Kubernetes Resources

Pod

• all containers of a pod are on the same node -> a pod never spans two nodes
• why pods and not individual containers? because the principle of Docker & K8s is one process = one container and it’s necessary to have something above containers that’d group containers that host the same app
• pod of containers allows you to run closely related processes together and pro- vide them with (almost) the same environment as if they were all running in a single container, but it also keeps them separated
• all containers are located in the same set of Linux namespaces -> they share the same IP address and port space

Logs

• docker logs <container id>
• cubectl logs <pod name> -c <container name>

Replication & Controllers

• liveness probe
  • mechanism to check if pods are healthy
  • part of YAML
• replication controller (or replica set) -> now deployment
  • checks, how many replicas of pods are
  • if a pod disappeares, it replication controller creates a new one
• horizontal scalling
  • set up higher amount of pods through command line $ kubectl scale rc kubia --replicas=10 or directly in the YAML

Service

• single, constant point of entry to a group of pods providing the same service
• points to pods using labels
• service has a static IP address, which never changes
  • when POD gets killed/deleted/whatever, ReplicaController creates a new one, which has new IP
    • service solves issue with pods changing IP addresses and makes sure that pods receive connection
    • multiple pods may provide the same service, hence the same IP is needed (in this case, the service IP)
• for a service to be accessible from outside, its type has to be set up as NodePort or LoadBalancer

Types Of Services

• ClusterIP
  • accessible only from inside the cluster
• NodePort
  • static port on each node’s IP
• LoadBalancer
  • exploses the service through external load balancer
• ExternalName
  • accessible through CNAME value

Volumes

• component of pod
• a volume is available to all containers in the pod, but it must be mounted in each container that needs to access it
• volumes are used to share data accross containers

Docker

• images are template to create containers, containers are instances created from images
• images are created from another images, two different images can have same parent image as their base
• check additinal information about containers:
  • enter the container -> docker inspect <container name>
  • in the continer -> ps aux