Notes: What is Kubernetes?

1. Definition

Kubernetes is an open-source container orchestration framework, originally developed by Google.
It manages applications composed of hundreds or thousands of containers across physical machines, virtual machines, cloud, or hybrid environments.

Growth of microservices → led to widespread container usage.
Managing large numbers of containers manually is complex and often infeasible.
Kubernetes provides:
- High availability – apps remain accessible with no downtime.
- Scalability – ensures performance and responsiveness.
- Disaster recovery – backups and restoration so no data is lost.

Cluster Structure:
- Master Node(s): Runs essential control processes.
  - API Server: Entry point for clients (UI, CLI, scripts).
  - Controller Manager: Monitors state, repairs issues, restarts failed containers.
  - Scheduler: Allocates containers to worker nodes based on resources.
  - etcd (Key-Value Store): Maintains cluster state and configuration (used for recovery).
- Worker Nodes: Run the actual workloads (apps in containers). Each has:
  - Kubelet: Ensures containers are running and communicates with master.
  - Deployed containers (often via Docker).
Networking: A virtual network spans all nodes, making the cluster act as one powerful machine.
Resilience: Production setups typically use multiple master nodes for fault tolerance.

Pod: Smallest deployable unit in Kubernetes; a wrapper for one or more containers.
- Typically one pod per application (DB, server, message broker, etc.).
- Each pod has its own IP address within the cluster.
- Pods are ephemeral (can die and be recreated).
Service:
- Provides a stable IP address to pods (since pods get new IPs if restarted).
- Acts as a load balancer between pods.
- Decouples app-to-app communication from pod lifecycle.

All configurations go through the API Server using YAML or JSON.
Declarative model: Define the desired state → Kubernetes ensures reality matches.
Example: A Deployment request may specify:
- my-app with 2 replicas.
- Each replica runs a container from a specified image.
- Environment variables and ports defined.
If a pod fails, the Controller Manager automatically recreates it to match the declared state.

Kubernetes = automation for container management at scale.
Ensures apps are always up, scalable, and recoverable.
Abstracts complexity: users define what they want, Kubernetes handles how it happens.