libcluster v2.5.0 Cluster.Strategy.Kubernetes

This clustering strategy works by loading all endpoints in the current Kubernetes namespace with the configured label. It will fetch the addresses of all endpoints with that label and attempt to connect. It will continually monitor and update its connections every 5s.

In order for your endpoints to be found they should be returned when you run:

kubectl get endpoints -l app=myapp

It assumes that all nodes share a base name, are using longnames, and are unique based on their FQDN, rather than the base hostname. In other words, in the following longname, <basename>@<domain>, basename would be the value configured in kubernetes_node_basename.

domain would be the value configured in mode and can be either of type :ip (the pod’s ip, can be obtained by setting an env variable to status.podIP) or :dns, which is the pod’s internal A Record. This A Record has the format <ip-with-dashes>.<namespace>.pod.cluster.local, e.g 1-2-3-4.default.pod.cluster.local.

Getting :dns to work requires a bit fiddling in the container’s CMD, for example:

# deployment.yaml
command: ["sh", "-c"]
args: ["POD_A_RECORD"]
args: ["export POD_A_RECORD=$(echo $POD_IP | sed 's/./-/g') && /app/bin/app foreground"]

# vm.args
-name app@<%= "${POD_A_RECORD}.${NAMESPACE}.pod.cluster.local" %>

(in an app running as a Distillery release).

The benefit of using :dns over :ip is that you can establish a remote shell (as well as run observer) by using kubectl port-forward in combination with some entries in /etc/hosts.

Defaults to :ip.

An example configuration is below:

config :libcluster,
  topologies: [
    k8s_example: [
      strategy: Elixir.Cluster.Strategy.Kubernetes,
      config: [
        mode: :ip,
        kubernetes_node_basename: "myapp",
        kubernetes_selector: "app=myapp",
        polling_interval: 10_000]]]

Link to this section Summary

Functions

child_spec(arg)

Returns a specification to start this module under a supervisor

init(opts)

Invoked when the server is started. start_link/3 or start/3 will block until it returns

start_link(opts)

Callback implementation for Cluster.Strategy.start_link/1

Link to this section Functions

child_spec(arg)

Returns a specification to start this module under a supervisor.

See Supervisor.

init(opts)

Invoked when the server is started. start_link/3 or start/3 will block until it returns.

args is the argument term (second argument) passed to start_link/3.

Returning {:ok, state} will cause start_link/3 to return {:ok, pid} and the process to enter its loop.

Returning {:ok, state, timeout} is similar to {:ok, state} except handle_info(:timeout, state) will be called after timeout milliseconds if no messages are received within the timeout.

Returning {:ok, state, :hibernate} is similar to {:ok, state} except the process is hibernated before entering the loop. See c:handle_call/3 for more information on hibernation.

Returning :ignore will cause start_link/3 to return :ignore and the process will exit normally without entering the loop or calling c:terminate/2. If used when part of a supervision tree the parent supervisor will not fail to start nor immediately try to restart the GenServer. The remainder of the supervision tree will be (re)started and so the GenServer should not be required by other processes. It can be started later with Supervisor.restart_child/2 as the child specification is saved in the parent supervisor. The main use cases for this are:

The GenServer is disabled by configuration but might be enabled later.
An error occurred and it will be handled by a different mechanism than the Supervisor. Likely this approach involves calling Supervisor.restart_child/2 after a delay to attempt a restart.

Returning {:stop, reason} will cause start_link/3 to return {:error, reason} and the process to exit with reason reason without entering the loop or calling c:terminate/2.

Callback implementation for GenServer.init/1.

start_link(opts)

Callback implementation for Cluster.Strategy.start_link/1.