[zeromq-dev] Check if message is dropped due to HWM at send in ZeroMQ PUB-SUB pattern

[Ranganath Gunawardane]

> Then do not use pub-sub but router-dealer. Well, my user case leans more towards a pub-sub pattern than req-reply. Let me explain. 
I am implementing a messaging service (a message bus) for multiple client processes to talk to each other. Every process sends messages with a particular 'to' tag. Also, every process subscribes to a unique tag to receive messages. This messaging service does not have a sense about the content of the message to implement a req-reply mechanism; that is upto the clients to handle at a 'business' level. 
Sending process do not need to worry about the receive getting it. But I at least need to tell the sending client whether they were successful in submitting the message to the messaging layer (without hitting send HWM etc.).
Because req-reply is out of question (a slow receiver cannot bog down the sender), the pub-sub seemed a better approximation. 

> From: ph at imatix.com
> Date: Tue, 30 Sep 2014 08:41:56 +0200
> To: zeromq-dev at lists.zeromq.org
> Subject: Re: [zeromq-dev] Check if message is dropped due to HWM at send in ZeroMQ PUB-SUB pattern
> 
> > I want to let my publisher know if he is dropping messages because he has hit the HWM.
> 
> Then do not use pub-sub but router-dealer. Pub-sub is designed for
> high volumes with many subscribers. There is no useful way to deal, in
> the publisher, with a single slow or blocked subscriber, and ZeroMQ
> pub-sub does not support that.
> 
> On Tue, Sep 30, 2014 at 1:36 AM, Ranganath Gunawardane
> <ranganath at outlook.com> wrote:
> > Hi All, I have implemented a message router using zmq, czmq with pub-sub
> > pattern.
> >
> > In my simple test set up, I am using a publisher-subscriber with a proxy. I
> > have a fast sender and a very slow receiver causing messages to hit HWM and
> > drop on send. My exception is that send would fail with 'message dropped'
> > error, but it is not the case. the zmq_msg_send() is not giving me any error
> > even though the messages get dropped (I can verify this by seeing gaps in
> > messages in subscriber end).
> >
> > I have looked into fixing slow subscriber problem by adding a sequence
> > number as you suggested, specially suicidal snail pattern in the guide. But
> > my use case is a bit different. I really do not care about the subscribers;
> > I want to let my publisher know if he is dropping messages because he has
> > hit the HWM.
> >
> > I gather that this is the nature of pub-sub. At the same time, from
> > documentation, it says, if I use ZMQ_DONTWAIT, and 'if the message cannot be
> > queued on the socket, function call shall fail'. Does it mean that I can use
> > this failure as a 'send failure due to buffer full'?
> >
> > Regards.
> > Ranganath
> >
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