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Running Erlang Releases without EPMD on OTP 23.1+

Erlang/OTP deployments that want to provide shell access or cluster nodes relied on something called the Erlang Port Mapper Daemon (EPMD), a separate process that handled all distribution mechanisms. Previous attempts at working around this requirement used third party dependencies such as epmdless and required specific configuration arguments to the VM to setup. This was not ideal and was easy to get wrong.

With Erlang/OTP 23.1 and relx 4.2 (included in Rebar3 3.14.3) it is now possible to conveniently run Erlang releases without relying on EPMD for opening a remote shell or clustering nodes. This is particularly going to make Erlang releases simpler to configure in locked down environments, and easier to use with containers.

EPMD is a separate process that is started, if not already running, when an Erlang node boots with distribution enabled – meaning -sname or -name is passed to the boot command. The new node talks to the EPMD process to register its name with a port that the node will also bind to. When a node wants to connect to another one it must look up the port the other node is listening on through EPMD. This is required both for clustering nodes together and connecting a remote shell on the command line – which is essentially the same as clustering because a new node is started before attaching to the remote node to bring up a shell on that node.

Bypassing EPMD has been made possible through a couple new options added to erl:

  • -erl_epmd_port P: Configures the port this node will listen on and use when connecting to remote nodes.
  • -dist_listen false: Setting this keeps the node from binding to the port, preventing it from acting as a server, and instead uses the port only for connecting to remote nodes. The use case for this is when opening a remote shell -remsh <node> (remote_console in the release script) to a local node.
  • -[s]name undefined: Passing undefined for the short or long name of a node generates a unique node name and sets additional flags, -dist_listen false -hidden -dist_auto_connect never.

An extra option has also been added to erl_call :

  • -address P: Sets the address and/or port to connect to the running Erlang node. This removes the need to look up the remote node or to even specify its name when running erl_call.
  • -R: Tells erl_call to use a dynamic random name for the hidden node it starts.

As of OTP 23.1+ the relx generated release script uses erl_call instead of the custom escript nodetool when the commands rpc and eval are used to run code on a running node.

These options are used automatically by the release shell script when starting a node, opening a remote shell or interacting with the running node through rpc and eval. The only change you as a user must make to run your release without epmd is to set the OS environment variable ERL_DIST_PORT.

When ERL_DIST_PORT is set in the environment the option to not start EPMD (-start_epmd false) is added to the erl arguments automatically, along with -erl_epmd_port ${ERL_DIST_PORT}. The ERL_DIST_PORT is also used by erl_call with -address ${ERL_DIST_PORT}.

epmdlessless is an example project that comes with a docker-compose.yml setup that makes it simple to play with these new features. Note that no changes are required to vm.args.src to disable EPMD or set a port to use, the only configuration done is through the environment variable ERL_DIST_PORT in docker-compose.yml:

    ...
    environment:
      - ERL_DIST_PORT=8001
    ...

Note that each node must listen on the same port because setting -erl_epmd_port P tells Erlang both what port to listen on and what port to use when connecting to other nodes.

After cloning the repo, build the Docker image and start 3 nodes (node_a, node_b, node_c):

$ docker-compose up

Then, use docker exec to open a remote shell on node_a and connect to node_b (be sure to disconnect from the remote shell with Ctrl-g followed by q, if you use q(). it will shutdown the remote node):

$ docker exec -ti node_a bin/epmdlessless remote_console
Erlang/OTP 23 [erts-11.1.3] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Eshell V11.1.3  (abort with ^G)
([email protected]_a)1> net_kernel:connect_node([email protected]_b).
true
([email protected]_a)2> erlang:nodes().
[[email protected]_b]
([email protected]_a)3> 
User switch command
 --> q

Do the same on node_c and also connect to node_b and you’ll see the full mesh is created with node_a:

$ docker exec -ti node_c bin/epmdlessless remote_console
Erlang/OTP 23 [erts-11.1.3] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Eshell V11.1.3  (abort with ^G)
([email protected]_c)1> net_kernel:connect_node([email protected]_b).
true
([email protected]_c)2> erlang:nodes().
[[email protected]_b,[email protected]_a]
([email protected]_c)3> 
User switch command
 --> q

Test the rpc command by running erlang:nodes() on node_b:

$ docker exec -ti node_b bin/epmdlessless rpc erlang nodes
[[email protected]_a, [email protected]_c]

Lastly, you can verify that none of the commands caused EPMD to be run:

$ docker exec -ti node_b erts-11.1.3/bin/epmd -names
epmd: Cannot connect to local epmd

Separate Ports: Where epmdless is Still Needed

It is important to note that this requires every node to listen on the same port, which requires every node to have its own IP. If this isn’t an option, or for whatever reason you need to use separate ports for nodes, then epmdless is an option to replace EPMD and track the node to port mapping. But if you are running with containers in an environment like Kubernetes you can now keep EPMD from running in each container by simply upgrading Erlang and Rebar3, and setting the ERL_DIST_PORT environment variable.