MQTT (MQ Telemetry Transport) is a lightweight publish/subscribe messaging protocol. It is useful for use with low power sensors, but is applicable to many scenarios. For more complete information on MQTT, see http://mqtt.org/.
Skynet's MQTT server is based on the Mosca project (https://github.com/mcollina/mosca) which supports the following persistent data stores: AMQP, Redis, MongoDB, ZeroMQ or just MQTT.
MQTT defines three levels of Quality of Service (QoS). The QoS defines how hard the broker/client will try to ensure that a message is received. Messages may be sent at any QoS level, and clients may attempt to subscribe to topics at any QoS level. This means that the client chooses the maximum QoS it will receive. For example, if a message is published at QoS 2 and a client is subscribed with QoS 0, the message will be delivered to that client with QoS 0. If a second client is also subscribed to the same topic, but with QoS 2, then it will receive the same message but with QoS 2. For a second example, if a client is subscribed with QoS 2 and a message is published on QoS 0, the client will receive it on QoS 0.
Higher levels of QoS are more reliable, but involve higher latency and have higher bandwidth requirements.
0: The broker/client will deliver the message once, with no confirmation.
1: The broker/client will deliver the message at least once, with confirmation required.
2: The broker/client will deliver the message exactly once by using a four step handshake.
Skynet MQTT currently only supports QoS 0 & 1.
All messages may be set to be retained. This means that the broker will keep the message even after sending it to all current subscribers. If a new subscription is made that matches the topic of the retained message, then the message will be sent to the client. This is useful as a "last known good" mechanism. If a topic is only updated infrequently, then without a retained message, a newly subscribed client may have to wait a long time to receive an update. With a retained message, the client will receive an instant update.
On connection, a client sets the "clean session" flag, which is sometimes also known as the "clean start" flag. If clean session is set to false, then the connection is treated as durable. This means that when the client disconnects, any subscriptions it has will remain and any subsequent QoS 1 or 2 messages will be stored until it connects again in the future. If clean session is true, then all subscriptions will be removed for the client when it disconnects.
Intergrate Skynet UUID / Token security on messaging
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