DevOps Classroomnotes 03/Dec/2022

Kubernetes Networking Model

Refer Here to understand the internals of kubernetes networking
K8s dictates following
- all Pods can communicate with all other Pods without using network address translation (NAT).
- all Nodes can communicate with all Pods without NAT.
- the IP that a Pod sees itself as is the same IP that others see it as.
All the Pods in the k8s cluster have a CIDR Range
To implement these k8s takes linux kernel networking features such as netfilter and iptables.

K8s Pods are atomic unit in k8s cluster.
Pods have containers which run applications
Consider the wordpress example. We need to have wordpress container and mysql container for wordpress to work
To run this with Pods
Now lets understand scaling.
Pod life cycle Refer Here
Pod Phases Refer Here
Pod restart policy: Always, Never, OnFailure
Lets write a spec which sleeps for 2 seconds (sleep 2)
- restart policy Never
- not specify restart policy in spec
Refer Here for the specs
Pods can run 3 types of containers
- Containers => Where we run our applications
- init containers:
  - These containers are created one by one and only after its completion, the normal containers are created.
  - We will use these containers for any initial setup or configuration kind of purposes
- ephemeral containers:
  - No guarantee containers, they are used rarely in the case of debugging or trouble shooting containers in Pod
Lets create a Pod with 2 init container which sleep for 5 seconds and then in container we run nginx.
Refer Here for the manifest written
Now create the pod
Writing YAML files to describe the status is referred as declarative approach, k8s also supports imperative approach

kubectl run nginx --image=nginx --restart=Never

* Refer Here for some example imperative commands

Refer Here for official docs
There are many cases where we would want to run multiple instance of a application.
In k8s we run application in Pod and to set mutlple instances we use replica sets or replication controllers.
Lets try to run 5 nginx Pods in our cluster
Refer Here for the manifest
We got an error with selector
To resolve this error we need to understand the concept of labels in k8s.
Refer Here for manifest with labels defined and selectors
Refer Here for the fix with matchLabels section removed
NOte: We will work on this tommorow

Refer Here
In k8s as part of metadata we can apply labels to the resources.
These labels help in querying resources based on conditions according labels defined
Lets create 2 pod specs with label specifications as shown above
Refer Here for the changes
Lets run some command line selectors