I'm designing a multi-node pub-sub pattern (each node is a separate computer
running Debian Linux). The number of nodes is configurable and can change over
time (i.e. through configuration I can add or remove nodes). I want to use
XPUB-XSUB proxies on each node as depicted below. The following link is for a
picture showing the whole topology:
https://docs.google.com/drawings/d/1WZ-h_3cP1wc0WEEBmtrhH6CDNakyNE4KTb-GApIV7Pk/edit?usp=sharing
I wasn't sure if this mailing list allows adding links to external pictures so
here's an attempt at a drawing in plain text (hopefully it doesn't get all
messed up).
From Proxy 1
From local PUBs on remote nodes
| | |
bind() connect() connect()
ipc:// tcp:// tcp://
| | |
+-----+------+ +--+------+--+
| XSUB | | XSUB |
+------------+ +------------+
| Proxy 1 | | Proxy 2 |
+------------+ +------------+
| XPUB | | XPUB |
+-----+------+ +-----+------+
| |
bind() bind()
tcp:// ipc://
| |
To Local SUBs To Local SUBs
and Proxy 2 on
remote nodes
The proxies use zmq_proxy_steerable(). I've already tested this and it works.
My question is regarding the XSUB socket on "Proxy 2". That's the socket that
connects to all the remote nodes. And since I want to be able to add or remove
nodes, how can I dynamically change the connections on that socket? I thought
of 3 ways I could potentially do this.
Run zmq_proxy_steerable() in one thread (Thread1) and run a second thread
(Thread2) where I could invoke zmq_connect/disconnect to change the
connections. I suspect that it is probably not safe for Thread2 to modify the
connections while zmq_proxy_steerable() is running in Thread1.
Thread2 could send a "PAUSE" to Thread1 to make it temporarily stop processing
messages. Then Thread2 invokes zmq_connect/disconnect to change the
connections. Finally, Thread2 sends "RESUME" to Thread1. Again, not sure this
is safe. Can the connections on a socket be changed while zmq_proxy_steerable()
is PAUSED?
Thread2 could write the list of nodes (IP addresses) to a data struct shared
with Thread1. Then Thread2 sends a TERMINATE to Thread1. Thread1 terminates
gracefully, reads the new list of nodes, changes the connections on the socket
accordingly, then goes back to running zmq_proxy_steerable(). With this I'm not
sure what would happen to in-flight messages. That is, when TERMINATE is
received by Thread1, does it drop any of the messages it's currently processing?
What's the best approach? Is there another (safer) way to do this?
Thanks,
Martin
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