Controller
Kubernetes controllers are integral components of the platform, responsible for maintaining the desired state of native resources within a cluster. Controllers are designed to manage built-in resources, such as ReplicaSets, Deployments, and Services.
Controllers follow the Kubernetes “controller pattern,” a control loop that monitors changes in the desired state and updates the cluster accordingly. For example, when a Deployment is created, the Deployment controller ensures that the specified number of replicas for a particular application is running. If a replica fails, the controller will create a new one to maintain the desired state.
Operator
Kubernetes operators provide a specialized method for resource management, enabling users to extend Kubernetes capabilities through Custom Resource Definitions (CRDs). They are designed to handle domain-specific tasks and resources, offering a high level of automation for application-specific needs.
Operators build upon the controller pattern, incorporating domain-specific expertise to manage more complex scenarios. For example, the kube-prometheus operator automates the deployment, management, and scaling of Prometheus, Alertmanager, and Grafana for Kubernetes monitoring. By using the kube-prometheus operator, engineers can easily establish a comprehensive monitoring solution for their cluster without needing to configure each component separately.
The difference
Deciding between using a controller or an operator depends on your specific needs and the level of automation you require. Controllers are ideal for managing native Kubernetes objects and maintaining the desired state of built-in resources. On the other hand, operators are designed for application-specific tasks and resources, providing a higher level of customization and domain-specific expertise.
Both controllers and operators are crucial for Kubernetes automation, each with its own strengths and use cases. Controllers are best suited for handling built-in resources, while operators excel at managing application-specific tasks and resources.