[zeromq-dev] Any way to get info on subscribers?

[Martin Sustrik]

Pieter,

>> This info is in principle not available. 0MQ abstracts from the notion
>> of individual connections. All you have is an opaque cloud of peers.
> 
> Does that cloud have to be opaque by definition?  For example, the
> publisher socket knows how many clients have connected to it, and
> could report this to the application via getsocketopt...

My rationale for hiding the info is that the semantics are not 
consistent across different topologies:

1. In direct PUB/SUB over TCP you get number of consuming apps.
2. With PUB/SUB over multicast you get no sane value.
3. With TCP and a device(s) in the middle you'll get more or less random 
number that has nothing to do with number of subscribing apps.

>> All in all, if you need to handle individual connections manually, you
>> should go for standard sockets rather than 0MQ.
> 
> 0MQ sockets do several useful things above TCP sockets:
> 
> * transport abstraction
> * message queuing
> * message bundling
> * automatic reconnection
> * message framing incl. multipart
> * routing patterns
> 
> Using TCP sockets forces one to recreate all this work, which is
> pretty horrid.  If one wants to use a custom routing pattern (e.g. to
> do content based routing), then 0MQ might offer "raw" sockets that do
> all the work except the routing, and allow the application to do that.
>  It still adds significant value over TCP.

Can be done. Of course, it means splitting the 0MQ codebase into two 
parts, meaning a *lot* of both design and implementation work.

Martin