Set the time in seconds after a QoS=1 or QoS=2 message has been sent that VerneMQ will wait before retrying when no response is received.
retry_interval = 20
This option default to 20 seconds.
This option defines the maximum number of QoS 1 or 2 messages that can be in the process of being transmitted simultaneously.
max_inflight_messages = 20
Defaults to 20 messages, use 0 for no limit. The inflight window serves as a protection for sessions, on the incoming side.
The maximum number of messages to hold in the queue above those messages that are currently in flight. Defaults to 1000. Set to -1 for no limit. This option protects a client session from overload by dropping messages (of any QoS).
max_online_messages = 1000
Defaults to 1000 messages, use -1 for no limit. This parameter was named max_queued_messages in 0.10.*. Note that 0 will totally block message delivery from any queue!
This option specifies the maximum number of QoS 1 and 2 messages to hold in the offline queue.
max_offline_messages = 1000
Defaults to 1000 messages, use -1 for no limit, use 0 if no messages should be stored.
In contrast to the session based inflight window, max_online_messages and max_offline_messages serves as a protection of queues, on the outgoing side.