Kubernetes Pod

Description:

What is a pod?

• Kubernetes Object
• Smallest unit in Kubernetes that you can create or deploy.
• a set of containers with shared namespaces and shared filesystem volumes
  • share network namespace, IP, network ports
  • containers in a pod are in same namespace, communicated with eachother over localhost
• Pod can be used:
  • for single container:
    • common use case
  • for mutiple containers that work together
    • tightly couples and need to share resources

Using pods

Working with pods:

• .
• .
• Kubernetes Pod Template

Static Pod:

• Managed by kubelet daemon on a specific node, without the kube-apiserver observing them → not managed by control plane
• bound to one kubelet in 1 node
• main use: components in Kubernetes Control Plane are static pods, they must be created before actual scheduling takes place
• defined in /etc/kubernetes/manifests/
  • the 4 components of control plane can be found here in control plane node
  • each pod has yaml as same as API
• name format: <static-pod-name>-<node-hostname>
• static pods are visible on kube-apiserver because kubelet creates a mirror pod on kube-apiserver for each static pod
  • but can be deleted but kubelet will recreate immediately

Pod lifecycle:

• pod lifetime
• pod phrases

State	Des
Pending	being scheduled or images being pulled from registry
Running:	after successfull attached to a node
Failed	a container terminated with failture and wont be restarting
Succeeded	all containers finished running successfully and they wont be restarting
Unknown	state cant be retrieved
CrashLoopBackOff	one of the containers in the pod exited unexpectedly even after it was restarted

Init containers

Side car containers:

• the secondary containers that run along with the main application container within the same pod
  • such as logging, monitoring, security, proxy, reverse proxy or data synchronization

---

API

• apiVersion: v1
• kind: Pod
• metadata
• spec: PodSpec
• status

PodSpec

• containers: []Container
• initContainers: []Container
• …
• volumes:
• nodeSelector: map[string]string
• nodeName
• affinity
  • nodeAffinity: NodeAffinity
  • podAffinity: PodAffinity
  • podAntiAffinity: PodAntiAffinity
• tolerations:
  • key
  • operator
  • value
  • effect
  • tolerationSecond
• schedulerName

Container:

• name:
• image:
• imagePullPolicy:

EphemeralContainer

LifecycleHandler

NodeAffinity

• preferredDuringSchedulingIgnoredDuringExecution ^9449a7
  • preference
    • matchExpressions
    • matchFields
  • weight
• requiredDuringSchedulingIgnoredDuringExecution ^f1dd99
  • nodeSelectorTerms:
    • matchExpressions
    • matchFields

PodAffinity

• preferredDuringSchedulingIgnoredDuringExecution
  • podAffinityTerm
    • topologyKey
    • labelSelector
    • matchLabelKeys
    • …
  • weight
• requiredDuringSchedulingIgnoredDuringExecution
  • topologyKey
  • labelSelector
  • …

PodAntiAffinity