Certified Kubernetes Application Developer (CKAD) Practice Test

What is the primary function of the Controller in a Kubernetes environment?

1. To distribute work across nodes

2. To maintain the desired state of the cluster

3. To store metadata

4. To run containerized applications

The correct answer is: To maintain the desired state of the cluster

The primary function of the Controller in a Kubernetes environment is to maintain the desired state of the cluster. Controllers are control loops that continuously monitor the state of the cluster and make adjustments to achieve the desired state specified in the resource definitions. For instance, if a deployment specifies that there should be three replicas of a pod running, the Controller ensures that this condition is met by creating or removing pod instances as needed. If a pod goes down, the Controller detects this change and will either restart the pod or replace it, as necessary, to fulfill the desired state configuration. This self-healing characteristic is one of the defining features of Kubernetes, allowing it to manage applications robustly and resiliently. In contrast, options regarding distributing work across nodes, storing metadata, or running containerized applications pertain to other components or functionalities within the Kubernetes ecosystem. Work distribution is managed by the Scheduler, metadata storage is handled by etcd (the key-value store used by Kubernetes), and running containerized applications is a fundamental purpose of Kubernetes itself, facilitated by the kubelet and other components. Nonetheless, it is the Controllers that are specifically tasked with responding to the current state of the system and making necessary modifications to improve or restore the desired state as defined by the user.