James,
I appreciate the example. I am having to use C and not C++. Do you have an
example with the core C code? And not czmq, I am using capnproto-c with
zmq. I could not get czmq to work with it.
Thank you for your help
On Wed, Sep 19, 2018 at 4:52 AM, James Harvey <jamesdillonhar...@gmail.com>
wrote:
> Also keep in mind the first byte of the message will be a 0/1 to give the
> context.
>
> So something like this:
>
> zmq::message_t subscription;
> pub_sock_->recv(&subscription);
> char* data = (char*)subscription.data();
>
> // Subscriptions should start with 0x1 and unsubscriptions 0x0
> if(*data != 0x1)
> {
> // unsubscribe
> }
> else
> {
> // subscribe
> }
>
> std::string sub(data + 1, subscription.size() -1));
>
>
>
> On Wed, Sep 19, 2018 at 11:13 AM Luca Boccassi <luca.bocca...@gmail.com>
> wrote:
>
>> If you call recv on an XPUB socket you'll get the subscriptions coming
>> in - no need to use XSUB, will work with SUB too
>>
>> On Tue, 2018-09-18 at 17:03 -0600, Carol Rice wrote:
>> > Thanks for the reply, Bill.
>> >
>> > I've read the zguide http://zguide.zeromq.org/page:
>> > all#The-Dynamic-Discovery-Problem
>> > 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.
>> >
>> > Carol
>> >
>> > On Tue, Sep 18, 2018 at 2:19 PM, Bill Torpey <wallstp...@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.ric...@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.
>> > > > _______________________________________________
>> > > > zeromq-dev mailing list
>> > > > zeromq-dev@lists.zeromq.org
>> > > > https://lists.zeromq.org/mailman/listinfo/zeromq-dev
>> > >
>> > > _______________________________________________
>> > > zeromq-dev mailing list
>> > > zeromq-dev@lists.zeromq.org
>> > > https://lists.zeromq.org/mailman/listinfo/zeromq-dev
>> > >
>> >
>> > _______________________________________________
>> > zeromq-dev mailing list
>> > zeromq-dev@lists.zeromq.org
>> > https://lists.zeromq.org/mailman/listinfo/zeromq-dev
>>
>> --
>> Kind regards,
>> Luca Boccassi_______________________________________________
>> zeromq-dev mailing list
>> zeromq-dev@lists.zeromq.org
>> https://lists.zeromq.org/mailman/listinfo/zeromq-dev
>>
>
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