ZeroMQ inproc implies one context

I’ve been tinkering with ZeroMQ a bit lately. Abstracting sockets like this is a great idea. However, the Ruby library, like sockets in general, is a bit light on guidance and the error messages aren’t of the form “Hey dumbie, you do it in this order!”

Here’s something that tripped me up today. ZeroMQ puts everything into a context. If you’re doing in-process communication (e.g. between two threads in Ruby 1.9), you need to share that context.

Doing it right:

# Create a context for all in-process communication
>> ctx =
# Set up a request socket (think of this as the client)
>> req = ctx.socket(ZMQ::REQ)
# Set up a reply socket (think of this as the server)
>> rep = ctx.socket(ZMQ::REP)
# Like a server, the reply socket binds
>> rep.bind('inproc://')
# Like a client, the request socket connects
>> req.connect('inproc://')
# ZeroMQ only knows about strings
>> req.send('1')
=> true
# Reply/server side got the message
>> p rep.recv
=> "1"
# Reply/server side sends response
>> rep.send("urf!")
=> true
# Request/client side got the response
>> req.recv
=> "urf!"

Doing it wrong:

# Create a second context
>> ctx2 =
# Create another client
>> req2 = ctx2.socket(ZMQ::REQ)
# Attempt to connect to a reply socket, but it doesn't 
# exist in this context
>> req2.connect('inproc://')
RuntimeError: Connection refused
	from (irb):16:in `connect'
	from (irb):16
	from /Users/adam/.rvm/rubies/ruby-1.9.2-p180/bin/irb:16:in `'

I believe what is happening here is that each ZMQ::Context gets a thread pool to manage message traffic. In the case of in-process messages, the threads only know about each other within the confines of a context.

And now you know, roughly speaking.