RabbitMQ MQTT Adapter

This is a protocol adapter that allows MQTT-capable clients to connect to a RabbitMQ broker. The adapter translates MQTT 3.1 methods into their AMQP equivalents and back.

Announcements regarding the adapter are periodically made on the RabbitMQ mailing list and blog.

Supported MQTT 3.1 features

Enabling the Plugin

The MQTT adapter is included in the RabbitMQ distribution. To enable it, use rabbitmq-plugins:

rabbitmq-plugins enable rabbitmq_mqtt

After the plugin has been enabled, RabbitMQ needs restarting.

How it Works

RabbitMQ MQTT plugin targets MQTT 3.1 and supports a broad range of MQTT clients. It also makes it possible for MQTT clients to interoperate with AMQP 0-9-1, AMQP 1.0, and STOMP clients.

The plugin builds on top of RabbitMQ exchanges and queues. Messages published to MQTT topics use a topic exchange (amq.topic by default) internally. Subscribers consume from RabbitMQ queues bound to the topic exchange. This both enables interoperability with other protocols and makes it possible to use the Management plugin to inspect queue sizes, message rates, and so on.

Subscription Durability

MQTT 3.1 assumes two primary usage scenarios:

This section briefly covers how these scenarios map to RabbitMQ queue durability and persistence features.

Transient (QoS0) subscription use non-durable, auto-delete queues that will be deleted when the client disconnects.

Durable (QoS1) subscriptions use durable queues. Whether the queues are auto-deleted is controlled by the client's clean session flag. Clients with clean sessions use auto-deleted queues, others use non-auto-deleted ones.

For transient (QoS0) publishes, the plugin will publish messages as transient (non-persistent). Naturally, for durable (QoS1) publishes, persistent messages will be used internally.

Queues created for MQTT subscribers will have names starting with mqtt-subscription-, one per subscription QoS level. The queues will have queue TTL depending on MQTT plugin configuration.

Plugin Configuration

Here is a sample configuration that sets every MQTT option:

[{rabbit,        [{tcp_listeners,    [5672]}]},
 {rabbitmq_mqtt, [{default_user,     <<"guest">>},
                  {default_pass,     <<"guest">>},
                  {allow_anonymous,  true},
                  {vhost,            <<"/">>},
                  {exchange,         <<"amq.topic">>},
                  {subscription_ttl, 1800000},
                  {prefetch,         10},
                  {ssl_listeners,    []},
                  %% Default MQTT with TLS port is 8883
                  %% {ssl_listeners,    [8883]}
                  {tcp_listeners,    [1883]},
                  {tcp_listen_options, [binary,
                                        {packet,    raw},
                                        {reuseaddr, true},
                                        {backlog,   128},
                                        {nodelay,   true}]}]}
].

Authentication

The default_user and default_pass options are used to authenticate the adapter in case MQTT clients provide no login credentials. If the allow_anonymous option is set to false then clients MUST provide credentials. The presence of client-supplied credentials over the network overrides the allow_anonymous option. Colons may not appear in usernames.

The vhost option controls which RabbitMQ vhost the adapter connects to. The vhost configuration is only consulted if no vhost is provided during connection establishment. You can optionally specify a vhost while connecting, by prepending the vhost to the username and separating with a colon. For example, connecting with /:guest is equivalent to the default vhost and username.

Host and Port

The tcp_listeners and tcp_listen_options options are interpreted in the same way as the corresponding options in the rabbit section, as explained in the broker configuration documentation.

TLS/SSL

The ssl_listeners option in the rabbitmq_mqtt config section controls the endpoint (if any) that the adapter accepts SSL connections on. The default MQTT SSL port is 8883. If this option is non-empty then the rabbit section of the configuration file must contain an ssl_options entry:

[{rabbit,        [
                  {ssl_options, [{cacertfile, "/path/to/tls/ca/cacert.pem"},
                                 {certfile,   "/path/to/tls/server/cert.pem"},
                                 {keyfile,    "/path/to/tls/server/key.pem"},
                                 {verify,     verify_peer},
                                 {fail_if_no_peer_cert, true}]}
                 ]},
 {rabbitmq_mqtt, [
                  {ssl_listeners,    [8883]}
                  {tcp_listeners,    [1883]}
                  ]}
].

See the SSL configuration guide for details.

Session Stickiness (Clean and Non-clean Sessions)

The subscription_ttl option controls the lifetime of non-clean sessions. This option is interpreted in the same way as the queue TTL parameter, so the value 1800000 means 30 minutes.

The prefetch option controls the maximum number of unacknowledged messages that will be delivered. This option is interpreted in the same way as the AMQP prefetch-count field, so a value of 0 means "no limit".

Custom Exchanges

The exchange option determines which exchange messages from MQTT clients are published to. If a non-default exchange is chosen then it must be created before clients publish any messages. The exchange is expected to be an AMQP topic exchange.

Limitations

Overlapping Subscriptions

Overlapping subscriptions from the same client (e.g. /sports/football/epl/# and /sports/football/#) can result in duplicate messages being delivered. Applications need to account for this.