Similar to this ticket - File Based Discovery plugin not working - I've setup a small, two node test cluster that uses docker and is configured to use file based discovery. The configuration directory is mounted/mapped to a directory on my docker host.
Starting two nodes and placing only their own IPs into the unicast_hosts.txt file results in neither server knowing about each other (as expected). However, adding the peer addresses into unicast_hosts.txt on both servers results in no change to the cluster. Discovery never happens.
I can only get the servers to join up if I restart one of the two nodes. Even after they "discovery" each other I cannot get the cluster to size itself dynamically using the contents of the file.
Wonder if others have ever found success with this configuration or if the issue is that ES is running in a docker container and is unable to see that the file has changed.