Kubernetes

Kubernetes

1. CONTAINERS

   • standarized, portable and consistent application environments
     • Control groups (cgroups)
       • Hardware resources management
         • Allow the host to share or limit resources for a process or container
     • Namespaces
       • Process, filesystem and networking isolation
         • Container processes are limited to see only what is in the same namespace
     • Union filesystems
       • Containers run from an image (.iso)

2. Components

   • Most common ones for basic deployments
     • Node
       • A physical or virtual machine
       • Where the service is deployed
     • Pod
       • the smallest unit of K8's
       • Runner environment over the Node
         • Abstraction over container
         • Helps so it only interacts with the kubernetes layer
           • Can run a variety of container technologies
       • There's usually one container (appliaction) per pod
       • Each Pod gets an IP of a K8 virtual network
         • Gets a new IP each time it restarts
     • Service
       • Static IP address
         • Can be attached to a Pod
           • If a Pod dies, the service keeps the IP address
         • Can be external (targets the node) or internal (targets the pod)
       • Load Balancer
     • Ingress
       • Does forwarding from requests to services
       • Ensures TLS and DNS
   • Meta-configuration
     • ConfigMap
       • External configuration for an application
         • config services URL, etc.
     • Secret
       • Contains meta-variables encoded in a base64 format
         • variables like credentials
   • Data Storage
     • Volumes
       • Data in Pods are volatile
       • Attaches a physical storage in the hard drive to a Pod
         • Storage can be done on the local machine or remotely (outisde of K8 cluster)
   • Replica and Schedule Management
     • Deployment
       • Blueprints for applications running on pods
         • Abstraction of Pods
       • Adjust the number of replicas of a Pod
     • StatefulSet
       • Avoid data inconsistencies on pods that need persistant data (like datbases)
         • Control which pods write on volumes
       • for STATEFUL applications

3. Architecture

   • Types of nodes
     • Master
       • 4 Processes necessary on Master Node
         • API Server
           • Interaction from user to cluster to deploy applications
           • Can be a UI, CLI (kubelet)
           • Cluster gateway which validates requests
         • Scheduler
           • Decides which node starts an application Pod
           • Schedules based on performance and resource usage on Pods
         • Controller Manager
           • Monitors and detects state changes like Pod failures
           • Tells Scheduler to re-schedule downed Pods
         • etcd
           • key value stored of the cluster state
           • Logs of cluster activities
             • Application data is not stored, only cluster related data
     • Slave
       • 3 Processes necessary on each node
         • Container runtime
           • Ex. Docker
         • Kubelet
           • Interacts with the container runtime and the pod
           • Assign resources to the container
         • Kube Proxy
           • Forwards requests from services to Pod
           • Performance of communication
       • also called Worker node
       • Does the application (service) processing

4. It is a Container Orchestrator

   • Makes microservices more available
     • Services that are Highly maintainable and testable, loosely coupled, independtly deployable and owned by a small team.
   • Multiple development environment scenarios
     • Cloud
     • On premises
     • Hybrid
   • Management of large scale conteneraized applications
     • Offering availability, scalability and disaster recovery of services

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