All clients are required to have a client name or ID. The client name is used by the MQTT broker to track subscriptions. Client names must also be unique. If you attempt to connect to an MQTT broker with the same name as an existing client, then the existing client connection is dropped. Because most MQTT clients will attempt to reconnect following a disconnect, this can result in a loop of disconnect and connect.
MQTT clients by default establish a clean session with a broker. A clean session is one in which the broker isn't expected to remember client subscriptions and may hold undelivered messages for the client.
The idea of the last will message is to notify a subscriber that the publisher is unavailable due to network outage. The last will message is set by the publishing client, and is set on a per topic basis which means that each topic can have its own last will message. The message is stored on the broker and sent to any subscribing client (to that topic) if the connection to the publisher fails.
Topics are structured in a hierarchy similar to folders and files in a file system using forward slashes. Topic names are
- Case sensitive
- UTF-8 strings
- Must consist of at least one character to be valid
There are no topics created on a broker by default, except for the $SYS topic. All topics are created by a subscribing or publishing client, and they are not permanent. A topic only exists if a client has subscribed to it, or a broker has a retained or last will messages stored for that topic.
The $SYS is a reserved topic and is used by most MQTT brokers to publish information about the broker. They are
read-only topics for the MQTT clients.
http://www.steves-internet-guide.com/mqtt-security-mechanisms/