For Fun And Practice, create an elixir application which can discover and connect to arbitrary elixir nodes running on the same host by polling EPMD. In particular, do not require the user to specify the names of other nodes ahead of time.
An erlang node is started with a given name (via --sname or --name) and is
assigned a port by the operating system. This port is registered with the Erlang
Port Mapper Daemon (EPMD) [1], which records a mapping from the node’s name
to its port, enabling nodes to connect and communicate with one another. For
example, to connect to name@host, a node must first contact the EPMD running
on host, which will return the port number associated with name. The node
can then contact name@host directly using its complete address. (Note that
host is resolved to an ip address in the usual way; EPMD has no role in that
process.)
In this way EPMD maintains a list of all nodes running on its host. We can query EPMD for this list in order to discover the names and addresses of other nodes on our host, and thereby automatically cluster all nodes on the same host.
Tinycluster calls :net_adm.world_list/2 [2], which retrieves the
list of nodes from EPMD, pings each to confrim it can be reached, and then
yields the name of all pingable nodes. We then connect to each in turn, thereby
clustering with all local nodes without knowing their names. As a user, if I
start several iex sessions on my laptop with the Tinycluster application
running on any of them, they will automatically discover and connect to one
another.
Since node connections are symmetric and transitive, only one node on a host needs to run tinycluster for the nodes to become completely connected. Since tinycluster polls, it will discover non-tinycluster nodes as they start. Running tinycluster on more than one node is fine too, however.