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Services

K8s services enable communication between components within and outside of K8s apps.
I.E. - groups of pods where...

  • some might be "frontends"
  • some might be "backends"
  • some connect to data sources
  • some might be service-oriented

Services help with communication, with loose coupling between components.

Types of Services

Node Port

A NodePort service...

  • is an object
  • lives in a node
  • listens to ports on the node
  • fwds req from a port to a port the pod
  • allows for pod-access from "outside" the k8s world
  • makes the pod available via a port on the node

Cluster IP

The ClusterIP Service...

  • creates a virtual ip in a cluster
  • lives in a cluster
  • enables communication between services

See a longer article on the topic of ClusterIP Services.

Load Balancer

LoadBalancer services...

  • provision a load-balancer
  • i.e distribute load across servers

External Communicaton

Lets take...

  • a container, in
  • a pod, with an internal pod network and IP (lets say 10.244.0.2 that is in a range of 10.244.0.0-2XX or something) in
  • a node with an IP (lets say 192.168.1.2)
  • a service in the node, the node port service
    • the service is an object
    • listens to ports on the node
    • fwds req from a port to a port the pod
    • its a node port service (see A Page on NodePorts)
  • my laptop with an ip (lets say 192.168.1.10)
  • NOTE: the laptop and the node have the same network

A K8s service, specifically the NodePort service, can "map" the outside world request to internal objects - pods.

A Method For Setting Up A Cluster And Services

Here's one way to go about developing pods, replica sets, deployments, and services:

  1. Create Deployments of pods && replica sets
  2. Create ClusterIP Services to manage communication between nodes
  3. Create LoadBalancer Services

Services Use Selectors

Services use selectors to "know" which k8s objects to connect to.
Two types of selectors work:

  • equality-based
    • by label key/value pair
  • set-based
    • like in, notin, exists

Use kubectl expose to create a service for a deployment

kubectl get deployments
# say there's a deployment called frontend

# create the service for the deployment!
kubectl expose deployment/frontend --port=80 --type=NodePort

kubectl get svc frontend -o yaml
# will return a yaml def of the service!

Expose can take some args:

  • port
  • targetPort

Services can be made without selectors

Perhaps a deploymenet is not ready, but the goals of the service are ready.
Create the service without the deployment.
Services could also "point to" another service! interesting.

Things To Do

  • get how many services exist in a namespace
  • id the...
    • service type of a service
    • TargetPort of the service
    • labels of a service
    • id how many endpoints are attached to a service
  • create a NodePort service with...
    • a name
    • the right type
    • targetport of 8080
    • nodePort of 30080
    • selecting pods with the label name: simple-webapp
Tags:
Kubernetes
k8s
services