I would like to have a better understanding of the behavior of this library. Specifically: let's say I have an open connection (over WSS, if this change anything) with an MQTT server. I publish a message with QoS=1. My understanding is that mqtt awaits for a PUBACK message. After the ack has been received, the done callback is called and the flow is ended. What is not clear to me is the low-level stuff: how much "time" do the library awaits for the ack? what happen if the ack doesn't come? the message is resent? the connection is closed/reopened? something else? Is this behavior tunable?
What happens when a message is published with QoS=1?
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Prior to answering your specific question I feel its worth outlining what the protocol requires (I'll highlight the key term). The MQTT 3.1.11 spec says:
The v5 spec tightens this:
So the only time the spec requires that the publish be resent is when the client reconnects. v3.1.1 does not prohibit resending at other times but I would not recommend doing this (see this answer for more info).
Looking specifically at mqtt.js I have scanned through the code and the only resend mechanism I can see is when the connection is established (backed up by this issue). So to answer your specific questions:
There is no limit; the callback is stored and called when the flow completes (for example).
Nothing. However in reality the use of TCP/IP means that if a message is not delivered then the connection should drop (and if the broker receives the message, but is unable to process it, then it should really drop the connection).
I guess you could implement a timed resend but this is unlikely to be a good idea (and doing so would breach the v5 spec). A better approach might be to drop the connection if a message is not acknowledged within a set time frame. However there really should be no need to do this.