How Leader Election Works in Kubernetes— By M. Sharma

Kubernetes simplifies the deployment and operational management of services running on clusters. However, it also simplifies the development of these services. In this post, we’ll see how you can use Kubernetes to easily perform leader elections in your distributed application. Distributed applications usually replicate the tasks of service for reliability and scalability, but often it is necessary to designate one of the replicas as the leader who is responsible for coordination among all of the replicas.

Typically in leader election, a set of candidates for becoming leaders is identified. These candidates all race to declare themselves the leader. One of the candidates wins and becomes the leader. Once the election is won, the leader continually “heartbeats” to renew their position as the leader, and the other candidates periodically make new attempts to become the leader. This ensures that a new leader is identified quickly if the current leader fails for some reason.

Implementing leader election usually requires either deploying software such as ZooKeeper, etcd or Consul and using it for consensus, or alternately, implementing a consensus algorithm on your own. We will see below that Kubernetes makes the process of using leader election in your application significantly easier.

Implementing leader election in Kubernetes

• The first requirement in leader election is the specification of the set of candidates for becoming the leader.
• Kubernetes already uses Endpoints to represent a replicated set of pods that comprise a service, so we will re-use this same object.

To perform leader election, we use two properties of all Kubernetes API objects:

• ResourceVersions — Every API object has a unique ResourceVersion, and you can use these versions to perform compare-and-swap on Kubernetes objects
• Annotations — Every API object can be annotated with arbitrary key/value pairs to be used by clients.

Given these primitives, the code to use master election is relatively straightforward, and you can find it here. Let’s run it ourselves.