[OASIS Issue Tracker] (MQTT-257) Flow Control

[OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org>]

    [ https://issues.oasis-open.org/browse/MQTT-257?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=59220#comment-59220 ] 

Raphael Cohen commented on MQTT-257:
------------------------------------

I've certainly seen real-world problems over the years with brokers sending more messages than a client can accept (mostly due to shockingly naive client code, eg logging every message with a naff library like log4j that uses to sync on every log call)! That said, flow control is difficult for most middle-ground developers to understand and manage, and it's not striaghtforward to implement. If we adopt shared subscriptions, then the problem is very much mitigated - a consumer can just add another processor connection for messages.

I do think we need to say something about the inflight window. It confuses a lot of people.

> Flow Control
> ------------
>
>                 Key: MQTT-257
>                 URL: https://issues.oasis-open.org/browse/MQTT-257
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: Bug
>          Components: futures
>            Reporter: Peter Niblett
>
> Developers sometimes ask if there's a way to slow down an MQTT message sender to stop it sending PUBLISH packets faster than the receiver (be it a server or client) is able to process them.
> The receiver can apply back pressure at the TCP/IP level (if it's a TCP/IP transport) or - for QOS 1 and QOS2 - it can delay sending acks so that eventually the sender's inflight message window will fill up, but these mechanisms are a bit crude, and the receiver has no in-band way of letting the sender know what rate is acceptable to it. 
> We need to decide whether this is a problem that affects a significant number of existing or envisaged real-life applications (as opposed to performance and stress tests)



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