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erlangdistributed-computing

Erlang - Nodes don't recognize

I'm trying to use distributing programming in Erlang.

But I had a problem, I can't communicate two Erlang's nodes to communicate.

I tried to put the same atom in the "Magical cookies", but it didn't work.

I tried to use command net:ping(node), but reponse was pang (didn't reconigze another node), or used nodes(), to see if my first node see the second node, but it didn't work again.

The first and second node is CentOS in VMWare, using bridge connection in network adaptor.

I entered command ping outside Erlang between VM's and they reconigze each one.

I start the first node, but the second node open process, but can't find the node pong.

(pong@localhost)8> tut17:start_pong().
true


(ping@localhost)5> c(tut17).
{ok,tut17}
(ping@localhost)6> tut17:start_ping(pong@localhost).
<0.55.0>

Thank you!

Solution

  • A similar question here.

    The distribution is provided by a daemon called Erlang Port Mapper Daemon. By default it listens on port 4369 so you need to make sure that that port is opened between the nodes. Additionally, each started Erlang VM opens an additional port to communicate with other VMs. You can see those ports with epmd -names:

    g@someserv1:~ % epmd -names
epmd: up and running on port 4369 with data:
name hbd at port 22200

    You can check if the port is opened by doing telnet to it, e.g.:

    g@someserv1:~ % telnet 127.0.0.1 22200
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
^]
Connection closed by foreign host.

    You can change the port to the port you want to check, e.g. 4369, and also the IP to the desired IP. Doing ping is not enough because it uses its own ICMP protocol which is different that TCP used by the Erlang distribution to communicate, e.g. ICMP may be allowed but TCP may be blocked.

    Edit:

    Please follow this guide Distributed Erlang to start an Erlang VM in distributed mode. Then you can use net_adm:ping/1 to connect to it from another node, e.g.:

    (hbd@someserv1.somehost.com)17> net_adm:ping('hbd@someserv2.somehost.com').
pong

    Only then epmd -names will show the started Erlang VM on the list.

    Edit2:

    Assume that there are tho hosts, A and B. Each one runs one Erlang VM. epmd -names run on each host shows for example:

    Host A:

    epmd: up and running on port 4369 with data:
name servA at port 22200

    Host B:

    epmd: up and running on port 4369 with data:
name servB at port 22300

    You need to be able to do:

    On Host A:

    telnet HostB 4369
telent HostB 22300

    On Host B:

    telnet HostA 4369
telnet HostA 22200

    where HostA and HostB are those hosts' IP addresses (.e.g HostA is IP of Host A, HostB is IP of Host B).

    If the telnet works correctly then you should be able to do net_adm:ping/1 from one host to the other, e.g. on Host A you would ping the name of Host B. The name is what the command node(). returns.