How Does Kubernetes Work?

Understanding Kubernetes Architecture

Control Plane Components

The control plane manages the overall cluster state:

kube-apiserver:

• API Server: Central communication hub for all Kubernetes components
• REST API: Exposes Kubernetes API for cluster management
• Authentication: Handles user authentication and authorization
• Validation: Validates and processes API requests

etcd:

• Distributed Database: Stores all cluster data and configuration
• Consistency: Ensures data consistency across the cluster
• Backup: Critical for cluster backup and disaster recovery
• High Availability: Can be deployed in HA mode for production

kube-scheduler:

• Pod Scheduling: Determines where to place new pods
• Resource Allocation: Considers resource requirements and constraints
• Affinity Rules: Applies node affinity and anti-affinity rules
• Load Balancing: Distributes pods across available nodes

kube-controller-manager:

• Controller Loop: Runs various controllers that maintain cluster state
• Node Controller: Monitors node health and handles node failures
• Replication Controller: Maintains desired number of pod replicas
• Service Controller: Manages service endpoints and load balancing

Node Components

Worker nodes run the actual application workloads:

kubelet:

• Node Agent: Primary node agent that runs on each node
• Pod Management: Creates, modifies, and deletes pods
• Health Monitoring: Monitors pod health and reports to API server
• Resource Management: Manages node resources and container runtime

kube-proxy:

• Network Proxy: Maintains network rules and enables communication
• Service Discovery: Implements service abstraction and load balancing
• Network Policies: Enforces network policies and security rules
• IP Tables: Manages IP tables rules for traffic routing

Container Runtime:

• Container Engine: Runs containers (Docker, containerd, CRI-O)
• Image Management: Pulls and manages container images
• Container Lifecycle: Starts, stops, and manages containers
• Resource Isolation: Provides resource isolation and security

How Kubernetes Manages Applications

Pod Lifecycle

Pods are the smallest deployable units in Kubernetes:

Pod Creation:

1. API Request: User submits pod creation request to API server
2. Scheduling: Scheduler determines optimal node for pod placement
3. Node Assignment: Pod is assigned to specific node
4. Container Creation: Kubelet creates containers on assigned node
5. Health Monitoring: Kubelet monitors pod health and reports status

Pod States:

• Pending: Pod accepted but not yet scheduled or running
• Running: Pod scheduled and all containers running
• Succeeded: All containers completed successfully
• Failed: At least one container failed or terminated
• Unknown: Pod state cannot be determined

Service Discovery and Load Balancing

Services provide stable networking for applications:

Service Types:

• ClusterIP: Internal cluster access (default)
• NodePort: External access via node ports
• LoadBalancer: Cloud provider load balancer integration
• ExternalName: DNS-based service discovery

Load Balancing:

• Round Robin: Distributes requests across available pods
• Session Affinity: Maintains client session consistency
• Health Checks: Removes unhealthy pods from load balancer
• Traffic Splitting: Supports canary deployments and A/B testing

Scaling and Self-Healing

Automatic scaling and recovery capabilities:

Horizontal Pod Autoscaler (HPA):

• Metrics Monitoring: Monitors CPU, memory, and custom metrics
• Automatic Scaling: Scales pods based on resource utilization
• Target Metrics: Maintains target average utilization
• Scale Limits: Configurable minimum and maximum replicas

Self-Healing:

• Health Checks: Regular health checks on pods and containers
• Automatic Restart: Restarts failed containers automatically
• Node Failure Handling: Reschedules pods from failed nodes
• Replica Management: Maintains desired number of replicas

Kubernetes Resource Management

Namespaces

Logical isolation within the cluster:

Default Namespaces:

• default: Default namespace for user resources
• kube-system: System components and add-ons
• kube-public: Publicly accessible resources
• kube-node-lease: Node lease objects

Namespace Benefits:

• Resource Isolation: Separate resources by project or team
• Access Control: Different RBAC policies per namespace
• Resource Quotas: Limit resource consumption per namespace
• Network Policies: Apply network policies at namespace level

Resource Quotas and Limits

Resource management and constraints:

Resource Quotas:

• CPU and Memory: Limit total CPU and memory usage
• Storage: Limit persistent volume claims
• Object Counts: Limit number of pods, services, etc.
• Namespace Scope: Applied at namespace level

Resource Limits:

• Requests: Minimum resources guaranteed to pod
• Limits: Maximum resources pod can consume
• Quality of Service: Determines pod scheduling priority
• OOM Handling: Out-of-memory handling and eviction

Networking in Kubernetes

Pod Networking

Network connectivity between pods:

Pod Network:

• Unique IP: Each pod gets unique cluster IP address
• Flat Network: All pods can communicate directly
• Network Plugins: CNI plugins provide networking implementation
• Cross-Node Communication: Pods can communicate across nodes

Service Networking:

• Virtual IP: Services get virtual IP addresses
• Load Balancing: Distributes traffic across service endpoints
• DNS Resolution: Automatic DNS resolution for services
• Port Mapping: Maps service ports to pod ports

Network Policies

Security and traffic control:

Policy Types:

• Ingress Rules: Control incoming traffic to pods
• Egress Rules: Control outgoing traffic from pods
• Namespace Isolation: Isolate traffic between namespaces
• Pod Selectors: Target specific pods or namespaces

Implementation:

• Network Plugins: Requires network plugin that supports policies
• Policy Enforcement: Applied at network level
• Default Policies: Allow or deny traffic by default
• Policy Chaining: Multiple policies can be applied

Storage in Kubernetes

Persistent Volumes

Persistent storage for applications:

Volume Types:

• Local Storage: Node-local storage
• Network Storage: NFS, iSCSI, cloud storage
• Cloud Volumes: AWS EBS, Azure Disk, GCP PD
• Distributed Storage: Ceph, GlusterFS

Volume Lifecycle:

• Provisioning: Dynamic or static volume provisioning
• Binding: Binding volumes to persistent volume claims
• Mounting: Mounting volumes to pods
• Reclaiming: Volume cleanup after pod deletion

ConfigMaps and Secrets

Configuration and sensitive data management:

ConfigMaps:

• Configuration Data: Non-sensitive configuration data
• Environment Variables: Inject configuration as environment variables
• Files: Mount configuration as files in pods
• Hot Reloading: Update configuration without pod restart

Secrets:

• Sensitive Data: Passwords, tokens, certificates
• Encryption: Encrypted storage in etcd
• Access Control: RBAC-based access to secrets
• External Integration: Integration with external secret managers

Security in Kubernetes

Authentication and Authorization

User and service authentication:

Authentication Methods:

• Client Certificates: X.509 certificates for API access
• Bearer Tokens: JWT tokens for service accounts
• OpenID Connect: Integration with identity providers
• Webhook: Custom authentication webhooks

Authorization:

• RBAC: Role-based access control
• ABAC: Attribute-based access control
• Node Authorization: Node-specific authorization
• Webhook Authorization: Custom authorization webhooks

Pod Security

Security policies for pods:

Pod Security Standards:

• Privileged: Unrestricted access (not recommended)
• Baseline: Minimal security restrictions
• Restricted: Maximum security restrictions

Security Context:

• RunAsUser: User ID for running containers
• RunAsGroup: Group ID for running containers
• ReadOnlyRootFilesystem: Read-only root filesystem
• Capabilities: Linux capabilities for containers

Monitoring and Observability

Metrics and Monitoring

Cluster and application monitoring:

Metrics Sources:

• kubelet: Node and pod metrics
• kube-state-metrics: Kubernetes object metrics
• cAdvisor: Container resource metrics
• Custom Metrics: Application-specific metrics

Monitoring Tools:

• Prometheus: Metrics collection and storage
• Grafana: Metrics visualization and dashboards
• AlertManager: Alerting and notification
• Custom Dashboards: Application-specific monitoring

Logging

Centralized logging for applications:

Log Collection:

• Container Logs: Standard output and error streams
• Node Logs: System and application logs
• Audit Logs: API server audit logs
• Event Logs: Kubernetes events

Logging Solutions:

• Fluentd: Log collection and forwarding
• Elasticsearch: Log storage and indexing
• Kibana: Log visualization and search
• Centralized Logging: Unified logging infrastructure

Best Practices

1. Resource Management

Efficient resource utilization:

• Resource Requests: Always specify resource requests
• Resource Limits: Set appropriate resource limits
• Horizontal Scaling: Use HPA for automatic scaling
• Resource Quotas: Implement namespace resource quotas

2. Security

Security-first approach:

• RBAC: Implement role-based access control
• Network Policies: Use network policies for traffic control
• Pod Security: Apply pod security standards
• Secret Management: Use external secret managers

3. Monitoring

Comprehensive monitoring:

• Health Checks: Implement liveness and readiness probes
• Metrics Collection: Collect application and infrastructure metrics
• Logging: Centralized logging for all applications
• Alerting: Set up alerts for critical issues

4. Backup and Recovery

Disaster recovery planning:

• etcd Backup: Regular backup of etcd data
• Application Backup: Backup application data and configurations
• Recovery Procedures: Document recovery procedures
• Testing: Regular testing of backup and recovery procedures