# kubernetes

## minikube
```
minikube start
--vm-driver='': Driver is one of: [virtualbox parallels vmwarefusion kvm2 vmware none] (defaults to auto-detect)

minikube service --url vote-app
```


## concepts
docker: container, image, tags

kubernetes:
-   nodes (formerly minions): machine (hardware or vm) units
-   cluster: set of nodes
-   pods: multiple containers, share one ip and volumes, run on same machine
-   services: set of pods
-   replication controllers: restart pods, load balancing, scaling
-   replica set: replacement for replication controller, `selector` is required
-   labels: key-value
-   volumes
-   secrets
-   namespaces
-   cluster: set of grouped nodes, balancing, fallback

## components
master node, part of cluster, watches over nodes:
-   api server: frontend
-   etcd: key-value store
-   scheduler: assigns containers to nodes
-   controller: notice and respond to changes

worker node:
-   container runtime: f.e. docker
-   kubelet: runs on nodes, watches containers

## rollout / deployment
`kubectl create` or `kubectl run`.
deployments create their own replicasets.

deployment strategies:
-   recreate: destroy and rebuild, downtime
-   rolling update, default: each pod

-   `kubectl apply -f <file>`
-   `kubectl rollout undo deployment/myapp-deployment`
-   `kubectl rollout status deployment/myapp-deployment`

## networking
-   pods get ip adresses
-   to prevent conflicts in the cluster need to install/configure s.th., not builtin

## services
exposes port which forwards to several pods of same type.
-   kinds
    -   `ClusterIP`: default, exposes on a cluster-internal ip
    -   `NodePort`: exposes publicly on each node at a static port
        (`curl <nodeIP>:<nodePort>`), includes ClusterIP service
    -   `LoadBalancer`: connects w/ cloud provider's load balancer
-   `targetPort`: on pod, what container provides, defaults to `port`
-   `port`: on service, like virtual server inside node, required
-   `nodePort`: on each node, 30.000-23.767, defaults to autogenerate
-   `clusterIP`: to be identified inside cluster
-   matches to pods via selector, clusterwide
-   balancing: random w/ `SessionAffinity`

## tools
### `kubeadm`
used to setup kubernetes cluster.

### `kubectx`
show and select contexts (clusters), f.e.
```
kubectx      # list all
kubectx foo  # switch to "foo"
kubectx -c   # show current
kubectx -h
```

### `kubens`
show and select namespaces inside context.
```
kubens      # list all
kubens foo  # switch to "foo"
kubens -c   # show current
kubens -h
```

### `kubectl`
```
kubectl config use-context docker-for-desktop
kubectl cluster-info
kubectl get nodes
# create pod, deploy image
kubectl run nginx --image=nginx
kubectl get pods
kubectl get pods -o wide
kubectl get pods -o yaml
kubectl describe pods
kubectl get deployments
kubectl delete deployment nginx
kubectl get all
```

## yaml
-   `apiVersion`: `v1` for pods and services, `app/v1` for replicasets and deployments
-   `kind`
-   `metadata:name`: cascades to subcomponents, that is pods will have name of replicaset or deployment, don't need to be unique, but probably a good idea
-   `metadata:labels`: used for selectors and custom categorizing

-   `kubectl create -f <file>`
-   `kubectl create -f .` processes all files in dir
-   `kubectl replace -f <file>` disruptive update
-   `kubectl apply -f <file>` create or in-place update
-   `kubectl diff -f <file>` show what `apply` would do

## namespaces
-   `kube-system`
-   `kube-public`
-   `default`: playground, don't use on real-life cluster
-   connect to service "db" via `connect("db")`, from other namespave via
    `connect("db.<namespace>.svc.cluster.local")`
-   specify namespace via `kubectl --namespace` or via yaml as `metadata:namespace`
-   `kubectl create namespace myapp-dev` or via yaml
-   `kubectl config set-context $(kubectl config current-context) --namespace=myapp-dev`
-   `kubectl get pods --all-namespaces`
-   `ResourceQuota` puts limits on namespaces


# unsorted
-   `Ingress` does what `Route` does in okd
-   `Helm` is a thing
-   `containerPort` in pod def, does it obsolete service port defs?