MQTT

MQTT is a light weight, client to server, publish / subscribe messaging protocol. MQTT has been specifically designed to reduce transport overhead (and thus network traffic) and code footprint on client devices. For this reason MQTT is ideally suited to constrained devices such as sensors and actuators and is quickly becoming the defacto standard communication protocol for IoT.

Apache ActiveMQ Artemis supports MQTT v3.1.1 (and also the older v3.1 code message format). By default there are acceptor elements configured to accept MQTT connections on ports 61616 and 1883.

See the general Protocols and Interoperability chapter for details on configuring an acceptor for MQTT.

The best source of information on the MQTT protocol is in the 3.1.1 specification.

Refer to the MQTT examples for a look at some of this functionality in action.

MQTT Quality of Service

MQTT offers 3 quality of service levels.

Each message (or topic subscription) can define a quality of service that is associated with it. The quality of service level defined on a topic is the maximum level a client is willing to accept. The quality of service level on a message is the desired quality of service level for this message. The broker will attempt to deliver messages to subscribers at the highest quality of service level based on what is defined on the message and topic subscription.

Each quality of service level offers a level of guarantee by which a message is sent or received:

• QoS 0: AT MOST ONCE

  Guarantees that a particular message is only ever received by the subscriber a maximum of one time. This does mean that the message may never arrive. The sender and the receiver will attempt to deliver the message, but if something fails and the message does not reach its destination (say due to a network connection) the message may be lost. This QoS has the least network traffic overhead and the least burden on the client and the broker and is often useful for telemetry data where it doesn't matter if some of the data is lost.

• QoS 1: AT LEAST ONCE

  Guarantees that a message will reach its intended recipient one or more times. The sender will continue to send the message until it receives an acknowledgment from the recipient, confirming it has received the message. The result of this QoS is that the recipient may receive the message multiple times, and also increases the network overhead than QoS 0, (due to acks). In addition more burden is placed on the sender as it needs to store the message and retry should it fail to receive an ack in a reasonable time.

• QoS 2: EXACTLY ONCE

  The most costly of the QoS (in terms of network traffic and burden on sender and receiver) this QoS will ensure that the message is received by a recipient exactly one time. This ensures that the receiver never gets any duplicate copies of the message and will eventually get it, but at the extra cost of network overhead and complexity required on the sender and receiver.

MQTT Retain Messages

MQTT has an interesting feature in which messages can be "retained" for a particular address. This means that once a retain message has been sent to an address, any new subscribers to that address will receive the last sent retain message before any others messages, this happens even if the retained message was sent before a client has connected or subscribed. An example of where this feature might be useful is in environments such as IoT where devices need to quickly get the current state of a system when they are on boarded into a system.

Will Messages

A will message can be sent when a client initially connects to a broker. Clients are able to set a "will message" as part of the connect packet. If the client abnormally disconnects, say due to a device or network failure the broker will proceed to publish the will message to the specified address (as defined also in the connect packet). Other subscribers to the will topic will receive the will message and can react accordingly. This feature can be useful in an IoT style scenario to detect errors across a potentially large scale deployment of devices.

Debug Logging

Detailed protocol logging (e.g. packets in/out) can be activated via the following steps:

1. Open <ARTEMIS_INSTANCE>/etc/logging.properties

2. Add org.apache.activemq.artemis.core.protocol.mqtt to the loggers list.

3. Add this line to enable TRACE logging for this new logger: logger.org.apache.activemq.artemis.core.protocol.mqtt.level=TRACE

4. Ensure the level for the handler you want to log the message doesn't block the TRACE logging. For example, modify the level of the CONSOLE handler like so: handler.CONSOLE.level=TRACE.

The MQTT specification doesn't dictate the format of the payloads which clients publish. As far as the broker is concerned a payload is just an array of bytes. However, to facilitate logging the broker will encode the payloads as UTF-8 strings and print them up to 256 characters. Payload logging is limited to avoid filling the logs with potentially hundreds of megabytes of unhelpful information.

Wild card subscriptions

MQTT addresses are hierarchical much like a file system, and they use a special character (i.e. / by default) to separate hierarchical levels. Subscribers are able to subscribe to specific topics or to whole branches of a hierarchy.

To subscribe to branches of an address hierarchy a subscriber can use wild cards. These wild cards (including the aforementioned separator) are configurable. See the Wildcard Syntax chapter for details about how to configure custom wild cards.

There are 2 types of wild cards in MQTT:

• Multi level (# by default)

  Adding this wild card to an address would match all branches of the address hierarchy under a specified node. For example: /uk/# Would match /uk/cities, /uk/cities/newcastle and also /uk/rivers/tyne. Subscribing to an address # would result in subscribing to all topics in the broker. This can be useful, but should be done so with care since it has significant performance implications.

• Single level (+ by default)

  Matches a single level in the address hierarchy. For example /uk/+/stores would match /uk/newcastle/stores but not /uk/cities/newcastle/stores.

Web Sockets

Apache ActiveMQ Artemis also supports MQTT over Web Sockets. Modern web browsers which support Web Sockets can send and receive MQTT messages.

MQTT over Web Sockets is supported via a normal MQTT acceptor:

<acceptor name="mqtt-ws-acceptor">tcp://localhost:1883?protocols=MQTT</acceptor>

With this configuration, Apache ActiveMQ Artemis will accept MQTT connections over Web Sockets on the port 1883. Web browsers can then connect to ws://<server>:1883 using a Web Socket to send and receive MQTT messages.

Automatic Subscription Clean-up

Sometimes MQTT clients don't clean up their subscriptions. In such situations the auto-delete-queues-delay and auto-delete-queues-message-count address-settings can be used to clean up the abandoned subscription queues. However, the MQTT session meta-data is still present in memory and needs to be cleaned up as well. The URL parameter defaultMqttSessionExpiryInterval can be configured on the MQTT acceptor to deal with this situation.

The default defaultMqttSessionExpiryInterval is -1 which means no session state will be expired. Otherwise it represents the number of milliseconds which must elapse after the client has disconnected before the broker will remove the session state.

MQTT session state is scanned every 5 seconds.