Can ZMQ publish message to specific client by pub-

Question

I am using pub/Sub Socket and currently the server subscribe byte[0] (all topics) while client subscribe byte[16] - a specific header as topic

However, I cannot stop client to subscribe byte[0] which can receive all other messages.

My application is a like a app game which has one single server using ZMQ as connection and many clients have a ZMQ sockets to talk with server.

What pattern or socket I should use in this case?

Thanks

Answer 1

" ... cannot stop client to subscribe byte[0] which can receive all other messages."

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Stopping a "subscribe to all" mode of the SUB client

For the ZMQ PUB/SUB Formal Communication Pattern archetype, the SUB client has to submit it's subscription request ( via zmq_setsockopt() ).

PUB-side ( a Game Server ) has got no option to do that from it's side.

There is no-subscription state right on a creation of a new SUB socket, thus an absolutely restrictive filter, thas no message pass through. ( For furhter details on methods for SUBSCRIBE / UNSUBSCRIBE ref. below )

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ZeroMQ specification details setting for this:

int zmq_setsockopt (       void   *socket,
                           int     option_name,
                     const void   *option_value,
                           size_t  option_len
                     );

Caution: only ZMQ_SUBSCRIBE
              ZMQ_UNSUBSCRIBE
              ZMQ_LINGER
         take effect immediately,
         other options are active only for subsequent socket bind/connects.

ZMQ_SUBSCRIBE: Establish message filter

The ZMQ_SUBSCRIBE option shall establish a new message filter on a ZMQ_SUB socket. Newly created ZMQ_SUB sockets shall filter out all incoming messages, therefore you should call this option to establish an initial message filter.

An empty option_value of length zero shall subscribe to all incoming messages.

A non-empty option_value shall subscribe to all messages beginning with the specified prefix.

Multiple filters may be attached to a single ZMQ_SUB socket, in which case a message shall be accepted if it matches at least one filter.

ZMQ_UNSUBSCRIBE: Remove message filter

The ZMQ_UNSUBSCRIBE option shall remove an existing message filter on a ZMQ_SUB socket. The filter specified must match an existing filter previously established with the ZMQ_SUBSCRIBE option. If the socket has several instances of the same filter attached the ZMQ_UNSUBSCRIBE option shall remove only one instance, leaving the rest in place and functional.

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How to enforce an ad-hoc, server-dictated, ZMQ_SUBSCRIBE restrictions?

This is possible via extending the messaging layer and adding a control-mode socket, that will carry server-initiated settings for the client ZMQ_SUB messages filtering.

Upon receiving a new, the server-dictated, ZMQ_SUBSCRIBE/ZMQ_UNSUBSCRIBE setting, the ZMQ_SUB client side code will simply handle that request and add zmq_setsockopt() accordingly.

FSA-driven grammars for this approach are rich of further possibilites, so will allow any Game Server / Game Community to smoothly go this way.

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What pattern or socket I should use?

ZeroMQ is rather a library of LEGO-style elements to get assembled into a bigger picture.

Expecting such a smart library to have a one-size-fits-all ninja-element is on a closer look an oxymoron.

So, to avoid a "Never-ending-story" of adding "although this ... and also that ..."

1. Review all requirements and & list features for the end-to-end scaleable solution,

2. Design a messaging concept & validate it to meet all the listed requirements & cover all features in [1]

3. Implement [2]

4. Test [3] & correct it for meeting 1:1 the end-to-end specification [1]

5. Enjoy it. You have done it end-to-end right.