ZMQ扩展Pub-Sub模式无订阅者时消息是否会发布？

Great question—this is a common point of confusion when working with ZeroMQ proxies, so let’s break it down clearly.

First, let’s recap the standard Pub-Sub behavior you already noted to set the baseline:

In standard ZeroMQ Pub-Sub, if there are no connected subscribers, the publisher doesn’t hold onto messages—they’re immediately dropped by the ZeroMQ layer, since there’s no recipient to route them to.

Now, for the XPUB/XSUB proxy scenario, the behavior depends entirely on whether your proxy’s XSUB socket is configured to actively subscribe to topics (or not):

Case 1: Default XPUB/XSUB Proxy (No Active Subscriptions on XSUB) #

By default, the XSUB socket acts only as a relay for subscription messages: it forwards subscription requests from the XPUB side (where actual subscribers connect) to the publishers. If there are no actual subscribers connected to the XPUB socket, the XPUB socket doesn’t send any subscription messages to the XSUB socket.

This means the publishers connected to the XSUB socket see no active subscriptions—just like in the standard Pub-Sub pattern. Any messages they publish will be dropped locally by their PUB socket, since there’s no matching subscription to route them to. The proxy never even receives these messages.

Case 2: Proxy Configures XSUB to Subscribe to All Topics #

If you explicitly configure the XSUB socket to subscribe to all topics (using zmq_setsockopt with ZMQ_SUBSCRIBE and an empty byte string), this changes everything:

• The publishers connected to the XSUB socket will now see an active subscription (from the XSUB itself). They’ll consistently publish messages, sending them to the proxy’s XSUB socket.
• The XSUB forwards these messages to the XPUB socket. But since there are no actual subscribers connected to XPUB, the XPUB socket has nowhere to send the messages—so it drops them at the proxy level.

In this scenario, you’ll see network traffic from the publisher to the proxy (which you can capture with Wireshark/tcpdump), but the messages still don’t reach any end subscriber.

How to Verify This in Practice #

If you want to confirm which behavior your proxy is using, you can:

• Use zmq_monitor on the publisher’s PUB socket to track message send events. If messages are being sent (even without subscribers), the XSUB is subscribed to topics.
• Capture network traffic between the publisher and proxy. If you see outgoing packets from the publisher when no subscribers are connected, that’s a sign the XSUB has active subscriptions.
• Check your proxy code: look for any calls to setsockopt(ZMQ_SUBSCRIBE, ...) on the XSUB socket.

内容的提问来源于stack exchange，提问作者castro