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Distributed Elixir on Kubernetes

Phoenix's PubSub comes with two main adapters - Phoenix.PubSub.PG2 and Phoenix.PubSub.Redis. While the Redis adapter relies on an external source of truth to synchronize messages between Erlang nodes, the PG2 adapter leverages distributed Erlang to do that, removing the SPOFiness of a Redis instance. In order to use PG2 however, nodes need to be connected into an Erlang cluster.

Configuring PG2 becomes a bit complicated in container orchestration systems like Kubernetes, because of the rather complex network setup. This repository contains a minimal Phoenix chat application, along with the Kubernetes setup that makes it possible to run a PG2-based pubsub to power Phoenix's channels.

How-to

First you'll need a K8s cluster - I've decided to use Kind, which runs the Kubernetes cluster as containers on your local Docker. To allow for nginx-ingress to be installed, the cluster needs to be configured appropriately using the cluster.yml file:

kind create cluster --config ./cluster.yaml

Once the cluster is created, let's check that it's our current context:

kubectl config current-context # should print kind-kind

If all is well and we're not working with a production cluster, we can start by installing the ingress controller:

kubectl apply -f https://raw.githubusercontent.com/kubernetes/ingress-nginx/master/deploy/static/provider/kind/deploy.yaml

Then wait for it to be ready:

kubectl wait --namespace ingress-nginx \
  --for=condition=ready pod \
  --selector=app.kubernetes.io/component=controller \
  --timeout=90s

Now let's build the Docker app image:

docker build -t shoutbox:latest ./shoutbox

To make the image available inside the cluster without an image registry, Kind allows to load the image into the cluster:

kind load docker-image shoutbox:latest

Finally, we can create Kubernetes resources, including permissions for fetching pods, the application service and an ingress:

kubectl apply -f deployment.yaml

Easy mode

All of the above is available as a single script - setup.sh (just make sure you're not logged into a production cluster, as the script fires kubectl):

./setup.sh

Demo

Go to http://localhost:4000 and you should see a demo app. Open it in multiple tabs and type in messages - they should appear across tabs, even though we're running 2 pods for that service.