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Non-standard MQTT options

Configure Non-Standard MQTT Options VerneMQ Supports.

Maximum Client Id Size

Set the maximum size for client ids, MQTT v3.1 specifies a limit of 23 characters.
max_client_id_size = 23
This option default to 23.

Maximum Topic Depth

Usually, you'll configure permissions on your topic structures using ACLs. In addition to that, topic_max_depth sets a global maximum value for topic levels. This protects the broker from clients subscribing to arbitrary deep topic levels.
topic_max_depth = 20
The default value for topic_max_depth is 10. As an example, this value will allow topics like a/b/c/d/e/f/g/h/i/k, that is 10 levels. A client running into the topic depth limit will be disconnected and an error will be logged.

Persistent Client Expiration

This option allows persistent clients (those with clean_session set to false) to be removed if they do not reconnect within a certain time frame.
This is a non-standard option. As far as the MQTT specification is concerned, persistent clients are persisted forever.
The expiration period should be an integer followed by one of h, d, w, m, y for hour, day, week, month, and year; or never:
persistent_client_expiration = 1w
This option defaults to never.

Message Size Limit

Limit the maximum publish payload size in bytes that VerneMQ allows. Messages that exceed this size won't be accepted.
max_message_size = 0
Defaults to 0, which means that all valid messages are accepted. MQTT specification imposes a maximum payload size of 268435455 bytes.