 1,
+}
+
+/// Represents a generic MQTT message.
+pub struct MqttMessage<'a> {
+    pub topic: &'a str,
+    pub message: Vec<u8, consts::U128>,
+    pub properties: Vec<minimq::Property<'a>, consts::U1>,
+}
+
+/// The payload of the MQTT response message to a settings update request.
+#[derive(Serialize)]
+pub struct SettingsResponse {
+    code: u8,
+    msg: String<heapless::consts::U64>,
+}
+
+impl<'a> MqttMessage<'a> {
+    /// Construct a new MQTT message from an incoming message.
+    ///
+    /// # Args
+    /// * `properties` - A list of properties associated with the inbound message.
+    /// * `default_response` - The default response topic for the message
+    /// * `msg` - The response associated with the message. Must fit within 128 bytes.
+    pub fn new<'b: 'a>(
+        properties: &[minimq::Property<'a>],
+        default_response: &'b str,
+        msg: &impl Serialize,
+    ) -> Self {
+        // Extract the MQTT response topic.
+        let topic = properties
+            .iter()
+            .find_map(|prop| {
+                if let minimq::Property::ResponseTopic(topic) = prop {
+                    Some(topic)
+                } else {
+                    None
+                }
+            })
+            .unwrap_or(&default_response);
+
+        // Associate any provided correlation data with the response.
+        let mut correlation_data: Vec<minimq::Property<'a>, consts::U1> =
+            Vec::new();
+        if let Some(data) = properties
+            .iter()
+            .find(|prop| matches!(prop, minimq::Property::CorrelationData(_)))
+        {
+            // Note(unwrap): Unwrap can not fail, as we only ever push one value.
+            correlation_data.push(*data).unwrap();
+        }
+
+        Self {
+            topic,
+            // Note(unwrap): All SettingsResponse objects are guaranteed to fit in the vector.
+            message: miniconf::serde_json_core::to_vec(msg).unwrap(),
+            properties: correlation_data,
+        }
+    }
+}
+
+impl From<Result<(), miniconf::Error>> for SettingsResponse {
+    fn from(result: Result<(), miniconf::Error>) -> Self {
+        match result {
+            Ok(_) => Self {
+                msg: String::from("OK"),
+                code: SettingsResponseCode::NoError as u8,
+            },
+
+            Err(error) => {
+                let mut msg = String::new();
+                if write!(&mut msg, "{:?}", error).is_err() {
+                    msg = String::from("Miniconf Error");
+                }
+
+                Self {
+                    code: SettingsResponseCode::MiniconfError as u8,
+                    msg,
+                }
+            }
+        }
+    }
+}

src/net/mod.rs

@@ -1,11 +1,18 @@
-use crate::hardware::{
+///! Stabilizer network management module
-    design_parameters::MQTT_BROKER, CycleCounter, EthernetPhy, NetworkStack,
+///!
-};
+///! # Design
+///! The stabilizer network architecture supports numerous layers to permit transmission of
+///! telemetry (via MQTT), configuration of run-time settings (via MQTT + Miniconf), and live data
+///! streaming over raw UDP/TCP sockets. This module encompasses the main processing routines
+///! related to Stabilizer networking operations.
+use heapless::{consts, String};
 
 use core::fmt::Write;
 
-use heapless::{consts, String};
+mod messages;
-use miniconf::minimq;
+mod mqtt_interface;
+use messages::{MqttMessage, SettingsResponse};
+pub use mqtt_interface::MqttInterface;
 
 /// Potential actions for firmware to take.
 pub enum Action {
@@ -16,101 +23,6 @@ pub enum Action {
     UpdateSettings,
 }
 
-/// MQTT settings interface.
-pub struct MiniconfInterface<S>
-where
-    S: miniconf::Miniconf + Default,
-{
-    pub mqtt: miniconf::MqttInterface<S, NetworkStack, minimq::consts::U256>,
-    clock: CycleCounter,
-    phy: EthernetPhy,
-    network_was_reset: bool,
-}
-
-impl<S> MiniconfInterface<S>
-where
-    S: miniconf::Miniconf + Default,
-{
-    /// Construct a new MQTT settings interface.
-    ///
-    /// # Args
-    /// * `stack` - The network stack to use for communication.
-    /// * `client_id` - The ID of the MQTT client. May be an empty string for auto-assigning.
-    /// * `prefix` - The MQTT device prefix to use for this device.
-    /// * `phy` - The PHY driver for querying the link state.
-    /// * `clock` - The clock to utilize for querying the current system time.
-    pub fn new(
-        stack: NetworkStack,
-        client_id: &str,
-        prefix: &str,
-        phy: EthernetPhy,
-        clock: CycleCounter,
-    ) -> Self {
-        let mqtt = {
-            let mqtt_client = {
-                minimq::MqttClient::new(MQTT_BROKER.into(), client_id, stack)
-                    .unwrap()
-            };
-
-            miniconf::MqttInterface::new(mqtt_client, prefix, S::default())
-                .unwrap()
-        };
-
-        Self {
-            mqtt,
-            clock,
-            phy,
-            network_was_reset: false,
-        }
-    }
-
-    /// Update the MQTT interface and service the network
-    ///
-    /// # Returns
-    /// An option containing an action that should be completed as a result of network servicing.
-    pub fn update(&mut self) -> Option<Action> {
-        let now = self.clock.current_ms();
-
-        // First, service the network stack to process and inbound and outbound traffic.
-        let sleep = match self.mqtt.network_stack().poll(now) {
-            Ok(updated) => !updated,
-            Err(err) => {
-                log::info!("Network error: {:?}", err);
-                false
-            }
-        };
-
-        // If the PHY indicates there's no more ethernet link, reset the DHCP server in the network
-        // stack.
-        if self.phy.poll_link() == false {
-            // Only reset the network stack once per link reconnection. This prevents us from
-            // sending an excessive number of DHCP requests.
-            if !self.network_was_reset {
-                self.network_was_reset = true;
-                self.mqtt.network_stack().handle_link_reset();
-            }
-        } else {
-            self.network_was_reset = false;
-        }
-
-        // Finally, service the MQTT interface and handle any necessary messages.
-        match self.mqtt.update() {
-            Ok(true) => Some(Action::UpdateSettings),
-            Ok(false) if sleep => Some(Action::Sleep),
-            Ok(_) => None,
-
-            Err(miniconf::MqttError::Network(
-                smoltcp_nal::NetworkError::NoIpAddress,
-            )) => None,
-
-            Err(error) => {
-                log::info!("Unexpected error: {:?}", error);
-                None
-            }
-        }
-    }
-}
-
 /// Get the MQTT prefix of a device.
 ///
 /// # Args

src/net/mqtt_interface.rs

@@ -0,0 +1,197 @@
+use crate::hardware::{
+    design_parameters::MQTT_BROKER, CycleCounter, EthernetPhy, NetworkStack,
+};
+
+use heapless::{consts, String};
+
+use super::{Action, MqttMessage, SettingsResponse};
+
+/// MQTT settings interface.
+pub struct MqttInterface<S>
+where
+    S: miniconf::Miniconf + Default + Clone,
+{
+    default_response_topic: String<consts::U128>,
+    mqtt: minimq::MqttClient<minimq::consts::U256, NetworkStack>,
+    settings: S,
+    clock: CycleCounter,
+    phy: EthernetPhy,
+    network_was_reset: bool,
+    subscribed: bool,
+    settings_prefix: String<consts::U64>,
+}
+
+impl<S> MqttInterface<S>
+where
+    S: miniconf::Miniconf + Default + Clone,
+{
+    /// Construct a new MQTT settings interface.
+    ///
+    /// # Args
+    /// * `stack` - The network stack to use for communication.
+    /// * `client_id` - The ID of the MQTT client. May be an empty string for auto-assigning.
+    /// * `prefix` - The MQTT device prefix to use for this device.
+    /// * `phy` - The PHY driver for querying the link state.
+    /// * `clock` - The clock to utilize for querying the current system time.
+    pub fn new(
+        stack: NetworkStack,
+        client_id: &str,
+        prefix: &str,
+        phy: EthernetPhy,
+        clock: CycleCounter,
+    ) -> Self {
+        let mqtt =
+            minimq::MqttClient::new(MQTT_BROKER.into(), client_id, stack)
+                .unwrap();
+
+        let mut response_topic: String<consts::U128> = String::from(prefix);
+        response_topic.push_str("/log").unwrap();
+
+        let mut settings_prefix: String<consts::U64> = String::from(prefix);
+        settings_prefix.push_str("/settings").unwrap();
+
+        Self {
+            mqtt,
+            settings: S::default(),
+            settings_prefix,
+            clock,
+            phy,
+            default_response_topic: response_topic,
+            network_was_reset: false,
+            subscribed: false,
+        }
+    }
+
+    /// Update the MQTT interface and service the network
+    ///
+    /// # Returns
+    /// An option containing an action that should be completed as a result of network servicing.
+    pub fn update(&mut self) -> Option<Action> {
+        // First, service the network stack to process any inbound and outbound traffic.
+        let sleep = match self.mqtt.network_stack.poll(self.clock.current_ms())
+        {
+            Ok(updated) => !updated,
+            Err(err) => {
+                log::info!("Network error: {:?}", err);
+                false
+            }
+        };
+
+        // If the PHY indicates there's no more ethernet link, reset the DHCP server in the network
+        // stack.
+        match self.phy.poll_link() {
+            true => self.network_was_reset = false,
+
+            // Only reset the network stack once per link reconnection. This prevents us from
+            // sending an excessive number of DHCP requests.
+            false if !self.network_was_reset => {
+                self.network_was_reset = true;
+                self.mqtt.network_stack.handle_link_reset();
+            }
+            _ => {}
+        };
+
+        let mqtt_connected = match self.mqtt.is_connected() {
+            Ok(connected) => connected,
+            Err(minimq::Error::Network(
+                smoltcp_nal::NetworkError::NoIpAddress,
+            )) => false,
+            Err(minimq::Error::Network(error)) => {
+                log::info!("Unexpected network error: {:?}", error);
+                false
+            }
+            Err(error) => {
+                log::warn!("Unexpected MQTT error: {:?}", error);
+                false
+            }
+        };
+
+        // If we're no longer subscribed to the settings topic, but we are connected to the broker,
+        // resubscribe.
+        if !self.subscribed && mqtt_connected {
+            // Note(unwrap): We construct a string with two more characters than the prefix
+            // strucutre, so we are guaranteed to have space for storage.
+            let mut settings_topic: String<consts::U66> =
+                String::from(self.settings_prefix.as_str());
+            settings_topic.push_str("/#").unwrap();
+
+            // We do not currently handle or process potential subscription failures. Instead, this
+            // failure will be logged through the stabilizer logging interface.
+            self.mqtt.subscribe(&settings_topic, &[]).unwrap();
+            self.subscribed = true;
+        }
+
+        // Handle any MQTT traffic.
+        let settings = &mut self.settings;
+        let mqtt = &mut self.mqtt;
+        let prefix = self.settings_prefix.as_str();
+        let default_response_topic = self.default_response_topic.as_str();
+
+        let mut update = false;
+        match mqtt.poll(|client, topic, message, properties| {
+            let path = match topic.strip_prefix(prefix) {
+                // For paths, we do not want to include the leading slash.
+                Some(path) => {
+                    if path.len() > 0 {
+                        &path[1..]
+                    } else {
+                        path
+                    }
+                }
+                None => {
+                    info!("Unexpected MQTT topic: {}", topic);
+                    return;
+                }
+            };
+
+            let message: SettingsResponse = settings
+                .string_set(path.split('/').peekable(), message)
+                .and_then(|_| {
+                    update = true;
+                    Ok(())
+                })
+                .into();
+
+            let response =
+                MqttMessage::new(properties, default_response_topic, &message);
+
+            client
+                .publish(
+                    response.topic,
+                    &response.message,
+                    // TODO: When Minimq supports more QoS levels, this should be increased to
+                    // ensure that the client has received it at least once.
+                    minimq::QoS::AtMostOnce,
+                    &response.properties,
+                )
+                .ok();
+        }) {
+            // If settings updated,
+            Ok(_) => {
+                if update {
+                    Some(Action::UpdateSettings)
+                } else if sleep {
+                    Some(Action::Sleep)
+                } else {
+                    None
+                }
+            }
+            Err(minimq::Error::Disconnected) => {
+                self.subscribed = false;
+                None
+            }
+            Err(minimq::Error::Network(
+                smoltcp_nal::NetworkError::NoIpAddress,
+            )) => None,
+
+            Err(error) => {
+                log::info!("Unexpected error: {:?}", error);
+                None
+            }
+        }
+    }
+
+    pub fn settings(&self) -> &S {
+        &self.settings
+    }
+}