emqx4.3/doc/protocol.md

MQTT Protocol Guide

Server or Broker

A program or device that acts as an intermediary between Clients which publish Application Messages and Clients which have made Subscriptions.

A Server Accepts Network Connections from Clients.
Accepts Application Messages published by Clients. Processes Subscribe and Unsubscribe requests from Clients.
Forwards Application Messages that match Client Subscriptions.

Client ----> Broker(Server) ----> Client

Publisher ----> Broker -----> Subscriber

Subscription and Session

Subscription

A Subscription comprises a Topic Filter and a maximum QoS. A Subscription is associated with a single Session. A Session can contain more than one Subscription. Each Subscription within a session has a different Topic Filter.

Session

A stateful interaction between a Client and a Server. Some Sessions last only as long as the Network

Connection, others can span multiple consecutive Network Connections between a Client and a Server.

Topic Name and Filter

An expression contained in a Subscription, to indicate an interest in one or more topics. A Topic Filter can include wildcard characters.

Packet Identifier