[zeromq-dev] Get a list of all topics

Carol Rice carol.rice08 at gmail.com
Wed Sep 19 01:03:07 CEST 2018

Thanks for the reply, Bill.

I've read the zguide  http://zguide.zeromq.org/page:
and it appears this is exactly what xpub and xsub does:

" We need XPUB and XSUB sockets because ZeroMQ does subscription forwarding
from subscribers to publishers. XSUB and XPUB are exactly like SUB and PUB
except they expose subscriptions as special messages. The proxy has to
forward these subscription messages from subscriber side to publisher side,
by reading them from the XPUB socket and writing them to the XSUB socket.
This is the main use case for XSUB and XPUB."

However, there is no example code for this is the zguide. Are there any
examples in C code for using xpub/xsub for exposing subscriptions as
special messages? It sounds like this is what I'm looking for, to see a
list of all the subscribed topics, or subscriptions.


On Tue, Sep 18, 2018 at 2:19 PM, Bill Torpey <wallstprog at gmail.com> wrote:

> Hi Carol:
> ZeroMQ actually doesn’t have the concept of “topics”, at least in the way
> that term is used with other middlewares.
> ZeroMQ does filtering on the prefix of a message, which is a bit of a
> different animal.  You can use that to create a topic-based addressing
> scheme similar to other middlewares, but it’s not an intrinsic part of
> ZeroMQ.
> When a SUB socket connects to a PUB socket, the list of prefixes that the
> SUB wants to receive are sent to the PUB socket as part of the connection
> process.  Then, when the PUB socket wants to send a message to a SUB
> socket, it compares the list of prefixes it maintains for the SUB socket
> against the beginning of the message.  If there’s a match, the message is
> sent, otherwise it is discarded.  (That list can change over time as well,
> and the SUB sends any changes to all the PUBs that it is connected to).
> If you follow that through, you can see that to get a list of all topics
> that are being subscribed to, you’d need to ask every subscriber and
> collect the results.  Publishers, on the other hand, are not associated
> with topics — the “topic” (actually filter) is associated with the SUB
> socket, although it is evaluated by the PUB socket when it has a message to
> send.  (The exception is when using a pgm: transport — since pgm is
> multicast, PUB sockets publish everything and it is the SUB socket that
> does the prefix matching when it receives a message).
> If you really need to know which “topics” exist on the network, you
> probably want to use XPUB/XSUB sockets (which expose the filters to the
> application), and perhaps also a central ZeroMQ broker.  The
> http://zguide.zeromq.org/page:all#The-Dynamic-Discovery-Problem chapter
> is a good starting point for learning about how that might work.
> Hope this helps.
> Bill
> > On Sep 18, 2018, at 3:25 PM, Carol Rice <carol.rice08 at gmail.com> wrote:
> >
> > I know for pub/sub sockets, I can set the subscriber filter to an empty
> string and receive messages from all topics.
> >
> > But I want to know if there is a way I can publish all available topics
> and receive a list of all the available topics to subscribe to?
> >
> > Thank you for your help.
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