Introduction

Leader election is a crucial pattern in distributed systems where multiple instances or nodes compete to perform certain tasks. In a Kubernetes cluster, leader election can be used to ensure that only one instance is responsible for executing leader-specific tasks at any given time. This blog post will explore how to implement a leader election mechanism in Kubernetes using lease locks.

Overview

The leader election mechanism implemented in Go code relies on Kubernetes coordination features, specifically Lease object in the coordination.k8s.io API Group. Lease locks provide a way to acquire a lease on a shared resource, which can be used to determine the leader among a group of nodes.

Repository

The example code, used for this blog is available on mjasion/golang-k8s-leader-example GitHub repository.

Code Walkthrough

The main function is the entry point of the program. It reads configuration values from environment variables and obtains the Kubernetes clientset by getting access to Kube-Api by ServiceAccount attached to Pod. The application is written to work in Kubernetes Pod, that’s why it is using rest.InClusterConfig() function.

The leader election configuration is set up using the LeaderElectionConfig struct from the Kubernetes client library. It specifies the lease lock, lease duration, renewal deadline, retry period, and callback functions for leader-specific tasks.

leaderElectionConfig := leaderelection.LeaderElectionConfig{
    Lock: &resourcelock.LeaseLock{
        LeaseMeta: metav1.ObjectMeta{
            Name:      lockName,
            Namespace: leaseNamespace,
        },
        Client: clientset.CoordinationV1(),
        LockConfig: resourcelock.ResourceLockConfig{
            Identity: os.Getenv("HOSTNAME"),
        },
    },
    LeaseDuration: time.Duration(leaseDuration) * time.Second,
    RenewDeadline: time.Duration(renewalDeadline) * time.Second,
    RetryPeriod:   time.Duration(retryPeriod) * time.Second,
    Callbacks: leaderelection.LeaderCallbacks{
        OnStartedLeading: onStartedLeading,
        OnStoppedLeading: onStoppedLeading,
    },
    ReleaseOnCancel: true,
}

The most important settings are the lease duration, renewal deadline, and retry period:

• The LeaseDuration specifies how long the lease is valid.
• The RenewDeadline specifies the amount of time that the current node has to renew the lease before it expires.
• The RetryPeriod specifies the amount of time that the current holder of a lease has last updated the lease.

The leader-specific tasks are performed in the onStartedLeading function, which is called when the current node becomes the leader. The updateServiceSelectorToCurrentPod function updates the service selector to include the current pod’s hostname.

func onStartedLeading(ctx context.Context) {
	log.Println("Became leader: ", os.Getenv("HOSTNAME"))
	clientset := getKubeClient()
	updateServiceSelectorToCurrentPod(clientset)
	go func() {
		for {
			select {
			case <-ctx.Done():
				log.Println("Stopped leader loop")
				return
			default:
				log.Println("Performing leader tasks...")
				time.Sleep(1 * time.Second)
			}
		}
	}()
}

The onStoppedLeading function is called when the current node stops being the leader. It can be used for cleanup tasks.

func onStoppedLeading() {
	log.Println("Stopped being leader")
}

A context and a wait group are created to manage goroutines. A goroutine is started to run the leader election using the leaderelection.RunOrDie function.

ctx, cancel := context.WithCancel(context.Background())
defer cancel()
wg := &sync.WaitGroup{}
wg.Add(1)

go func() {
    defer wg.Done()
	leaderelection.RunOrDie(ctx, leaderElectionConfig)
}()

cancel()
wg.Wait()

The program also sets up a Gin router and defines a root endpoint that returns the hostname of the current node, to easily check which Pod is being the leader.

Demo 1 - Deploying a single Pod

In this demo, we will deploy a single Pod to a Kubernetes cluster and observe how the leader election works.

As you can see here, the pod is elected as a leader and performs leader-specific tasks. The lease object contains the information about the current leader in the HOLDER column.

NAME                 HOLDER                               AGE
k8s-leader-example   k8s-leader-example-8dd646bb7-dsfmq   11s

Demo 2 - Deploying multiple Pods and killing the leader

In this demo, we will deploy multiple Pods to a Kubernetes cluster and observe how the leader election works. The settings used for this demo are as follows:

The leader election mechanism will attempt to renew the lease every 5 seconds. If the lease is not renewed within 5 seconds, the leader election mechanism will attempt to acquire the lease. If the lease is not acquired within 1 second, the leader election mechanism will retry to acquire the lease.

Running command kubectl get lease --watch allows to observe the leader election process. The lease object contains first the information about the previous leader, when the leader is killed, and then the information about the new leader.

Conclusion

Implementing leader election in Kubernetes using lease locks is an effective way to ensure that only one instance or node performs leader-specific tasks at a time. In this blog post, we explored the provided Go code that demonstrates how to implement leader election in a Kubernetes cluster.

By incorporating leader election into your distributed system, you can enhance its reliability and prevent conflicts that may arise from multiple instances attempting to execute the same tasks simultaneously.