[zeromq-dev] Point-to-Point Client/Server Request/Response

[Martin Sustrik]

Hi Alex,

> Studying the excellent design documentation. Wanted to get an 
> understandng how a conventional point-to-point conversation would be 
> mapped in 0MQ solution space with practicality left aside. Assuming a 
> number n of private (firewalled) clients each sending 'requests' to a 
> single public server and waiting for their appropriate 'responses', we 
> would have:
> 
> 1. Server with Global Queue Q-REQ
> 2. Each Client would create a Local Exchange E-REQ
> 3. Each Client would bind their E-REQ to Q-REQ to send their request.
> 
> It is the next steps on how the server will handle the respones to the 
> each of the clients that is not so clear. Would we have:-
> 
> 4. Server creates an instance of a Global Exchange E-RESi...n for each 
> client
> 5. Each  Client would create a Local Queue Q-RES
> 6. Each Client would bind their Q-RES to their corresponding E-RESx to 
> recieve their response.
> 
> Obviously, it feels like there should be a more elegant solution. Or is 
> it a case of trying to fit a square peg in a round hole.

You are right. That's the only solution at the moment. (0MQ was designed 
primarily for pub/sub model so req/rep is, well... ugly.

However, the solution is being implemented right now. Actually, there's 
a "routing" branch in SVN that implements it, however, it has 
experimental status at the moment. For more info have a look here:

http://lists.zeromq.org/pipermail/zeromq-dev/2009-April/000633.html

Once the routing is in place, req/rep is easy and efficient:

1. Server declares a global queue and global exchange
2. Each client has it's own unique identity (a string)
3. Client binds it's local queue to the global exchange and its local 
exchange to the global queue.
4. Client subscribes for messages tagged by it's identity.
5. Client sends requests tagged by it's identity.
6. Server reads requests and generates replies; it tags replies by the 
identity taken from the request.
7. Client reads the replies (only the correct ones are delivered because 
of the subscription.

Thoughts?
Martin