# Exposing Kubernetes Ports

> Markdown source of <https://docs.calebsargeant.com/computing/kubernetes/exposing-ports/>.

## Service Types

### Exposing Containers

- `kubectl expose` creates a **service** for existing pods

- A **service** is a stable address for pod(s)

- If we want to connect to pod(s), we need a **service**

- CoreDNS allows us to resolve **services** by name

- There are different types of **services**

  > - ClusterIP
  > - NodePort
  > - LoadBalancer
  > - ExternalName

### Basic Service Types

- ClusterIP (default)

  > - Single, internal virtual IP allocated
  > - Only reachable from within the cluster (nodes and podes)
  > - Pods can reach service on apps port number

- NodePort

  > - High port allocated on each node
  > - Port is open on every node's IP
  > - Anyone can connect (if they can reach the node)
  > - Other pods need to be updated to this port

- These services are always available in Kubernetes

### More Service Types

- LoadBalancer

  > - Controls a LB endpoint external to the cluster
  > - Only available when infra provider gives ou a LB (AWS ELB, etc)
  > - Creates NodePort+ClusterIP services, tells LB to send to NodePort

- ExternalName

  > - Adds CNAME DNS record to CoreDNS only
  > - Not used for Pods, but for giving pods a DNS name to use for something outside Kubernetes

## Creating a ClusterIP Service

- Open two shell windos so we can watch this

  > - `kubectl get pods -w`

- In second window, lets start a simple http server using sample code

  > - `kubectl create deployment httpenv --image=bretfisher/httpenv`

- Scale it to 5 replicas

  > - `kubectl scale deployment/httpenv --replicas=5`

- Lets create a ClusterIP service (default)

  > - `kubectl expose deployment/httpenv --port 8888`

### Inspecting ClusterIP Service

- Look up what IP was allocated

  > - `kubectl get service`

- Remember this IP is cluster internal only, how do we curl it?

- If you're on DOcker Desktop (Host OS is not container OS)

  > - `kubectl run --generator=run-pod/v1 tmp-shell --rm -it --image bretfisher/netshoot -- bash`
  > - `curl httpenv:8888`

- If you're on Linux host

  > - curl \[ip of service\]:8888

## Creating a NodePort and LoadBalancer Service

### Create a NodePort Service

- Lets expose a NodePort so we can access it via the host IP (including localhost)

  > - `kubetcl expose deployment/httpenv --port 8888 -name httpenv-np --type NodePort`

- Did you know that a NodePort service also creates a ClusterIP?

- These three services are additive, each one creates the ones above it:

  > - ClusterIP
  > - NodePort
  > - LoadBalancer

### Add a LoadBalancer Service

- If you're on Docker Desktop, it provides a built-in LoadBalancer that publishes the --port on localhost

  > - `kubectl expose deployment/httpenv --port 8888 --name httpenv-lb --type LoadBalancer`
  > - `curl localhost:8888`

- If you're on kubeadm, minikube, or microk8s

  > - No built-in LB
  > - You can still run the command, it'll just stay at "pending" (but its NodePort works)

## Kubernetes Services DNS

- Starting with 1.11, internal DNS is provided by CoreDNS

- Like Swarm, this is DNS-Based Service Discovery

- So far we've been using hostnames to access services

  > - `curl <hostname>`

- But that only works for Services in the same Namespace

  > - `kubectl get namespaces`

- Services also have a FQDN

  > - `curl <hostname>.<namespace>.svc.cluster.local`