Initial commit: Elixir MQTT system stats daemon

- MQTT client with configurable broker connection
- Periodic system stats publishing (30s interval)
- Command listening on MQTT topic with logging
- Systemd service configuration
- NixOS module for declarative deployment

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

.formatter.exs

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+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

.gitignore

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+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+system_stats_daemon-*.tar
+
+# Temporary files, for example, from tests.
+/tmp/

README.md

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+# System Stats Daemon
+
+An Elixir application that runs as a systemd daemon to:
+1. Publish system stats to MQTT every 30 seconds
+2. Listen for commands over MQTT and log them to syslog
+
+## Configuration
+
+Edit `config/config.exs` to configure MQTT connection:
+
+```elixir
+config :system_stats_daemon, SystemStatsDaemon.MqttClient,
+  host: "localhost",
+  port: 1883,
+  client_id: "system_stats_daemon",
+  username: nil,
+  password: nil,
+  stats_topic: "system/stats",
+  command_topic: "system/commands",
+  publish_interval: 30_000
+```
+
+## Building
+
+```bash
+mix deps.get
+mix compile
+```
+
+## Running
+
+```bash
+# Development
+mix run --no-halt
+
+# Production release
+MIX_ENV=prod mix release
+_build/prod/rel/system_stats_daemon/bin/system_stats_daemon start
+```
+
+## Systemd Installation
+
+1. Build production release
+2. Copy binary to `/usr/local/bin/`
+3. Copy `system_stats_daemon.service` to `/etc/systemd/system/`
+4. Enable and start:
+
+```bash
+sudo systemctl enable system_stats_daemon
+sudo systemctl start system_stats_daemon
+```
+
+## Features
+
+- Publishes "Hello world" stats every 30 seconds to `system/stats` topic
+- Listens on `system/commands` topic and logs received messages
+- Configurable MQTT connection settings
+- Runs as systemd daemon with auto-restart
+- Logs to system journal
+

config/config.exs

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+import Config
+
+config :system_stats_daemon, SystemStatsDaemon.MqttClient,
+  host: "mqtt.home",
+  port: 1883,
+  client_id: "system_stats_daemon",
+  username: "mqtt",
+  password: "pleasework",
+  stats_topic: "system/stats",
+  command_topic: "system/commands",
+  publish_interval: 30_000
+
+config :logger, :console,
+  format: "$time $metadata[$level] $message\n",
+  metadata: [:request_id]

lib/system_stats_daemon.ex

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+defmodule SystemStatsDaemon do
+  @moduledoc """
+  Documentation for `SystemStatsDaemon`.
+  """
+
+  @doc """
+  Hello world.
+
+  ## Examples
+
+      iex> SystemStatsDaemon.hello()
+      :world
+
+  """
+  def hello do
+    :world
+  end
+end

lib/system_stats_daemon/application.ex

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+defmodule SystemStatsDaemon.Application do
+  # See https://hexdocs.pm/elixir/Application.html
+  # for more information on OTP Applications
+  @moduledoc false
+
+  use Application
+
+  @impl true
+  def start(_type, _args) do
+    children = [
+      {SystemStatsDaemon.MqttClient, []}
+    ]
+
+    # See https://hexdocs.pm/elixir/Supervisor.html
+    # for other strategies and supported options
+    opts = [strategy: :one_for_one, name: SystemStatsDaemon.Supervisor]
+    Supervisor.start_link(children, opts)
+  end
+end

lib/system_stats_daemon/mqtt_client.ex

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+defmodule SystemStatsDaemon.MqttClient do
+  use GenServer
+  require Logger
+
+  @moduledoc """
+  MQTT client for publishing system stats and handling commands
+  """
+
+  def start_link(opts) do
+    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
+  end
+
+  def init(_opts) do
+    config = Application.get_env(:system_stats_daemon, __MODULE__)
+    Logger.info("Starting MQTT client with config: #{inspect(config)}")
+    
+    # Use a unique client ID to avoid conflicts
+    client_id = "#{config[:client_id]}_#{:rand.uniform(1000)}"
+    
+    connection_opts = [
+      client_id: client_id,
+      server: {Tortoise.Transport.Tcp, host: to_charlist(config[:host]), port: config[:port]},
+      handler: {Tortoise.Handler.Logger, []},
+      user_name: config[:username],
+      password: config[:password],
+      subscriptions: [{config[:command_topic], 0}]
+    ]
+
+    case Tortoise.Connection.start_link(connection_opts) do
+      {:ok, _pid} ->
+        Logger.info("MQTT client connected successfully")
+        
+        # Send immediate hello message
+        hello_msg = %{
+          message: "Hello - system_stats_daemon started",
+          timestamp: DateTime.utc_now() |> DateTime.to_iso8601(),
+          hostname: get_hostname()
+        }
+        Tortoise.publish(client_id, config[:stats_topic], Jason.encode!(hello_msg), qos: 0)
+        
+        schedule_stats_publish(config[:publish_interval])
+        {:ok, %{config: config, client_id: client_id}}
+        
+      {:error, reason} ->
+        Logger.error("Failed to connect to MQTT broker: #{inspect(reason)}")
+        {:stop, reason}
+    end
+  end
+
+  def handle_info(:publish_stats, state) do
+    publish_stats(state.config, state.client_id)
+    schedule_stats_publish(state.config[:publish_interval])
+    {:noreply, state}
+  end
+
+  def handle_info(_msg, state) do
+    {:noreply, state}
+  end
+
+  def terminate(reason, state) do
+    Logger.info("MQTT client terminating: #{inspect(reason)}")
+    :ok
+  end
+
+  defp publish_stats(config, client_id) do
+    stats = %{
+      message: "Hello from system_stats_daemon",
+      timestamp: DateTime.utc_now() |> DateTime.to_iso8601(),
+      hostname: get_hostname()
+    }
+    
+    payload = Jason.encode!(stats)
+    
+    case Tortoise.publish(client_id, config[:stats_topic], payload, qos: 0) do
+      :ok ->
+        Logger.info("Published stats: #{payload}")
+      {:error, reason} ->
+        Logger.error("Failed to publish stats: #{inspect(reason)}")
+    end
+  end
+
+  defp schedule_stats_publish(interval) do
+    Process.send_after(self(), :publish_stats, interval)
+  end
+
+  defp get_hostname do
+    case :inet.gethostname() do
+      {:ok, hostname} -> List.to_string(hostname)
+      _ -> "unknown"
+    end
+  end
+end

mix.exs

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+defmodule SystemStatsDaemon.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :system_stats_daemon,
+      version: "0.1.0",
+      elixir: "~> 1.18",
+      start_permanent: Mix.env() == :prod,
+      deps: deps(),
+      releases: releases()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger],
+      mod: {SystemStatsDaemon.Application, []}
+    ]
+  end
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      {:tortoise, "~> 0.9.5"},
+      {:jason, "~> 1.4"}
+    ]
+  end
+
+  defp releases do
+    [
+      system_stats_daemon: [
+        include_executables_for: [:unix],
+        applications: [runtime_tools: :permanent]
+      ]
+    ]
+  end
+end

mix.lock

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+%{
+  "cowlib": {:hex, :cowlib, "2.13.0", "db8f7505d8332d98ef50a3ef34b34c1afddec7506e4ee4dd4a3a266285d282ca", [:make, :rebar3], [], "hexpm", "e1e1284dc3fc030a64b1ad0d8382ae7e99da46c3246b815318a4b848873800a4"},
+  "emqtt": {:hex, :emqtt, "1.14.4", "f34fd1e612e3138e61e9a2d27b0f9674e1da87cc794d30b7916d96f6ee7eef71", [:rebar3], [{:cowlib, "2.13.0", [hex: :cowlib, repo: "hexpm", optional: false]}, {:getopt, "1.0.3", [hex: :getopt, repo: "hexpm", optional: false]}, {:gun, "2.1.0", [hex: :gun, repo: "hexpm", optional: false]}], "hexpm", "9065ba581ea899fde316b7eafd03f3c945044c151480bf3adabc6b62b0e60dad"},
+  "gen_state_machine": {:hex, :gen_state_machine, "3.0.0", "1e57f86a494e5c6b14137ebef26a7eb342b3b0070c7135f2d6768ed3f6b6cdff", [:mix], [], "hexpm", "0a59652574bebceb7309f6b749d2a41b45fdeda8dbb4da0791e355dd19f0ed15"},
+  "getopt": {:hex, :getopt, "1.0.3", "4f3320c1f6f26b2bec0f6c6446b943eb927a1e6428ea279a1c6c534906ee79f1", [:rebar3], [], "hexpm", "7e01de90ac540f21494ff72792b1e3162d399966ebbfc674b4ce52cb8f49324f"},
+  "gun": {:hex, :gun, "2.1.0", "b4e4cbbf3026d21981c447e9e7ca856766046eff693720ba43114d7f5de36e87", [:make, :rebar3], [{:cowlib, "2.13.0", [hex: :cowlib, repo: "hexpm", optional: false]}], "hexpm", "52fc7fc246bfc3b00e01aea1c2854c70a366348574ab50c57dfe796d24a0101d"},
+  "jason": {:hex, :jason, "1.4.4", "b9226785a9aa77b6857ca22832cffa5d5011a667207eb2a0ad56adb5db443b8a", [:mix], [{:decimal, "~> 1.0 or ~> 2.0", [hex: :decimal, repo: "hexpm", optional: true]}], "hexpm", "c5eb0cab91f094599f94d55bc63409236a8ec69a21a67814529e8d5f6cc90b3b"},
+  "syslog": {:hex, :syslog, "1.1.0", "6419a232bea84f07b56dc575225007ffe34d9fdc91abe6f1b2f254fd71d8efc2", [:rebar3], [], "hexpm", "4c6a41373c7e20587be33ef841d3de6f3beba08519809329ecc4d27b15b659e1"},
+  "tortoise": {:hex, :tortoise, "0.9.9", "2e467570ef1d342d4de8fdc6ba3861f841054ab524080ec3d7052ee07c04501d", [:mix], [{:gen_state_machine, "~> 2.0 or ~> 3.0", [hex: :gen_state_machine, repo: "hexpm", optional: false]}], "hexpm", "4a316220b4b443c2497f42702f0c0616af3e4b2cbc6c150ebebb51657a773797"},
+}

nixos-module.nix

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+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.services.system-stats-daemon;
+in
+{
+  options.services.system-stats-daemon = {
+    enable = mkEnableOption "System Stats MQTT Daemon";
+
+    package = mkOption {
+      type = types.package;
+      description = "The system-stats-daemon package to use";
+    };
+
+    mqttHost = mkOption {
+      type = types.str;
+      default = "localhost";
+      description = "MQTT broker hostname";
+    };
+
+    mqttPort = mkOption {
+      type = types.int;
+      default = 1883;
+      description = "MQTT broker port";
+    };
+
+    mqttUsername = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = "MQTT username (null for no auth)";
+    };
+
+    mqttPassword = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = "MQTT password (null for no auth)";
+    };
+
+    statsTopic = mkOption {
+      type = types.str;
+      default = "system/stats";
+      description = "MQTT topic for publishing stats";
+    };
+
+    commandTopic = mkOption {
+      type = types.str;
+      default = "system/commands";
+      description = "MQTT topic for receiving commands";
+    };
+
+    publishInterval = mkOption {
+      type = types.int;
+      default = 30000;
+      description = "Interval between stats publications (milliseconds)";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    systemd.services.system-stats-daemon = {
+      description = "System Stats MQTT Daemon";
+      after = [ "network.target" ];
+      wantedBy = [ "multi-user.target" ];
+
+      environment = {
+        SYSTEM_STATS_MQTT_HOST = cfg.mqttHost;
+        SYSTEM_STATS_MQTT_PORT = toString cfg.mqttPort;
+        SYSTEM_STATS_MQTT_USERNAME = mkIf (cfg.mqttUsername != null) cfg.mqttUsername;
+        SYSTEM_STATS_MQTT_PASSWORD = mkIf (cfg.mqttPassword != null) cfg.mqttPassword;
+        SYSTEM_STATS_STATS_TOPIC = cfg.statsTopic;
+        SYSTEM_STATS_COMMAND_TOPIC = cfg.commandTopic;
+        SYSTEM_STATS_PUBLISH_INTERVAL = toString cfg.publishInterval;
+      };
+
+      serviceConfig = {
+        Type = "exec";
+        User = "root";
+        Group = "root";
+        ExecStart = "${cfg.package}/bin/system_stats_daemon start";
+        ExecStop = "${cfg.package}/bin/system_stats_daemon stop";
+        Restart = "always";
+        RestartSec = 5;
+        StandardOutput = "journal";
+        StandardError = "journal";
+        SyslogIdentifier = "system_stats_daemon";
+        WorkingDirectory = "${cfg.package}";
+
+        # Security settings
+        NoNewPrivileges = true;
+        PrivateTmp = true;
+        ProtectHome = true;
+        ProtectSystem = false; # Need access to system stats
+      };
+    };
+  };
+}

rel/env.sh.eex

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+#!/bin/sh
+
+# Configure environment for release
+export MIX_ENV=prod
+export RELEASE_DISTRIBUTION=none

system_stats_daemon.service

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+[Unit]
+Description=System Stats MQTT Daemon
+After=network.target
+
+[Service]
+Type=exec
+User=root
+Group=root
+ExecStart=/opt/system_stats_daemon/bin/system_stats_daemon start
+ExecStop=/opt/system_stats_daemon/bin/system_stats_daemon stop
+Restart=always
+RestartSec=5
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=system_stats_daemon
+WorkingDirectory=/opt/system_stats_daemon
+
+# Security settings - still apply restrictions where possible
+NoNewPrivileges=true
+PrivateTmp=true
+ProtectHome=true
+# Don't use ProtectSystem=strict since we need to read system stats
+ProtectSystem=false
+
+[Install]
+WantedBy=multi-user.target

test/system_stats_daemon_test.exs

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+defmodule SystemStatsDaemonTest do
+  use ExUnit.Case
+  doctest SystemStatsDaemon
+
+  test "greets the world" do
+    assert SystemStatsDaemon.hello() == :world
+  end
+end

test/test_helper.exs

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+ExUnit.start()