Why Does My Rails ffi-rzmq Subscriber Not Receive ZeroMQ Messages?

Below are a few common points to check and try. In our experience similar symptoms typically aren’t due to a bug in ffi‑rzmq/versions but rather due to subtleties with ZeroMQ’s asynchronous and subscription behavior. Here are some pointers:

1. Subscription Setup Delay
   ZeroMQ’s SUB sockets require a little time after connecting for the subscription to “take.” If the publisher sends messages before the subscriber’s filtering has been fully set up, those messages are lost. Try adding a short sleep (e.g. 500ms–1 second) right after connecting/subscribing and before entering your receive loop:

   RUBYCopied
   subscriber.connect(zmq_url)
   subscriber.setsockopt(ZMQ::SUBSCRIBE, '')
   sleep 1  # wait for the subscription to become active

2. Ensure the Correct Endpoint
   Verify that ENV['ZMQ_HOST'] is set to the IP or hostname that can actually reach the Quake Live server. Although the PUB socket is bound to tcp://0.0.0.0:27960, your subscriber needs to connect using an IP/hostname that is routable from the client machine (for example an external IP if required).

3. Message Arrival Timing
   Given that the code appears to be working (authentication succeeds, no errors), it might simply be that the messages you expect aren’t being sent after your connection is established. You can test by ensuring that some messages are published after your subscriber is already connected.

4. Consider Using a Poller
   Although your blocking call should work as expected, you might consider using ZeroMQ’s poller to ensure you aren’t missing messages in unexpected ways. It also helps with clean shutdown if you plan to interrupt the loop:

   RUBYCopied
   poller = ZMQ::Poller.new
   poller.register(subscriber, ZMQ::POLLIN)
   loop do
     poller.poll(1000)  # timeout in ms
     if poller.readables.include?(subscriber)
       message = ''
       subscriber.recv_string(message)
       Rails.logger.info "Received message: #{message}"
     end
   rescue => e
     Rails.logger.error "Error: #{e.message}"
   end

5. Double-check Your Credentials
   Although your logs appear to show a successful authentication, ensure that the credentials are being correctly passed to the subscriber (note the use of .b for binary conversion). If you’re using environment variables, double-check that they’re set properly in your Rails environment.

6. Testing with Different Clients or Languages
   As you mentioned testing a Python version also didn’t work, it reinforces that the issue may not be on the ffi‑rzmq side. Test the same connection details with a third client (for example, using the official ZeroMQ examples in C) to isolate whether the issue is on the publisher or subscriber side.

In summary, while ffi‑rzmq isn’t known to have issues with PLAIN authentication on its recent versions, try adding a subscription delay and verifying connection details. These adjustments often resolve cases where the subscriber appears connected but never sees any data.

Give these changes a try and let me know if you still run into issues!