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
    • so different container cant have same port
• 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

• depicts the state of the WORST container

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

Container states:

• waiting
• running
• terminated

How pods handle problems with containers

• Transclude of Kubernetes-Pod#^32aa8f

• Following sequence:
  1. Initital crash: kubernetes attempts an immediate restart based on Pod’s restartPolicy
  2. repeated crashes: applies an exponential backoff delay for subsequent restarts, described in restartPolicy
  3. CrashLoopBackOff state: backoff delay mechanism is in effect
  4. Backoff reset: if a container runs successfully for a certain duration

container restarts

container restarts and resilience

Pod-level container restart policy

• if exit code is 0: only restart for restartPolicy=Always
• if exit code is 1: restart for restartPolicy=Always and OnFailure

Kubernetes Init Container

Kubernetes Sidecar Container

Kubernetes Ephemeral Container

---

API

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

PodSpec

• --- containers ---
• containers: []Container
• initContainers: []Container ^92329b
  • spec for Kubernetes Init Container
• …
• --- volumes ---
• volumes:
• --- scheduling ---
• nodeSelector: map[string]string
• nodeName
• affinity
  • nodeAffinity: NodeAffinity
  • podAffinity: PodAffinity
  • podAntiAffinity: PodAntiAffinity
• tolerations: [] Toleration
  • key
  • operator
  • value
  • effect
  • tolerationSecond
• schedulerName
• --- lifecycle ---
• …
• restartPolicy: ^32aa8f
  • ”Always” | “Never” | “OnFailure”
• terminationGracePeriodSeconds
• …
• --- hostname and name resolution ---
• --- hosts namespaces ---
• --- service account ---
• --- security context ---
• --- alpha level ---
• hostUsers
• resources: ResourceRequiremets
  • .
  • limits: map[string]Quantity
  • requests: map[string]Quantity

Container:

• name:
• --- image ---
• image:
• imagePullPolicy: string
  • Image pull policy
• --- entrypoint ---
• --- port ---
• --- env var ---
• --- volumes ---
• --- resources ---
• resources:
  • claims
  • limits
  • requests

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