[zeromq-dev] Question about multiple Pub/Sub sockets

[Martin Sustrik]

gonzalo diethelm wrote:
>> Yes. You are right. If there are multiple senders and multiple
> receivers
>> you have to have a forwarder process in the middle otherwise you would
>> have to manually setup all the connections between senders and
> receivers.
> 
> Ok. Any thoughts about this forwarder being a single point of failure?

It's a tradeoff. Either you have centralised connection management and 
SPoF or you have to manage connection by hand.

If you want to avoid the SPoF you can use hot-hot failover, i.e. stream 
messages through 2 instances of zmq_forwarder and discard the duplicates 
on the recevier side.

> 
>> A side note: You may want to have "event types" managed automatically,
>> guarantee the ordering etc. In such a case use a single PUB socket and
>> tag the events by event type. For instance, message containing event
> of
>> type A may look something like this:
>>
>> A|my-event-specific-data
>>
>> On the SUB side you can subscribe for a particular message types:
>>
>> zmq_setsockopt (s, ZMQ_SUBSCRIBE, "A|", 2);
>> zmq_setsockopt (s, ZMQ_SUBSCRIBE, "B|", 2);
> 
> Perfect, this was my plan to begin with. Quick question: does it matter
> if the payload after the event tag contains binary characters? I am
> using Google Protocol Buffers to marshal my data, so the payload could
> contain anything, but I will have a fixed-length printable event tag
> (such as your "A|", "B|", etc) prepended to the payload. Will it work,
> in the sense that 0MQ will match only on those first N printable bytes
> at the beginning?

No, it doesn't matter. The message is opaque binary BLOB and the 
subscription is a binary BLOB as well. You'll get the message as long as 
the subscription BLOB is found at the beginning of the message BLOB.

Martin