[zeromq-dev] Formally modelling the publish-subscribe pattern

John Lång john.lang at mykolab.com
Tue Nov 3 15:23:45 CET 2020


Hi,

Now I have a new question.

My distributed system alternates between occasionally sending large 
messages and frequently sending small messages. The small messages are 
exactly 128 bytes and the large messages are in the order of 12 
megabytes. It's not ultimately decided at this point if the size of the 
large messages can vary or if it they all share the same constant size. 
The small messages either contain payload data or the size of the next 
large message which should immediately follow the small one. I'm using 
this size for (re)allocating memory for storing the contents of the 
large message. As I mentioned previously, I'm using the PUB-SUB pattern.

The problem is this: What happens if process A sends the small message 
containing the size of the large message to process B, but before B has 
received this message, A already begins sending the large message. Is it 
possible that the high water mark is exceeded and thus the large message 
gets dropped? How about if A continues sending new unrelated small 
messages and there happens to be enough space for them in the queue? Do 
these messages get through to B?

Best regards,
John

John Lång kirjoitti 15.10.2020 klo 12.23:
>
> Hello,
>
> (Hopefully this message didn't get duplicated...) As part of my 
> master's thesis, I'm building a formal model for my distributed system 
> using PROMELA. The system I'm modelling uses 0MQ publish-subscribe 
> pattern. I have some questions about the pattern. I'd really 
> appreciate all answers to these questions! I'm now looking at the 
> informal specification at https://rfc.zeromq.org/spec/29/
>
> The specification mostly looks clear. However, I'm a bit confused 
> about these two bullet points:
>
>   * SHALL create a queue when initiating an outgoing connection to a
>     subscriber, and SHALL maintain the queue whether or not the
>     connection is established.
>   * SHALL create a queue when a subscriber connects to it. If this
>     subscriber disconnects, the PUB socket SHALL destroy its queue and
>     SHALL discard any messages it contains.
>
> What is the difference between initiating an outgoing connection to a 
> subscriber and a subscriber connecting to a publisher? In the C++ 
> source code of my system, a publisher binds to a PUB socket and a 
> subscriber connects to the address of a publisher. I guess this sounds 
> more like the second point, but I'm not certain.
>
> Did I understand correctly that this message queue between a publisher 
> and a subscriber works FIFO? It says that for outgoing messages, 
> "SHALL silently drop the message if the queue for a subscriber is 
> full." Does this imply that those messages that fit in the queue are 
> delivered in the order they were sent in? I take it that the queue 
> being full means that the high water mark has been exceeded.
>
> What is a binary comparison? Is it the same as bitwise comparison? To 
> me, "binary comparison" sounds like just comparing two things with 
> each other.
>
> Currently, I'm modelling my 0MQ publish-subscribe connections as 
> arrays in PROMELA. After all, the specification for the publisher says 
> that processing outgoing messages "SHALL NOT block on sending". 
> Another reason for my decision is that I have multiple publishers and 
> subscribers and a fixed message queue length, so a three-dimensional 
> array is handy for accessing the messages. Is there a way for 
> achieving sensible channel semantics for 0MQ publish-subscribe pattern?
>
> I wonder if there's already a formal specification for 0MQ 
> publish-subscribe pattern out there somewhere... I should probably do 
> some more research to see if I can find related work on this matter.
>
> Best regards,
> John Lång
>
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