Kubernetes

Kubernetes (aka K8s): A container orchestration tool used in enterprise applications

Main Components

Pod: Abstraction of containers; can be thought of as an abstraction of an application
Service: Service is like a static IP address for each Pod & a load balancer all in one; used for communication without needing to hardcode an IP address that will end up changing
Ingress: Used to route traffic into the cluster
ConfigMap: Used for key-value storage of config that you’d like to not have hardcoded in properties files in applications
Secrets: Encrypted key-value storage
- Q: Not sure if this is good to use, as it’s only base64 encoded? I feel like runtime-injected secrets would be better?
Volumes: Similar to Docker, used for data persistence for stateful applications like Databases
Deployments: Pod blueprints including replication behavior
- Not to be used with Stateful applications like Databases; should only be used with stateless applications
StatefulSet: Like a deployment, but for Stateful applications
- According to Nana’s tutorial, it’s very tedious and maybe not as used in the real world?

In K8s, the Worker Node is a server that runs the applications stored in the Pods
- Runs multiple pods
- Usually K8s clusters have multiple Worker Nodes running as replicas, load balanced by Services
Each node has to have 3 processes running on it in order to function correctly with K8s:
- Container runtime (e.g., Docker)
- Kubelet: The K8s process that interacts with both the container and the underlying VM (or physical machine)
  - Responsible for taking the configuration and running the pod, managing Node resources for the various pods, etc…
- Kube Proxy: Forwards requests through services in a performant manner

Master Node: Server that handles the higher level coordination of the cluster
Most K8s clusters are made up of multiple Master Nodes for fault tolerance
Need less physical resources (CPU, RAM, Storage) than Worker Nodes
Tasks include
- Scheduling pod
- Monitoring overall health of cluster
- Rescheduling / restarting Pods when necessary
- Helping new Nodes to join the cluster
4 Process that must run on every Master Node
- Kubernetes API Server: Like a cluster Gateway; all K8s clients interact with this API server which receives instructions to pass down to the Worker Nodes
  - Also is a gateway for Authentication for interacting with the Cluster
- Scheduler: Starts / schedules Pods on Worker Nodes in an intelligent manner without you needing to know which Worker Node the Pod will live on
- Controller Manager: Detects cluster state changes (like Pod crashes)
- etcd: Key-value store of cluster state
  - Like the Cluster Brain, stores this data to then be retrieved by the API Server when we’re looking at dashboards, etc…
  - Application data (like DB data for the app itself) is NOT stored in etcd, just cluster information

minikube: Open source tool to replicate production K8s clusters by running all Master and Worker processes on a single node running in a VM set up via some sort of Hypervisor (e.g., VirtualBox)
- Mainly used for just starting up a mini, local cluster, or deleting it; all interaction / configuration with the cluster happens a client like kubectl
kubectl: CLI for locally interacting with Cluster running anywhere (including in minikube)
- Interacts with cluster via the Master Node API Server process
- Considered the most powerful of all K8s clients