ExMQTT

An Elixir wrapper around the Erlang emqtt library.

Why this package?

• supports MQTT v3.0, v3.1.1, and v5.0
• supports clean_session/clean_start
• simplifies usage to just defining opts and implementing a message handler

Installation

The package can be installed by adding exmqtt to your list of dependencies in mix.exs:

def deps do
  [
    {:exmqtt, github: "NeatoRobotics/exmqtt", branch: "master"}
  ]
end

Sometimes you might have dependency issues because of the emqtt dependencies using git links and tags (see: emqx/emqtt#100), so you might need to do:

def deps do
  [
    {:exmqtt, github: "NeatoRobotics/exmqtt", branch: "master"},
    {:gun, "~> 1.3.0", override: true},
    {:cowlib, "~> 2.6.0", override: true}
  ]
end

Note: This is not available in hex, and there are no plans to do so unless emqtt starts consistently and reliably publishing to hex (they do publish to hex but not consistently and reliably).

Usage

Starting the client

You can use the GenServer or the Supervisor like so:

ExMQTT.start_link(opts)

or

ExMQTT.Supervisor.start_link(opts)

You probably just want to add either to your application's supervision tree.

Using the client

ExMQTT.publish(message, topic, qos)

ExMQTT.subscribe(topic, qos)

ExMQTT.unsubscribe(topic)

ExMQTT.publish_sync(message, topic, qos)

ExMQTT.subscribe_sync(topic, qos)

ExMQTT.unsubscribe_sync(topic)

opts

{:name, atom}
{:owner, pid}
{:message_handler, module}
{:puback_handler, module}
{:publish_handler, module}
{:disconnect_handler, module}
{:host, binary}
{:hosts, [{binary, :inet.port_number()}]}
{:port, :inet.port_number()}
{:tcp_opts, [:gen_tcp.option()]}
{:ssl, boolean}
{:ssl_opts, [:ssl.ssl_option()]}
{:ws_path, binary}
{:connect_timeout, pos_integer}
{:bridge_mode, boolean}
{:client_id, iodata}
{:clean_start, boolean}
{:username, iodata}
{:password, iodata}
{:protocol_version, 3 | 4 | 5}
{:keepalive, non_neg_integer}
{:max_inflight, pos_integer}
{:retry_interval, timeout}
{:will_topic, iodata}
{:will_payload, iodata}
{:will_retain, boolean}
{:will_qos, pos_integer}
{:will_props, %{atom => term}}
{:auto_ack, boolean}
{:ack_timeout, pos_integer}
{:force_ping, boolean}
{:properties, %{atom => term}}
{:subscriptions, [{topic :: binary, qos :: non_neg_integer}]}
{:start_when, {mfa, retry_in :: non_neg_integer}}

Note:

• The opts are mostly the same as :emqtt.option(), but they are different, so use the type defs in this library
• opts.ssl_opts are erlang's :ssl.option()
• opts.handler_functions type is defined here
• opts.start_when is for controller the GenServer's handle_continue/2 callback, so you can add an init condition. This is handy for example if you need to wait for the network to be ready before you try to connect to the MQTT broker. The value is a tuple {start_when, retry_in} where start_when is a {module, function, arguments} (MFA) tuple for a function that resolves to a boolean which determines when we actually finish init, and retry_in is the time to sleep (in ms) before we try again.

Example opts for SSL connection:

[
  host: "127.0.0.1",
  port: 8883,
  protocol_version: 5,
  ssl: true,
  client_id: "client-02",
  username: "user-01",
  password: "mysecretprivates",
  clean_start: false,
  ssl_opts: [
    cacertfile: '/etc/mqtt/certs/all-ca.crt',
    keyfile: '/etc/mqtt/certs/client.key',
    certfile: '/etc/mqtt/certs/client.crt'
  ],
  start_when: {{MyProject.Network, :connected?, []}, 2000},
  message_handler: {MyApp.MQTTMessageHandler, []},
  subscriptions: [
    {"foo/#", 1},
    {"baz/+", 0}
  ]
]

Message Handler module

defmodule MyApp.MQTTMessageHandler do
  @behaviour ExMQTT.MessageHandler

  @impl true
  def handle_message(["foo", "bar"], message, _extra) do
    # Matches on "foo/bar"
  end

  def handle_message(["foo", "bar" | _rest], message, _extra) do
    # Matches on "foo/bar/#"
  end

  def handle_message(["baz", buzz], message, _extra) do
    # Matches on "baz/+"
  end

  def handle_message(topic, message, _extra) do
    # Catch-all
  end
end