Controllers

• Replica Set:

  • Much like Replication Controller
  • Replica Set can be preserved and revisited

• Stateful Sets: For easier understanding,

  • Stateful Sets are much like Replication Controllers with persistent storage
  • note: This definition is not 100% right, but its ok for now

• Jobs & Cron Jobs:

  • Job/Cron Job speaks about specification which will run some application/script/code which will execute and finish

• Note:

  • Like any other k8s object, Controllers also have
    • Specification
    • Status
  • Here the Specification is more about controlling the objects/workloads.
    • Can define workload
    • how to manage the workload

Service

• Consider this Scenario, where we have app server and db server

• In this app server will communicate with db server to store/retrieve the data

• Now due to application load you have scaled pods to 3

• Now lets assume the node on which mysql pod is running has failed, In that case we get a new sql pod which has new ip address

• In this case application stops working as db is not connected.

• To Resolve this , K8s uses Service which is used for communicating to the database pod.

• Now lets assume pod has failed and new mysql pod is created

• When a service is created, in the specification label information has to be provided.

• K8s service will forward traffic to the pods matching the labels given in the service spec

• Service is a Layer-4 load balancer

• In K8s layer-7 load balancing is supported by ingress.