[zeromq-dev] programmable message routing

[Martin Sustrik]

Hi Ferenc,

> Let's have three node. Node1 defines E1 with global scope while node2 
> and node3 create local Q queues and all of them bind the local Q queue 
> to E1 exchange. If the node1 sends message to E1 all of the nodes will 
> receive these messages.
> 
> I would like to implement the following message routing:
> First E1 sends message to the first node and if there was an 'error' (or 
> any specified event happend) then it sends to the next node.
> 
> If I understand correctly the demux class includes message routing 
> currently. However when I create exchange I could not define different 
> demux.
> 
> I read in the list archive that the implementation of the complex 
> routing scenatios like topic-based routing is under the way, however I 
> would be interested in the solution which gives more control to 
> developer in the routing. Can you tell me what is the plan related to 
> this topic? Is there any place to the contribution?

I am not sure what exactly does your scenario look like, but I suspect 
you may want load-balancing (i.e. fair distribution of messages between 
several queues). If that's the case have a look at 0.3.3 branch in 
Subversion. There, you cas specify a 'load-balance' flag when creating 
the exchange that will cause exchange to distribute messages between the 
queues in round-robin manner.

As for more complex routing, yes, it's on our roadmap. If you want to 
contribute here we'll be happy to collaborate with you.

Basically, the idea of generalised routing is that there'll be another 
layer on top of existing 0MQ implementation that will define special 
section of the message (a.k.a. message header) containing data that can 
be used by routing algorithm. The layer should expose an API that would 
allow 0MQ users to specify the message header (this can be a simple 
integer, string or even a table of name-value pairs).

Once you have that you are able to write an exchange implementation that 
will check the message header and dispatch the message to appropriate 
destinations.

For algorithms we want to use for message matching have a look here:

http://www.zeromq.org/whitepapers:message-matching

Of course, even different algorithms may be used given that they are 
extra-fast :)

Regards and let us know your thoughts.
Martin