Erlang cluster on Kubernetes: HTTP Service

Because this is going to be a cluster of Erlang nodes, there’s (obviously) going to be more than one instance. It makes sense to add some kind of way to have some kind of “whoami” page, so that we can clearly see which node we’re talking to.

Cowboy

To do this, I added a simple cowboy server to the erlclu release. I added the dependency to rebar.config and added cowboy to the applications list in erlclu.app.src.

Then I added a simple router to erlclu_app.erl:

start(_StartType, _StartArgs) ->
    Dispatch = cowboy_router:compile([
        {'_', [{"/", home_handler, []}]}
    ]),
    {ok, _} = cowboy:start_clear(erlclu_listener,
        [{port, 8080}],
        #{env => #{dispatch => Dispatch}}
    ),
    erlclu_sup:start_link().

home_handler looks like this:

-module(home_handler).

-export([init/2]).

init(Req0, Opts) ->
    Headers = #{<<"content-type">> => <<"text/plain">>},
    Body = io_lib:format("~p~n", [node()]),
    Req = cowboy_req:reply(200, Headers, Body, Req0),
    {ok, Req, Opts}.

It outputs the Erlang node name. I’ll add more information later.

Service/Ingress

Then I added a Service and an Ingress. The service is fairly ordinary:

apiVersion: v1
kind: Service
metadata:
  name: erlclu
  namespace: erlclu
spec:
  selector:
    app: erlclu
  type: ClusterIP
  ports:
  - name: http
    port: 8080
    targetPort: 8080

The ingress (I’m using Traefik) defines an HTTPS ingress that refers to the above service. It uses cert-manager to provision the certificate.

apiVersion: networking.k8s.io/v1
kind: Ingress

metadata:
  name: erlclu
  namespace: erlclu
  annotations:
    # Traefik: secure, plz.
    traefik.ingress.kubernetes.io/router.entrypoints: websecure
    traefik.ingress.kubernetes.io/router.tls: "true"
    # Ask cert-manager to issue a TLS certificate.
    cert-manager.io/cluster-issuer: k3s-ca-cluster-issuer

spec:
  tls:
  - hosts:
      - erlclu.k3s.differentpla.net
    secretName: erlclu-http-tls
  rules:
  - host: erlclu.k3s.differentpla.net
    http:
      paths:
      - path: /
        pathType: Prefix
        backend:
          service:
            name: erlclu
            port:
              name: http

I also updated the deployment to start 5 replicas:

...
spec:
  replicas: 5
...

I added the host to my custom CoreDNS server. By navigating to erlclu.k3s.differentpla.net and hitting Refresh, I can see that it’s hitting the 5 different nodes. They’re not clustered yet. I’ll fix that shortly.