How to monitor a remote erlang node which was down and is restarting

Question

My application runs in an erlang cluster - with usually two or more nodes. There's active monitoring between the nodes (using erlang:monitor_node) which works fine - I can detect and react to the fact that a node that was up is now down.

But how do I then find out that the node has restarted and is back in business? I can of course periodically ping the node until it is back up, but is there a better way that I've simply missed? Is process groups a better way of achieving this?

(Edited to add)

I think the answer to perform a technique like election of a supervisor is the thought process I was missing. I'll look into that and mark this question as done....

Answer 1

But how do I then find out that the node has restarted and is back in business? I can of course periodically ping the node until it is back up, but is there a better way that I've simply missed? Is process groups a better way of achieving this?

Just an idea, but how about having the restarting node itself explicitly inform the supervisor/monitoring node that it has finished restarting and that it is available again?

You could use a recurring "heartbeat message" for this purpose, or come up with a custom message specifically meant to be sent once after successful initialization. Something along the lines of:

start(SupervisorPID) ->
  SuperVisorPID ! {hello, MyPID};
  mainloop().

Answer 2

You could create a global_group then use the global_group:monitor_nodes(true) to monitor the other nodes within the same global group. The process that is monitoring the nodes will get nodeup and nodedown messages.