init.pp

# = Class: mosquitto
class mosquitto (
  $backends = undef,
  $hash     = undef,
  $version  = undef,
  ) {

  $pkg = "${hash}-mosquitto-${version}"

  ::nix::deploy { 'mosquitto':
    hash    => $hash,
    pkg     => 'mosquitto',
    version => $version,
  }

  service { 'mosquitto':
    ensure    => running,
    enable    => true,
    require   => [
      Nix::Deploy['mosquitto'],
      File['/etc/mosquitto/mosquitto.conf'],
      User['mosquitto'],
      Systemd::Unit_file['mosquitto.service'],
    ]
  }

  user { 'mosquitto':
    ensure     => present,
  }

  ::systemd::unit_file { 'mosquitto.service':
    content => template('mosquitto/service.erb'),
  }

  file { '/etc/mosquitto':
    ensure => directory,
    mode   => '0755',
    owner  => 'root',
    group  => 'root',
  }

  file { '/etc/mosquitto/mosquitto.conf':
    ensure  => present,
    mode    => '0644',
    owner   => 'root',
    group   => 'root',
    notify  => Service['mosquitto'],
    content => template('mosquitto/mosquitto.conf.erb'),
  }

}

service.erb

[Unit]
Description=Mosquitto MQTT Server
After=network.target

[Service]
Type=simple
User=mosquitto
ExecStart=/nix/store/<%= @pkg %>/bin/mosquitto -c /etc/mosquitto/mosquitto.conf
LimitNOFILE=1000000

Restart=always

[Install]
WantedBy=multi-user.target