[zeromq-dev] How to distinguish peers using socket_monitor

[Bill Torpey]

Hi Milosz:

I dislike it when someone asks a “How do I do X”? question and the replies are all about how NOT to do X, but in this case I can’t help it — sorry:

- You may want to re-think REQ/REP.  In practice, REQ/REP has been found to be somewhat fragile (https://stackoverflow.com/questions/26915347/zeromq-reset-req-rep-socket-state <https://stackoverflow.com/questions/26915347/zeromq-reset-req-rep-socket-state>).  With recent versions of the library the ZMQ_REQ_RELAXED socket option may help.

- The zmq_socket_monitor functionality was never intended for driving state changes in an application, but rather for logging and trouble-shooting purposes.  In particular, the zmq_socket_monitor events are asynchronous (the docs state in several places "Warning: there is no guarantee that the FD is still valid by the time your code receives this event.”).  In addition, the underlying connect/disconnect events are themselves asynchronous (i.e., calling zmq_connect simply starts the connection process — it may take an arbitrary length of time to complete).

If you need to track connection state reliably, you are going to have to implement some connection-tracking mechanism “on top of” ZeroMQ, possibly using application-level heartbeats, timeouts, etc.  This can get quite messy, but as far as I know is the only way.  

Hope this helps.

Bill Torpey

> On Jun 3, 2019, at 5:41 AM, Milosz Malczak <milosz.malczak at cern.ch> wrote:
> 
> I am creating a distributed system with one server and many peers. For communication I am using ZeroMQ, REQ-REP pattern. In order to be able to detect that the connection was lost I am using socket_monitor. The problem is that, when I get the message about disconnection, I don't know how I can tell which peer was actually disconnected. The only information that allows to distinguish the connections is the file descriptor, which I receive from the socket_monitor, but I don't know which connection/peer it belongs to.
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