How can i detect that publisher is disconnected with ZeroMQ and Node.js

Question

I am using Node.js + ZeroMQ for subscribing to a certain feed using the PUB/SUB pattern. How could i detect the condition where my publisher is disconnected? (I am connected as a subscriber)

Another thing: is there a way to get automatically messages from the past when i first connected to the publisher?

Thanks in advance

Answer 1

You could publish a heartbeat and if your subscriber misses one or more in-a-row you can assume that you lost the connection and try to reconnect.

To get the messages from the past you need to use a different pattern, like REQuesting those missing messages. In this case you need a way to identify which messages are missing.

Answer 2

In ZeroMQ's default pubsub model, there's no way for the subscriber to get messages from the past. See the ZeroMQ documentation , where you find statements like

If you start the SUB socket (ie, establish a connection to a PUB socket) after the PUB socket has started sending out data, you will lose whatever it published before the connection was made. If this is a problem, set up your architecture so the SUB socket starts first, then the PUB socket starts publishing.

and

Pub-sub is like a radio broadcast; you miss everything before you join, and then how much information you get depends on the quality of your reception.