learn/kubernetes/components.md

Worker Node

can be a physical server, virtual machine, etc

made up of........

pods

• a wrapper over a container so that Kubernetes can replace them if necessary
• there is usually 1 application per pod (or 1 application with a helper)

pods are EPHEMERAL (they can die very easily) ★ for example, if the app contained in the pod's wrapped container crashes, runs out of resources, etc

when this happens, a new pod is created in it's place, with a new address. To work with this, we attach a.....

Service

• a static IP address that is attatched to each pod/app
• an app (stored in a node) will have it's own service/database, and every pod will have it's own service
• the service lifetime is not connected to the pod's, so when the pod is replaced, the same internal IP can be used
• also works as a load balancer when allocating requests to pods

There are two types of services

1. internal
   • this is the default type
   • can not be accessed externally
2. external
   • you must specify this when creating a service
   • can communicate externally (i.e. via browser for a web app's endpoint)
   • can be accessed via http://[node_ip_addr][service_port_number]

what if you want to access something, but it needs to be via some sort of domain? use.....

Ingress

• exposes HTTP and HTTPS routes from outside the cluster to services within the cluster
• you set rules, which will then decide how requests are routed (forewarded to services or otherwise)

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ConfigMap

• external configurations for your application (i.e. database urls)
• connected to pods so that they have access to this data
• this allows you to just change the value in the configMap without re-building the pods
• stored in plaintext

HOWEVER This type of storage is NOT secure, so kubernetes offers.....

Secret

• just like configMap, but for secrets (i.e. usernames, passowrd, etc)
• stored in base64-encoded format
• is meant to be encrypted using third-party tools (perhaps given by the cloud provider)

BOTH ConfigMap and Secret can be used inside of pods like env vars or as a properties file

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Volume

• used for data storage ★ imagine there is a "database" pod. If that pod restarts, your data is gone
• attaches physical storage to a pod
• can be on the same machine the pod is running on or a remote source outside of the cluster
• kubernetes does not manage data persistance