DevOps Classroom Series – 10/Apr/2021

Introducing the problem

  • Consider we have replicaset where we deployed the application with 4 replicas Preview
  • Now when we get a new release from dev team, we build a new docker image and upload it to the docker hub or any private registry
  • How can we apply the new image to replicaset?
    • one approach is to change the replicaset and apply new changes (new docker image) but this will have downtime
  • We need a solution where we can apply the new docker images with replications without having downtime
  • This can be achieved by kubernetes Deployment controller object

Deployment

  • A deployment is a k8s object that acts a wrapper around Replica Set and makes it easier to use.
  • The major motivation for deployments is to maintain the history of revisions.
  • Every time a change is made to the replica set or the underlying pods a new version of the ReplicaSet is recorded by Deployment. This way using a Deployment makes it easy to rollback to previous state or version
  • Hierarchy of Deployment, ReplicaSet, Pods and container Preview
  • Strategy: In this strategy section we can define strategy to replace old pods with new ones when you are updating . There are two kinds of strategies
    • RollingUpdate
    • Recreate
  • RollingUpdate:
    • We use this strategy to update deployment without having a downtime
    • In Rolling update we specify two important values
      • maxUnavailable: This is max number of Pods that can be unavailable during the update. default value for maxUnavailable is 25%
      • maxSurge: this is number of Pods that can be scheduled/created above the desired number of Pods. default value for maxUnavailable is 25%
  • Recreate: In this strategy all the existing pods will be killed b4 creating new pods, so there will be some down time
  • Lets create a simple deployment
  • Refer Here for the changeset
  • Lets apply the template Preview Preview
  • Labels and annotations on the deployment
    • k8s adds an annotation with the deployment.kubernetes.io/revision key which contains information about how many revisions have been there for particular deployment Preview
  • Added a simple service to access jenkins Refer Here for spec
  • Now make some changes to simulate new application version Refer Here for the changes
  • Now lets apply the changes using kubectl apply with –record flag. This flag esures that the the update to the deployment is recorded in the history of deployment Preview Preview Preview
  • Lets undo the deployment Preview Preview

DaemonSets

  • DaemonSets are used to manage the creation of particular pod on all or set of selected nodes in a cluster
  • Use cases for DaemonSet
    • Logging: To manage running a log collection pod on all nodes and then process the logs in a log processing pipeline (sending logs to central logging server)
    • Local data caching
    • Monitoring
  • Lets write one daemon set
    • Refer Here for the changeset
    • lets apply the template Preview

Jobs

  • A job is supervisor in k8s that can be used to manage pods to run some script/task and exit gracefully

  • The pods created by the job arenot deleted following the completion of job. The pods run to completion and stay with a Completed status

  • Lets create a k8s job configuration which will run the alpine pod to sleep for 2 minutes (sleep 2m) with restartPolicy OnFailure

  • Refer Here for the changeset Preview Preview

  • Exercise: Try to create a cron job spec which run every 15 minutes using same pod spec

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