DevOps Classroomnotes 09/Jul/2022

Service Discovery

The service-discovery tools help solve the problem of finding which process are listening at which address.
Real discovery in k8s starts with a Service Object.
Service object is a way to create a named label selector

k8s Service

K8s service when created gets a cluster ip which is virtual in nature, when any other resource tries to access the service using cluster ip it forwards to request to one of the pod matching labels
An easier way to create the service is by using kubectl expose
k8s service uses a label selecter which will find all the pods with matching labels and will load-balance across all the pods
since the cluster ip is virtual, its stable and it is appropriate to give it a DNS address.
k8s provides a DNS service exposed to Pods running in cluster.
Lets create a k8s service Refer Here for the changes
use kubectl to create and describe the service
Create a test pod and try to access the service using curl command to check if the loadbalancing is happening or not
The full dns name for the service would be service-name.namespace.svc.cluster.local => nginx-svc.default.svc.cluster.local
Service should not forward the request to faulty pods, as this might impact application access, so lets see what can be done over here
- Readiness Checks/Probes:
  - This is to check whether the application in container running in Pod is ready to serve requests or not
  - If this check fails the k8s removes the Ip address of Pods from all endpoints in Services
- Liveness Checks/Probes:
  - This is to check whether or not application in container is running or not.
  - K8s restarts containers if this check fails based on restart policy
Refer Here for writing checks or probes
Refer Here for the sample probes
Accesing the service from outside cluster: For this in k8s service we have 3 options
- Node Port: Where you expose service on some port of the node
- Load Balancer Integration: Generally in all the managed clusters like AKS, EKS, GKE cluster is configured to integrate with external load balancers, so this can be used to expose the service
- ExternalName: Will be a DNS record which you can add to existing DNS servers

Kubernetes Deployment

We know that Replicaset manage pods.
Deployment manages replica set.
K8s is a self-healing system. The top level deployment object manages replicaset, when you adjust number of replicas it will not match desired state so it will scale up or down
Deployment allows us to deploy the newer versions of the applications by ensuring it supports all the necessary options to minimize/make zero down time deployments and rollout to a new version or roll back to the older version.
Refer Here for the changeset & Refer Here for the fix for wrong indentation
Now lets apply a new version (changing the container name to httpd)
Refer Here for the changes done
To go back to the previous version
Deploy the new version with change reason. Refer Here