[zeromq-dev] ZMQ_XPUB_VERBOSE & unsubscribe

John Muehlhausen jgm at jgm.org
Mon Nov 5 23:21:13 CET 2012


The *bold* patch below causes only subscription requests to be verbose.
 Using the patch, unsubscribe requests are not verbose, even with
ZMQ_XPUB_VERBOSE set.  (It should be noted that my fix for issue 456 was
only on the subscribe side.)

There is a use-case where only subscriptions should be verbose, from the
perspective of the publisher:

When anyone new comes: send them (and everyone) a snapshot (i.e., from the
last value cache)
When the last person leaves: release resources associated with the topic


If unsubscribe is also verbose, then we need to consider how to know when
an unsubscribe message is the "last" one.  The only way would be to count
subscriptions, then count unsubscriptions.  The problem is that a whole
slew of unsubscriptions can be represented by only one unsubscribe message,
as in the case of a socket disconnect of a xpub/xsub router.  So I don't
think counting works.

If there is a real use-case for unsubscribe to be verbose also then I'd
like to recommend a new socket option to enable this behavior.  Otherwise
I'd like to propose this patch.  Either way I'll make the appropriate issue
on Jira.  Apologies if creating an issue is the right way to start this
conversation.

-John

void zmq::xpub_t::xread_activated (pipe_t *pipe_)
{
    //  There are some subscriptions waiting. Let's process them.
    msg_t sub;
    while (pipe_->read (&sub)) {

        //  Apply the subscription to the trie.
        unsigned char *const data = (unsigned char*) sub.data ();
        const size_t size = sub.size ();
        if (size > 0 && (*data == 0 || *data == 1)) {
            bool unique;
            if (*data == 0)
                unique = subscriptions.rm (data + 1, size - 1, pipe_);
            else
                unique = subscriptions.add (data + 1, size - 1, pipe_);

            //  If the subscription is not a duplicate store it so that it
can be
            //  passed to used on next recv call.
*            // if (options.type == ZMQ_XPUB && (unique || verbose))*
*            if (options.type == ZMQ_XPUB && (unique || (*data && verbose)))
*
                pending.push_back (blob_t (data, size));
        }

        sub.close ();
    }
}
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