mod_mqtt

MQTT is a machine-to-machine (M2M)/“Internet of Things” connectivity protocol. It was designed as an extremely lightweight publish/subscribe messaging transport. It is useful for connections with remote locations where a small code footprint is required and/or network bandwidth is at a premium. For example, it has been used in sensors communicating to a broker via satellite link, over occasional dial-up connections with healthcare providers, and in a range of home automation and small device scenarios

MQTT uses a simple message broker to route messages from publishers to multiple subscribers.

A quick overview of MQTT

Publish/subscribe

With MQTT messages are published to topics. All subscribers to a topic will then receive the message. A topic is a string, much like a file-path, for example: truck/0001/temperature

A subscriber can directly subscribe to a topic, or can use wildcards to subscribe to related topics. For this the wildcards + and # can be used. + matches exactly one word between the slashes, and # can be used at the end of a pattern to match all sub-topics.

Examples of subscription patterns:

• truck/0001/temperature matches the temperature publications of truck 0001.
• truck/+/temperature matches all temperature publications for all trucks.
• +/+/temperature matches all temperature publications for all trucks and other things with a temperature
• truck/0001/# matches all publishes to truck/0001 and all its sub-topics

Retained messages

A publisher can publish a retained message to a topic. When publishing all current topic subscribers will receive the message. Above that, if a new subscription is made to the topic, then all retained messages are sent to the new subscriber.

Quality of service

MQTT has three levels for the quality of message delivery. These are used when sending messages between machines. The levels are:

• Level 0: send the message, no reception acknowledgments are reported.
• Level 1: on receipt a single ack is sent back to the publisher
• Level 2: a double handshake is performed

For most communication level 0 is used.

Wills

A client can set a last will message and topic. This is a message that will be published to the topic at the moment the client is unexpectedly disconnected.

MQTT in Zotonic

Zotonic has a central MQTT message broker. Optionally clients can connect to this broker using the normal MQTT protocol.

The broker is used for internal publish/subscribe support.

Each open HTML page can also have a local (simplified) broker. The system can relay messages between the brokers on open pages and the central broker in Zotonic. In this way it is possible for HTML pages to have their own local publish/subscribe system and also subscribe or publish to topics on the central broker.

As the central broker is shared between sites it is even possible to publish/subscribe between different sites. In the future it will be possible to bridge the brokers between servers.

Predefined topics

Currently the following topics are defined:

Topics and namespaces

To make it easier to write generic software, without changing topic names, some namespace conventions and mappings are introduced.

The following topics are expanded:

Note that there are not automatic subscriptions for user topics. All subscriptions need to be added explicitly.

Access control

All topics have access control added. For this an extra #acl_mqtt{} ACL object is defined, with the actions publish and subscribe; see notification#acl_mqtt. Modules can observe the usual notification#acl_is_allowed notification to allow access to MQTT topics:

your_site.erl

observe_acl_is_allowed(#acl_is_allowed{object = #acl_mqtt{topic = [ <<"my">>, <<"topic">> ]}}, _Context) ->
    %% Allow anonymous access on this topic
    true;
observe_acl_is_allowed(#acl_is_allowed{}, _Context) ->
    undefined.

Subscribing modules

Modules can automatically subscribe to topics. This is done by adding specially named functions.

For example, the following function subscribes to the topic test/#:

-include_lib("kernel/include/logger.hrl").

-export([
    'mqtt:test/#'/2
]).

-spec 'mqtt:test/#'( map(), z:context() ) -> ok.
'mqtt:test/#'(Message, Context) ->
    ?LOG_DEBUG("mqtt:test on site ~p received ~p", [ z_context:site(Context), Message ]),
    ok.

Here Message is a map with the received MQTT message (of type publish):

#{
    type => publish,
    pool => Pool,                 % A MQTT pool (and topic tree) per site
    topic => Topic,               % Unpacked topic for the publish [ <<"foo">>, <<"bar">> ]
    topic_bindings => Bound,      % Variables bound from the topic
    message => Msg,               % The MQTT message itself
    publisher_context => PublisherContext
}

The Context is the context of the process/user that subscribed to the message. Use the publisher_context for the Context (and ACL permissions) of the publisher.

Erlang API

Subscribe a function F in a module M to a topic:

-spec subscribe(z_mqtt:topic(), mfa(), pid(), z:context()) -> ok | {error, eacces | term()}.
z_mqtt:subscribe([ <<"my">>, <<"topic">>, '#' ], {M, F, []}, self(), Context)

This will subscribe the function, with the current process (self()) as the managing process. If the process exits then the subscription is removed.

Access control applies and the result {error, eacces} will be returned if access is denied, ok will be returned on a succesful subscription.

Subscribe the current process to a topic:

-spec subscribe(z_mqtt:topic(), z:context()) -> ok | {error, eacces | term()}.
z_mqtt:subscribe(Topic, Context)

When the process stops it will automatically be unsubscribed. The process will receive messages {mqtt_msg, map()}, where the map() is like the map in the section above.

Subscribe another process to a topic:

-spec subscribe(z_mqtt:topic(), pid(), z:context()) -> ok | {error, eacces | term()}.
z_mqtt:subscribe(Topic, Pid, Context)

To unsubscribe, use z_mqtt:unsubscribe with the same arguments as during subscription.

To publish a message:

-spec publish( z_mqtt:topic(), term(), z:context() ) -> ok | {error, term()}.
z_mqtt:publish(Topic, Payload, Context)

With options (qos or retain):

-spec publish( z_mqtt:topic(), term(), z_mqtt:publish_options(), z:context() ) -> ok | {error, term()}.
z_mqtt:publish(Topic, Payload, #{ qos => 1, retain => true }, Context)

Or, with a complete MQTT message:

-spec publish( mqtt_packet_map:mqtt_packet(), z:context()) -> ok | {error, term()}.
Msg = #{
    type => publish,
    qos => 0,
    topic => [ <<"~client">>, <<"public">>, <<"hello">> ],
    payload => #{ key => 1, foo => <<"bar">> }
},
z_mqtt:publish(Msg, Context)

JavaScript API

Every browser page has a local Cotonic broker. Use cotonic.broker.subscribe and cotonic.broker.publish for messages within that browser context:

cotonic.broker.subscribe("example/+name", function (msg, bindings) {
    console.log(bindings.name, msg.payload);
});

cotonic.broker.publish("example/world", { greeting: "Hello" });

The browser broker is connected to the Zotonic server through the origin bridge. Prefix a server-side topic with bridge/origin/ when publishing or subscribing in the browser. For example, this subscribes to the server topic public/hello:

cotonic.broker.subscribe("bridge/origin/public/hello", function (msg) {
    console.log(msg.payload);
});

Messages sent by Erlang to a ~client topic are routed back to that client. This publishes to the browser-local topic example/notice belonging to the current client:

z_mqtt:publish(
    [ <<"~client">>, <<"example">>, <<"notice">> ],
    #{ text => <<"Updated">> },
    Context
).

subscribe accepts options including qos; publish accepts qos and retain. QoS defaults to 0. Access control is checked on the server after bridge topic mappings have been applied.

Enabling the MQTT listener

MQTT can listen on a port for incoming connections. Per default the listener is enabled.

Configuration

The MQTT listener is configured in the zotonic.config. Use bin/zotonic configfiles to see where this file is located.

If this file is missing then it can be copied from ~apps/zotonic_launcher/priv/zotonic.config.in.

Per default it listens on MQTT port 1883 and MQTT with TLS on port 8883:

%%% IP address for MQTT connections - defaults to 'listen_ip'
%%% Use 'none' to disable.
   %% {mqtt_listen_ip, any},

%%% IPv6 address for MQTT connections - defaults to 'listen_ip6'
%%% Use 'none' to disable.
   %% {mqtt_listen_ip6, any},

%%% Port number for MQTT connections
   %% {mqtt_listen_port, 1883},

%%% Port number for MQTT ssl connections
   %% {mqtt_listen_ssl_port, 8883},

Authentication

All connections must authenticate using a username and password. The username is prefixed with the hostname of the user’s site, for example: foobar.com:myusername. In this way Zotonic knows which site the user belongs to.

If no matching site can be found, or if no hostname is given, then Zotonic will try to authenticate against the default site.

Accepted Events

This module handles the following notifier callbacks:

• observe_acl_is_allowed: Apply module ACL checks for the requested action/object.
• observe_action_event_type: Register the {mqtt, ...} action event type so wire actions can subscribe through MQTT.
• observe_module_activate: On module activation, scan the module for exported mqtt: observer callbacks and subscribe them as MQTT handlers using a sudo context.