API Server
What Is an API Server?
An API Server is a software component. It essentially takes and handles requests via an Application Programming Interface (API). The Kubernetes cluster’s main point of entry is the API server. It exposes the Kubernetes API and manages access to cluster objects like Pods, Deployments, Services, and ConfigMaps. Other Kubernetes components and external clients interact with the cluster state using this interface.
How Does an API Server Work?
Kubernetes API servers process requests through a controlled sequence. First, they receive HTTP requests from clients. These clients include kubectl, controllers, and applications. The servers authenticate the requesters. They check authorization for the requested operations. Admission controls validate and modify supported requests. They complete this step before the API servers accept the requests.
After validation, the servers process the operations. They persist accepted changes in the etcd data store. For read requests, they retrieve the requested information. They return appropriate responses. These processes provide a consistent interface for managing Kubernetes resources.
Key Characteristics of an API Server
- Central API access: They provide a common interface for cluster operations.
- Authentication: They verify the identity of requesters.
- Authorization: They evaluate permissions for requested actions.
- Admission control: They apply additional policies to supported requests.
- State persistence: They coordinate resource storage through etcd.
Benefits of an API Server
- Consistent communication: They give clients a standardized way to interact with Kubernetes.
- Access protection: They enforce identity checks and permission controls.
- Policy enforcement: They support validation and mutation through admission mechanisms.
- Component coordination: They allow controllers and other components to observe and modify cluster resources.
- Operational automation: They support scripts, deployment tools, and custom controllers.
API Server in Cloud Computing
Managed Kubernetes services rely on API servers. They use them to coordinate cluster management operations. Developers submit workload configurations through the Kubernetes API. Controllers use the same interface. They monitor resource changes and reconcile the desired state. Cloud providers manage the control-plane infrastructure in managed offerings. Divisions of responsibilities vary by service.
API servers also form important security boundaries. Network access restrictions, authentication settings, role-based access control, and admission policies help protect cluster operations. These controls determine authorized users. They dictate allowed actions.