How MQTT keeps a connection open even when no messages are being sent?

MQTT uses a TCP/IP connection. This connection is normally left open by the client so that is can send and receive data at any time. If no data flows over an open connection for a certain time period then the client will generate a PINGREQ and expect to receive a PINGRESP from the broker.

Does MQTT guarantee delivery?

There is no guarantee of delivery. The recipient does not acknowledge receipt of the message and the message is not stored and re-transmitted by the sender. QoS level 0 is often called “fire and forget” and provides the same guarantee as the underlying TCP protocol.

What in MQTT handles receiving published messages and sending them on to any clients who have subscribed?

When a client sends a message to an MQTT broker for publication, the broker reads the message, acknowledges the message (according to the QoS Level), and processes the message. Processing by the broker includes determining which clients have subscribed to the topic and sending the message to them.

What is the purpose of the keepalive message?

A keepalive (KA) is a message sent by one device to another to check that the link between the two is operating, or to prevent the link from being broken.

Is MQTT always connected?

The MQTT connection is always between one client and the broker. Clients never connect to each other directly. To initiate a connection, the client sends a CONNECT message to the broker.

Why is MQTT reliable?

MQTT may be a lightweight protocol, but it is used in some of the complex scenarios that demand reliable delivery of messages. Clients can configure different levels of Quality of Service (QoS) to ensure reliable message delivery. There are three levels of QoS in MQTT: QoS 0: At most once delivery.

How does MQTT Broker help in the message communication between the client and server?

An MQTT broker acts as a go-between for the clients who are sending messages and the subscribers who are receiving those messages. In a post office analogy, the broker is the post office itself. All messages have to go through the broker before they can be delivered to the subscriber.

How publisher and subscriber works in MQTT protocol?

MQTT uses the pub/sub pattern to connect interested parties with each other. It does it by decoupling the sender (publisher) with the receiver (subscriber). The publisher sends a message to a central topic which has multiple subscribers waiting to receive the message.

What happens if we publish first before subscribe to a topic?

A message published before a given subscription was created will usually not be delivered for that subscription. Thus, a message published to a topic that has no subscription will not be delivered to any subscriber.