Elect a kubernetes leader for life using leases for ruby.

• elects a new leader when the old leader pod is deleted
• elects a new leader when the old leader pod fails to update it's lease (see race condition issue)
• waits until the current pod is the leader, then continues reporting "I am the leader" metric
• lease is a simple crd that does not do anything under the hood, except get GCed when the owning pod is deleted
• leader continuously updates the lease to signal that it's healthy
• follower determines the leader is dead when lease is not updated (avoid az outage zombie pod issues)

similar to kubernetes go implementation:

• https://github.com/kubernetes/client-go/blob/master/tools/leaderelection/leaderelection.go
• https://github.com/kubernetes/client-go/blob/master/tools/leaderelection/resourcelock/leaselock.go

Works best with:

• statsd for metrics, for example dogstatsd-ruby
• kubeclient

gem install kubernetes_leader_election

Thread.abort_on_exception = true
$stdout.sync = true
require "logger"
logger = Logger.new STDOUT

# setup
require "kubernetes_leader_election"
origin = "https://#{ENV.fetch('KUBERNETES_SERVICE_HOST')}:#{ENV.fetch('KUBERNETES_SERVICE_PORT_HTTPS')}"
kubeclient = Kubeclient::Client.new(
  "#{origin}/apis/coordination.k8s.io",
  "v1",
  auth_options: {bearer_token_file: '/var/run/secrets/kubernetes.io/serviceaccount/token'},
  ssl_options: {ca_file: '/var/run/secrets/kubernetes.io/serviceaccount/ca.crt'}
)

# wait for leader to be elected
is_leader = false
elector = KubernetesLeaderElection.new("my-app", kubeclient, logger: logger)
Thread.new { elector.become_leader_for_life { is_leader = true } }
sleep 1 until is_leader

# do leader things
puts "I'm the leader now"
sleep

KubernetesLeaderElection.new("my-app", kubeclient, options)

see example/ folder

Needs permissions:

- apiGroups: ["coordination.k8s.io"]
  resources: ["leases"]
  verbs: ["create"]
- apiGroups: ["coordination.k8s.io"]
  resources: ["leases"]
  resourceNames: ["my-app"]
  verbs: ["get", "patch", "delete"]

- name: POD_NAME
  valueFrom:
    fieldRef:
      fieldPath: metadata.name
- name: POD_UID
  valueFrom:
    fieldRef:
      fieldPath: metadata.uid
- name: POD_NAMESPACE
  valueFrom:
    fieldRef:
      fieldPath: metadata.namespace

When using service account tokens from disk, then provide a method that builds a kubeclient instead of a kubeclient object. Ideally cache for <=1h so the token never expires.

kubeclient = -> { cache.fetch(:kubclient) { Kubeclient.new(...) } }
elector = KubernetesLeaderElection.new("my-app", kubeclient, logger: logger)

Michael Grosser
michael@grosser.it
License: MIT