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Cosmas Nyairo
Cosmas Nyairo

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My CKAD Prep

#kubernetes #ckad #containers #devops

Table of Contents

Notes

Before we dive into the templates and commands, here are some notes about Kubernetes that you should keep in mind:

Cluster is a collection of nodes

Master node has:

  • api sever (frontend for kube)
  • etcd (key store for data used to manage cluster)
  • scheduler (assigns containers to nodes)
  • controller (bring up containers when they go down)

Worked node has:

  • kubelet (agent in each node in cluster that ensures containers running on nodes as expected)
  • runtime (software to run containers in the background i.e docker)

Multi container pods can communicate to each other through localhost

Resources in a namespace can refer to each other by their names

pods have a 1 to 1 relationship with containers
entrypoint in docker -> command in kubernetes
cmd in docker -> args in kubernetes
kube system - resources for internal purposes for kubernetes
kube public - resources to be made available to all users
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kubeapiserver path is /etc/kubernetes/manifests/kube-apiserver.yaml
To check settings passed to kubeapi server: ps -aux | grep authorization

To view api resources and their shortnames:

kubectl api-resources
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To enable alpha versions: --runtime-config=api/version

Run watch crictl ps to wait for kube api server to come back online

To handle api deprecations:

kubectl convert -f <old-file> --output-version <new-api-version> > <new-file>
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Now, let's dive into the templates and commands you can use for resources.

Pod

Single instance of an application (smallest object we can create in k8)

We scale pods up or down

kubectl get pods -o wide
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kubectl get pods -A
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kubectl label pod/<pod-name> <label>=<value>
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kubectl get pods,svc
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kubectl get pods --no-headers | wc -l
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kubectl run <pod-name> --image=<image-name>  -n <namespace> --dry-run=client -o yaml > test.yaml
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kubectl set image pod <pod-name> <container-name>=<image>
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kubectl get pod <pod-name> -o yaml > pod-definition.yaml
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kubectl exec -it podname -- commandtorun
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kubectl replace --force -f app.yaml
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kubectl apply -f test.yaml
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kubeclt edit pod <pod-name>
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kubectl explain pods --recursive | less
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kubectl explain pods --recursive | grep envFrom -A<number-of-lines>
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kubectl get pods --selector label=value
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kubectl get pods --selector label=value,label2=value2
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  • Selectors match labels described i.e match labels on the pod or service to match labels of a pod

  • Annotation records details for informatory purposes e.g build information, tool used.

Replication Controller and Replicaset 

Replication controller is in v1 while replicaset in apps/v1

Replica set uses selector to determine pods to watch and manage even existing pods
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kubectl create -f definition.yaml
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kubectl get replicationcontroller
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kubectl get rs
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kubectl delete replicaset <replicaset-name>
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kubectl edit replicaset <replicaset-name>
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kubectl set image rs <replica-set> <container-name>=<image>
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kubectl describe replicaset <replicaset-name>
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kubectl apply -f definition.yaml
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kubectl replace -f definition.yaml
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kubectl scale -replicas=10 -f definition.yaml
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kubectl scale --replicas=0 replicaset/<replicaset-name>
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Deployments 

kubectl create deployment nginx --image=nginx
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kubectl scale deploy/webapp --replicas=3
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kubectl get deploy
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kubectl get deploy -o wide
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kubectl delete deployment <deployment -name>
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kubectl create deploy redis-deploy --image=redis --replicas=2 -n dev-ns
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  • Deployment strategies:
    • Recreate - Remove all applications running on older verision and bring up applications running on newer verision
    • Rolling update (default strategy) - Remove a single application and bring up a new one one by one until the newer version is running on all applications
    • A new replica set is created under the hood when we do deployment upgrades
    • We use record flag to save commands used to create/update deployments
    • We use the to revision flag to rollback to a specific revision 
kubectl set image deploy/deploymentname <container-name>=<image>
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kubectl set image deploy/deploymentname <container-name>=<image> --record
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kubectl rollout restart deploy/deploymentname
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kubectl rollout status deploy/deploymentname
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kubectl rollout history deploy/deploymentname
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kubectl rollout undo deploy/deploymentname
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kubectl rollout undo deploy/deploymentname --to-revision=1
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Namespaces 

kubectl create ns <namespace-name>
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kubectl config set-context $(kubectl config current-context) -n <namespace-name>
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Services

Enable communication between components within the application

Types:

  • Node Port
    • Node Port Definition file
    • Map a port on node to a port on the pod
    • The node's port Can only be in the range 30000 to 32767
    • Node port -> Service -> Target port (Pod's port)
    • Node port and service port are not mandatory, if not provided, node port is allocated an available ip in the range 30000 to 32767 while service port is assumed to be same as port
    • Acts as loadbalancer if we have multiple pods with the same label, it uses a random algorithm to select which pod to send requests to.

  • Cluster Ip

  • Load Balancer

    • Builds on top of node port and allows balancing of requests to the service to it's target applications

To connect to another service in a different namespace: we use the following syntax:

<service-name>.<namespace>.<svc>.<domain>
test-service.test-ns.svc.cluster.local
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kubectl expose resource <resource-name> --type=<type> --port=<port> --target-port=<target-port> --name=<service-name>
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kubectl create service <type> <service-name> --tcp=<port>:<port>
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Config Map 


  kubectl create configmap \
      app-config --from-literal=APP_COLOR=RED \
                 --from-literal=APP_TYPE=AAB \
                 --from-literal=APP_ENV=PROD \
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  kubectl create configmap \
      app-config --from-file=app_config.properties
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  • Declarative: 
  kubectl create -f test-config-map.yaml
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  kubectl get configmaps
  kubectl get cm
  kubectl describe configmap
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Secrets 

  kubectl create secret generic \
      app-secret --from-literal=DB_HOST=mysql \
                 --from-literal=DB_PORT=1000
                 --from-literal=DB_NAME=test \
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  kubectl create secret generic \
      app-secret --from-file=test.env
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  • Declarative 
  kubectl create -f test-secret.yaml
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  kubectl get secrets
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  kubectl get secret <secret-name> -o yaml
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  kubectl describe secrets
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Security Context

We can add security context on the pod level or the container level. If both are specified, the container level takes precedence.

Cabapilites are supported only on the container level

Service Account 

kubectl create serviceaccount test-sa
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kubectl get serviceaccount
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kubectl describe serviceaccount test-sa
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kubectl create token SERVICEACCOUNTNAME
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Taints and Tolerations 

kubectl taint nodes NODENAME app=red:taint-effect
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kubectl taint nodes NODENAME app=red:taint-effect-
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taint-effects include: NoSchedule|NoExecute|PreferNoSchedule

Node Selectors 

kubectl label nodes NODENAME type=test
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We cannot apply advanced filters e.g not , or

Node Affinity

Node affinity types:

  • requiredDuringSchedulingIgnoredDuringExecution
  • PreferredDuringSchedulingIgnoredDuringExecution
  • requiredDuringSchedulingrequiredDuringExecution
During Scheduling During Execution
1 Required Ignored
2 Preferred Ignored
3 Required Required

Multicontainer Pods

  • Multicontainer Pods Definition file

  • Sidecar containers -> help the main container e.g to logging agent to send logs to log server

  • Adapter containers -> process data for the main container e.g converts logs to readable format before sending to logging server

  • Ambassador containers -> proxy requests from the main container e.g send requests to db on main container's behalf

  • Init Containers Definition file

  • Init containers -> Process inside init container must finish before other containers start.

  • If the init container fails, the pod is restarted until the init container succeeds

Observability

Readiness Probe:

  • Perform test to check if the container is up before marking the container as ready.
  • For readiness, we can do http calls, tcp calls or run a command that when succesfull, we mark the container as ready

Liveness Probe:

  • Periodically test if application within container is healthy.

  • For liveness, we can do http calls, tcp calls or run a command that when they fail, we mark the container as unhealthy and it's restarted

  • Readiness & Liveness Probe Definition file

Logging:

Show logs

kubectl logs podname
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Show live logs

kubectl logs -f podname
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For multi container we specify container name

kubectl logs -f podname container-name
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Metric Server:

  • We can have 1 metric server per cluster
  • Receives metrics from nodes and pods and stores them in memory ( we can't see historical data with metric server)
  • To install on cluster,we clone the metric server repo and run kubectl create -f repourl 
kubectl top node
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kubectl top pod
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Jobs and Cronjobs

Job - Tasks that can be run and exit after they've finalized

kubectl get jobs
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Cronjob - To run jobs periodically

kubectl get cronjob
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Ingress

  • Ingress Definition file

  • Enables users access application through an externally accessible url that we can configure to route to different services in the cluster based on url path while implementing ssl as well

  • We need to expose it so it can be accessible outside the cluster

  • Ingress controller:

    • Does not come with kubernetes as default. We need to deploy if first.
    • Examples are istio, nginx, haproxy

  • Ingress resources:

    • Rules and configs applied to ingress controller to forward traffic to single applications, via paths or via domain name 
kubectl get ingress
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kubectl create ingress <ingress-name> --rule="host/path=service-name:port"
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kubectl create ingress world --rule=world.universe.mine/europe*=europe:80 --rule=world.universe.mine/asia*=asia:80  --class=nginx -n world
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Network Policies 

kubectl get netpol
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Storage 

  kubeclt delete pvc <pvc-claim>
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Authentication

All user access is managed by the api server

We can store user credentials as:

  • Static Password File: 
    # Add this to kubeapi server service or pod definition file
    --basic-auth-file=user-credentials.csv
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    curl -v -k <api-url> -u "user:password"
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  • Static Token File: 
   # Add this to kubeapi server service or pod definition file
   --token-auth-file=user-credentials.csv
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    curl -v -k <api-url> --header "Authorization: Bearer <TOKEN>"
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We can use kubeconfig to manage which clusters we can access

kubectl config view --kubeconfig=my-custom-file
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kubectl config use-context developer-development-playground
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kubectl config get-context developer-development-playground
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Authorization

Authorization modes:

  • Node authorizer -> handles node requests (user should have name prefixed system node)
  • Attribute based authotization -> Assosiate users/group of users with a set of permissions(difficult to manage)
  • Role bases access control -> We define roles and associate users with specific roles
  • Webhook -> Outsource authorization to 3rd party tools
  • AlwaysAllow -> Allows all requests without doing authorization checks (default)
  • AlwaysDeny -> Denys all requests

On kubeapiserver, we specify modes to use --authorization-mode=Node,RBAC,Webhook

Role based access control:

kubectl get roles
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kubectl create role test --verb=list,create --resource=pods
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kubectl describe role <role-name>
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kubectl get rolebindings
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kubectl create rolebinding test-rb --role=test --user=user1 --group=group1
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kubectl describe rolebindings <rolebindings-name>
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kubectl auth can-i create deploy
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kubectl auth can-i create deploy --as dev-user --namespace dev
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kubectl api-resources --namespaced=false
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We can create roles scoped on clusters. We can also create cluster roles on namespace scoped resources

Cluster Role based access control:

kubectl create clusterrole test --verb=* --resource=*
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kubectl create clusterrolebinding test-rb --clusterrole=test --user=user --group=group
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Admission Controllers

  • Implement security measures to enforce how a cluster is used.
  • It can validate, change or reject requests from users
  • It can also perform operations in the backend 
kube-apiserver -h | grep enable-admission-plugins
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On kubeapiserver, to enable an admission controller we update the --enable-admission-plugins=NodeRestriction,NamespaceLifecycle
On kubeapiserver, to disable an admission controller we update the --disable-admission-plugins=DefaultStorageClass

We can create custom admission contollers:

  • We use the Mutating and Validating webhooks that we configure to a server hosting our admission webhook service.
    If a request is made, it goes sends an admission review object to admission webhook server that responds whith an admission review object of whether the result is allowed or not

  • We then deploy our admission webhook server

  • We then configure to reach out to the service and validate or mutate requests by creating a validating configuration object

Custom Resource Definition

An extension of the Kubernetes API that isn't available in the Kubernetes installation

Custom Controller

Process or code running in a loop and monitoring the kubernetes cluster and listening to events of specific objects

We build the custom controller then provide kubeconfig file the controller would need to authenticate to the kubernetes api

Thanks!

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