Kubernetes Major Workloads

Pod

• Refer Here for official docs
• Pods will have one or more containers
• Each Pod gets an IP Address which is shared to the container(s)
• Pods will two types of containers in it
  • init containers: They are created in a sequence and they are expected to finish in some finite time. Generally we do precondition checks over here
  • containers: These are where we run our application images and they should run forever and they are created in parallel
• In Pod the desired state is containers, when containers fail k8s will continuously restart containers
• There is one more type of container called as ephemeral container which is used for debugging purposes
  

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Controllers

ReplicaSet

• Refer Here
  

Deployment

• Refer Here for deployment

Daemonset

• This will create a pod on every node (or selected nodes) in k8s cluster
• Refer Here

Statefulset

• Statefulsets are used to run stateful applications and they have capability to manage volumes as well

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Label

• this is a key value pair which can be attached to any k8s object
• k8s can query objects with the help of labels

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Service

Refer Here for official docs

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Kubernetes API Server and Clients

• kubectl uses config in ~/.kube/config to get the cluster information and secrets to connect with api server Refer Here
• kubectl command line tool cheatsheet Refer Here
• kuberentes also provides client libraries in various languages Refer Here
• kubectl has two styles of resource creation
  • imperative:
    • we construct a command to create a resource
    • quick and convient way to perform one time or infrequent operations.
  • declarative
    • We write a manifest in yaml format with our desired state
    • Declarative way is useful for automation, repetition and having history of changes
• Kubernetes api versioning scheme Refer Here
• To group releated resources k8s use apiGroups Refer Here
  • core
  • apps
  • batch
  • …
• API Version: APIGROUP/version for all groups other than core. For core the APIVERSION = version
• When we are writing manifest we specify (generally)
  • apiVersion: Api version of Resource
  • kind: Type of Resource
  • metadata: name and labels
  • spec: here we define what we want
• Once we execute this kuberentes adds the fifth field status