What you don't want to do is read everything offered into memory faster than you can write it to disk / database. You could potentially fill the memory until you run out and crash. Instead control the flow by not reading until you are finished with the previous message (or better, allow N unacknowledged messages to be read, where N is just enough so that the application is as fast as it can be).
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That's the normal use case for QoS 1 and 2 messages: the client must acknowledge each received message otherwise the server will try to resend it. From the MQTT V3.1 Protocol Spec: http://public.dhe.ibm.com/software/dw/webservices/ws-mqtt/mq...
What you don't want to do is read everything offered into memory faster than you can write it to disk / database. You could potentially fill the memory until you run out and crash. Instead control the flow by not reading until you are finished with the previous message (or better, allow N unacknowledged messages to be read, where N is just enough so that the application is as fast as it can be).